Java Interview Questions have been designed especially to get you acquainted with the nature of questions you may encounter during your interview for the subject of Java Programming Language. As per my experience, good interviewers hardly planned to ask any particular question during your interview, normally questions start with some basic concept of the subject and later they continue based on further discussion and what you answer:
Q: What do you know about Java?
A: Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX.
Q: What are the supported platforms by Java Programming Language?
A: Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc.
Q: List any five features of Java?
A: Some features include Object Oriented, Platform Independent, Robust, Interpreted, Multi-threaded
Q: Why is Java Architectural Neutral?
A: It’s compiler generates an architecture-neutral object file format, which makes the compiled code to be executable on many processors, with the presence of Java runtime system.
Q: How Java enabled High Performance?
A: Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode, which is a program that contains instructions that must be interpreted into instructions that can be sent directly to the processor.
Q: Why Java is considered dynamic?
A: It is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.
Q: What is Java Virtual Machine and how it is considered in context of Java’s platform independent feature?
A: When Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.
Q: List two Java IDE’s?
A: Netbeans, Eclipse, etc.
Q: List some Java keywords(unlike C, C++ keywords)?
A: Some Java keywords are import, super, finally, etc.
Q: What do you mean by Object?
A: Object is a runtime entity and it’s state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication.
Q: Define class?
A: A class is a blue print from which individual objects are created. A class can contain fields and methods to describe the behavior of an object.
Q: What kind of variables a class can consist of?
A: A class consist of Local variable, instance variables and class variables.
Q: What is a Local Variable
A: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and it will be destroyed when the method has completed.
Q: What is a Instance Variable
A: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded.
Q: What is a Class Variable
A: These are variables declared with in a class, outside any method, with the static keyword.
Q: What is Singleton class?
A: Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.
Q: What do you mean by Constructor?
A: Constructor gets invoked when a new object is created. Every class has a constructor. If we do not explicitly write a constructor for a class the java compiler builds a default constructor for that class.
Q: List the three steps for creating an Object for a class?
A: An Object is first declared, then instantiated and then it is initialized.
Q: What is the default value of byte datatype in Java?
A: Default value of byte datatype is 0.
Q: What is the default value of float and double datatype in Java?
A: Default value of float and double datatype in different as compared to C/C++. For float its 0.0f and for double it’s 0.0d
Q: When a byte datatype is used?
A: This data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.
Q: What is a static variable?
A: Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
Q: What do you mean by Access Modifier?
A: Java provides access modifiers to set access levels for classes, variables, methods and constructors. A member has package or default accessibility when no accessibility modifier is specified.
Q: What is protected access modifier?
A: Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.
Q: What do you mean by synchronized Non Access Modifier?
A: Java provides these modifiers for providing functionalities other than Access Modifiers, synchronized used to indicate that a method can be accessed by only one thread at a time.
Q: According to Java Operator precedence, which operator is considered to be with highest precedence?
A: Postfix operators i.e () [] . is at the highest precedence.
Q: Variables used in a switch statement can be used with which datatypes?
A: Variables used in a switch statement can only be a byte, short, int, or char.
Q: When parseInt() method can be used?
A: This method is used to get the primitive data type of a certain String.
Q: Why is String class considered immutable?
A: The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.
Q: Why is StringBuffer called mutable?
A: The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make alot of modifications to Strings of characters then StringBuffer should be used.
Q: What is the difference between StringBuffer and StringBuilder class?
A: Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.
Q: Which package is used for pattern matching with regular expressions?
A: java.util.regex package is used for this purpose.
Q: java.util.regex consists of which classes?
A: java.util.regex consists of three classes: Pattern class, Matcher class and PatternSyntaxException class.
Q: What is finalize() method?
A: It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.
Q: What is an Exception?
A: An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread's method invocation stack.
Q: What do you mean by Checked Exceptions?
A: It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
Q: Explain Runtime Exceptions?
A: It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
Q: Which are the two subclasses under Exception class?
A: The Exception class has two main subclasses : IOException class and RuntimeException Class.
Q: When throws keyword is used?
A: If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method's signature.
Q: When throw keyword is used?
A: An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.
Q: How finally used under Exception Handling?
A: The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
Q: What things should be kept in mind while creating your own exceptions in Java?
A: While creating your own exception:
· All exceptions must be a child of Throwable.
· If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
· You want to write a runtime exception, you need to extend the RuntimeException class.
Q: Define Inheritance?
A: It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.
Q: When super keyword is used?
A: If the method overrides one of its superclass's methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field
Q: What is Polymorphism?
A: Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
Q: What is Abstraction?
A: It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.
Q: What is Abstract class
A: These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.
Q: When Abstract methods are used?
A: If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.
Q: What is Encapsulation?
A: It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.
Q: What is the primary benefit of Encapsulation?
A: The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.
Q: What is an Interface?
A: An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
Q: Give some features of Interface?
A: It includes:
· Interface cannot be instantiated
· An interface does not contain any constructors.
· All of the methods in an interface are abstract.
Q: Define Packages in Java?
A: A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.
Q: Why Packages are used?
A: Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations, etc., easier.
Q: What do you mean by Multithreaded program?
A: A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
Q: What are the two ways in which Thread can be created?
A: Thread can be created by: implementing Runnable interface, extending the Thread class.
Q: What is an applet?
A: An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
Q: An applet extend which class?
A: An applet extends java.applet.Applet class.
Q: Explain garbage collection in Java?
A: It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.
Q: Define immutable object?
A: An immutable object can’t be changed once it is created.
Q: Explain the usage of this() with constructors?
A: It is used with variables or methods and used to call constructer of same class.
Q: Explain Set Interface?
A: It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.
Q: Explain TreeSet?
A: It is a Set implemented when we want elements in a sorted order.
Q: What is Comparable Interface?
A: It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.
Q: Difference between throw and throws?
A: It includes:
· Throw is used to trigger an exception where as throws is used in declaration of exception.
· Without throws, Checked exception cannot be handled where as checked exception can be propagated with throws.
Q: Explain the following line used under Java Program:
public static void main (String args[ ])
A: The following shows the explanation individually:
· public: it is the access specifier.
· static: it allows main() to be called without instantiating a particular instance of a class.
· void: it affirns the compiler that no value is returned by main().
· main(): this method is called at the beginning of a Java program.
· String args[ ]: args parameter is an instance array of class String
Q: Define JRE i.e. Java Runtime Environment?
A: Java Runtime Environment is an implementation of the Java Virtual Machine which executes Java programs. It provides the minimum requirements for executing a Java application;
Q: What is JAR file?
A: JAR files is Java Archive fles and it aggregates many files into one. It holds Java classes in a library. JAR files are built on ZIP file format and have .jar file extension.
Q: What is a WAR file?
A: This is Web Archive File and used to store XML, java classes, and JavaServer pages. which is used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static Web pages etc.
Q: Define JIT compiler?
A: It improves the runtime performance of computer programs based on bytecode.
Q: What is the difference between object oriented programming language and object based programming language?
A: Object based programming languages follow all the features of OOPs except Inheritance. JavaScript is an example of object based programming languages
Q: What is the purpose of default constructor?
A: The java compiler creates a default constructor only if there is no constructor in the class.
Q: Can a constructor be made final?
A: No, this is not possible.
Q: What is static block?
A: It is used to initialize the static data member, It is excuted before main method at the time of classloading.
Q: Define composition?
A: Holding the reference of the other class within some other class is known as composition.
Q: What is function overloading?
A: If a class has multiple functions by same name but different parameters, it is known as Method Overloading.
Q: What is function overriding?
A: If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding.
Q: Difference between Overloading and Overriding?
A: Method overloading increases the readability of the program. Method overriding provides the specific implementation of the method that is already provided by its super class parameter must be different in case of overloading, parameter must be same in case of overriding.
Q: What is final class?
A: Final classes are created so the methods implemented by that class cannot be overridden. It can’t be inherited.
Q: What is NullPointerException?
A: A NullPointerException is thrown when calling the instance method of a null object, accessing or modifying the field of a null object etc.
Q: What are the ways in which a thread can enter the waiting state?
A: A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
Q: How does multi-threading take place on a computer with a single CPU?
A: The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
Q: What invokes a thread's run() method?
A: After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
Q: Does it matter in what order catch statements for FileNotFoundException and IOException are written?
A: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
Q: What is the difference between yielding and sleeping?
A: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.
Q: Why Vector class is used?
A: The Vector class provides the capability to implement a growable array of objects. Vector proves to be very useful if you don't know the size of the array in advance, or you just need one that can change sizes over the lifetime of a program.
Q: How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
A: Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
Q: What are Wrapper classes?
A: These are classes that allow primitive types to be accessed as objects. Example: Integer, Character, Double, Boolean etc.
Q: What is the difference between a Window and a Frame?
A: The Frame class extends Window to define a main application window that can have a menu bar.
Q: Which package has light weight components?
A: javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
Q: What is the difference between the paint() and repaint() methods?
A: The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
Q: What is the purpose of File class?
A: It is used to create objects that provide access to the files and directories of a local file system.
Q: What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
A: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
Q: Which class should you use to obtain design information about an object?
A: The Class class is used to obtain information about an object's design and java.lang.Class class instance represent classes, interfaces in a running Java application.
Q: What is the difference between static and non-static variables?
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
Q: What is Serialization and deserialization?
A: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
Q: What are use cases?
A: It is part of the analysis of a program and describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance.
Q: Explain the use of sublass in a Java program?
A: Sub class inherits all the public and protected methods and the implementation. It also inherits all the default modifier methods and their implementation.
Q: How to add menushortcut to menu item?
A: If there is a button instance called b1, you may add menu short cut by calling b1.setMnemonic('F'), so the user may be able to use Alt+F to click the button.
Q: Can you write a Java class that could be used both as an applet as well as an application?
A: Yes, just add a main() method to the applet.
Q: What is the difference between Swing and AWT components?
A: AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button.
Q: What's the difference between constructors and other methods?
A: Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
Q: Is there any limitation of using Inheritance?
A: Yes, since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation.
Q: When is the ArrayStoreException thrown?
A: When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.
Q: Can you call one constructor from another if a class has multiple constructors?
A: Yes, use this() syntax.
Q: What's the difference between the methods sleep() and wait()?
A: The code sleep(2000); puts thread aside for exactly two seconds. The code wait(2000), causes a wait of up to two second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
Q: When ArithmeticException is thrown?
A: The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.
Q: What is a transient variable?
A: A transient variable is a variable that may not be serialized during Serialization and which is initialized by its default value during de-serialization,
Q: What is synchronization?
A: Synchronization is the capability to control the access of multiple threads to shared resources. synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.
Q: What is the Collections API?
A: The Collections API is a set of classes and interfaces that support operations on collections of objects.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
Q: The immediate superclass of the Applet class?
A: Panel is the immediate superclass. A panel provides space in which an application can attach any other component, including other panels.
Q: Which Java operator is right associative?
A: The = operator is right associative.
Q: What is the difference between a break statement and a continue statement?
A: A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.
Q: If a variable is declared as private, where may the variable be accessed?
A: A private variable may only be accessed within the class in which it is declared.
Q: What is the purpose of the System class?
A: The purpose of the System class is to provide access to system resources.
Q: List primitive Java types?
A: The eight primitive types are byte, char, short, int, long, float, double, and boolean.
Q: What is the relationship between clipping and repainting under AWT?
A: When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.
Q: Which class is the immediate superclass of the Container class?
A: Component class is the immediate super class.
Q: What class of exceptions are generated by the Java run-time system?
A: The Java runtime system generates RuntimeException and Error exceptions.
Q: Under what conditions is an object's finalize() method invoked by the garbage collector?
A: The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.
Q: How can a dead thread be restarted?
A: A dead thread cannot be restarted.
Q: Which arithmetic operations can result in the throwing of an ArithmeticException?
A: Integer / and % can result in the throwing of an ArithmeticException.
Q: Variable of the boolean type is automatically initialized as?
A: The default value of the boolean type is false.
Q: Can try statements be nested?
A: Yes
Q: What are ClassLoaders?
A: A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class.
Q: What is the difference between an Interface and an Abstract class?
A: An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation.
Q: What will happen if static modifier is removed from the signature of the main method?
A: Program throws "NoSuchMethodError" error at runtime .
Q: What is the default value of an object reference declared as an instance variable?
A: Null, unless it is defined explicitly.
Q: Can a top level class be private or protected?
A: No, a top level class can not be private or protected. It can have either "public" or no modifier.
Q: Why do we need wrapper classes?
A: We can pass them around as method parameters where a method expects an object. It also provides utility methods.
Q: What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. Exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist.
Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block or a finally block.
Q: When a thread is created and started, what is its initial state?
A: A thread is in the ready state as initial state after it has been created and started.
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q: What is runtime polymorphism or dynamic method dispatch?
A: Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass.
Q: What is Dynamic Binding(late binding)?
A: Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding means that the code associated with a given procedure call is not known until the time of the call at run-time.
Q: Can constructor be inherited?
A: No, constructor cannot be inherited.
Q: What are the advantages of ArrayList over arrays?
A: ArrayList can grow dynamically and provides more powerful insertion and search mechanisms than arrays.
Q: Why deletion in LinkedList is fast than ArrayList?
A: Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.
Q: How do you decide when to use ArrayList and LinkedList?
A: If you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList should be used. If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList should be used.
Q: What is a Values Collection View ?
A: It is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.
Q: What is dot operator?
A: The dot operator(.) is used to access the instance variables and methods of class objects.It is also used to access classes and sub-packages from a package.
Q: Where and how can you use a private constructor?
A: Private constructor is used if you do not want other classes to instantiate the object and to prevent subclassing.T
Q: What is type casting?
A: Type casting means treating a variable of one type as though it is another type.
Q: Describe life cycle of thread?
A: A thread is a execution in a program. The life cycle of a thread include:
· Newborn state
· Runnable state
· Running state
· Blocked state
· Dead state
Q: What is the difference between the >> and >>> operators?
A: The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
Q: Which method of the Component class is used to set the position and size of a component?
A: setBounds() method is used for this purpose.
Q: What is the range of the short type?
A: The range of the short type is -(2^15) to 2^15 - 1.
Q: What is the immediate superclass of Menu?
A: MenuItem class
Q: Does Java allow Default Arguments?
A: No, Java does not allow Default Arguments.
Q: Which number is denoted by leading zero in java?
A: Octal Numbers are denoted by leading zero in java, example: 06
Q: Which number is denoted by leading 0x or 0X in java?
A: Hexadecimal Numbers are denoted by leading 0x or 0X in java, example: 0XF
Q: Break statement can be used as labels in Java?
A: Yes, an example can be break one;
Q: Where import statement is used in a Java program?
A: Import statement is allowed at the beginning of the program file after package statement.
Q: Explain suspend() method under Thread class>
A: It is used to pause or temporarily stop the execution of the thread.
Q: Explain isAlive() method under Thread class?
A: It is used to find out whether a thread is still running or not.
Q: What is currentThread()?
A: It is a public static method used to obtain a reference to the current thread.
Q: Explain main thread under Thread class execution?
A: The main thread is created automatically and it begins to execute immediately when a program starts. It ia thread from which all other child threads originate.
Q: Life cycle of an applet includes which steps?
A: Life cycle involves the following steps:
· Initialization
· Starting
· Stopping
· Destroying
· Painting
Q: Why is the role of init() method under applets?
A: It initializes the applet and is the first method to be called.
Q: Which method is called by Applet class to load an image?
A: getImage(URL object, filename) is used for this purpose.
Q: Define code as an attribute of Applet?
A: It is used to specify the name of the applet class.
Q: Define canvas?
A: It is a simple drawing surface which are used for painting images or to perform other graphical operations.
Q: Define Network Programming?
A: It refers to writing programs that execute across multiple devices (computers), in which the devices are all connected to each other using a network.
Q: What is a Socket?
A: Sockets provide the communication mechanism between two computers using TCP. A client program creates a socket on its end of the communication and attempts to connect that socket to a server.
Q: Advantages of Java Sockets?
A: Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications. It cause low network traffic.
Q: Disadvantages of Java Sockets?
A: Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.
Q: Which class is used by server applications to obtain a port and listen for client requests?
A: java.net.ServerSocket class is used by server applications to obtain a port and listen for client requests
Q: Which class represents the socket that both the client and server use to communicate with each other?
A: java.net.Socket class represents the socket that both the client and server use to communicate with each other.
Q: Why Generics are used in Java?
A: Generics provide compile-time type safety that allows programmers to catch invalid types at compile time. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types.
Q: What environment variables do I need to set on my machine in order to be able to run Java programs?
A: CLASSPATH and PATH are the two variables.
Q: Is there any need to import java.lang package?
A: No, there is no need to import this package. It is by default loaded internally by the JVM.
Q: What is Nested top-level class?
A: If a class is declared within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Nested top-level class is an Inner class.
Q: What is Externalizable interface?
A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.
Q: If System.exit (0); is written at the end of the try block, will the finally block still execute?
A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.
Q: What is daemon thread?
A: Daemon thread is a low priority thread, which runs intermittently in the back ground doing the garbage collection operation for the java runtime system.
Q: Which method is used to create the daemon thread?
A: setDaemon method is used to create a daemon thread.
Q: Which method must be implemented by all threads?
A: All tasks must implement the run() method
Q: What is the GregorianCalendar class?
A: The GregorianCalendar provides support for traditional Western calendars
Q: What is the SimpleTimeZone class?
A: The SimpleTimeZone class provides support for a Gregorian calendar .
Q: What is the difference between the size and capacity of a Vector?
A: The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.
Q: Can a vector contain heterogenous objects?
A: Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.
Q: What is an enumeration?
A: An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.
Q: What is difference between Path and Classpath?
A: Path and Classpath are operating system level environment variales. Path is defines where the system can find the executables(.exe) files and classpath is used to specify the location of .class files.
Q: Can a class declared as private be accessed outside it's package?
A: No, it's not possible to accessed outside it's package.
Q: What are the restriction imposed on a static method or a static block of code?
A: A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.
Q: Can an Interface extend another Interface?
A: Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
Q: Which object oriented Concept is achieved by using overloading and overriding?
A: Polymorphism
Q: What is an object's lock and which object's have locks?
A: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock.
Q: What is Downcasting?
A: It is the casting from a general to a more specific type, i.e. casting down the hierarchy.
Q: What are order of precedence and associativity and how are they used?
A: Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.
Q: If a method is declared as protected, where may the method be accessed?
A: A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
Q: What is the difference between inner class and nested class?
A: When a class is defined within a scope of another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.
Q: What restrictions are placed on method overriding?
A: Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides.
Q: What is constructor chaining and how is it achieved in Java?
A: A child object constructor always first needs to construct its parent. In Java it is done via an implicit call to the no-args constructor as the first statement.
Q: Can a double value be cast to a byte?
A: Yes, a double value can be cast to a byte.
Q: How does a try statement determine which catch clause should be used to handle an exception?
A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.
Q: What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type.
Q. What if the main method is declared as private?
A: The program compiles properly but at runtime it will give “Main method not public.” message.
Q. What is meant by pass by reference and pass by value in Java?
A:Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.
Q. If you’re overriding the method equals() of an object, which other method you might also
consider?
A: hashCode()
Q. What is Byte Code?
Or
Q. What gives java it’s “write once and run anywhere” nature?
A: All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said
to be platform independent.
Q. Expain the reason for each keyword of public static void main(String args[])?
A: public – main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.
static : Java environment should be able to call this method without creating an instance of the class , so this method must be
declared as static.
void : main does not return anything so the return type must be void
The argument String indicates the argument type which is given at the command line and arg is an array for string given during command
line.
Q. What are the differences between == and .equals()?
Or
Q. what is difference between == and equals?
Or
Q. Difference between == and equals method
Or
Q. What would you use to compare two String variables – the operator == or the
method equals()?
Or
Q. How is it possible for two String objects with identical values not to be equal under
the == operator?
A: The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.
== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.
public class EqualsTest {
public static void main(String[] args) {
String s1 = “abc”;
String s2 = s1;
String s5 = “abc”;
String s3 = new String(”abc”);
String s4 = new String(”abc”);
System.out.println(”== comparison : ” + (s1 == s5));
System.out.println(”== comparison : ” +(s1 == s2));
System.out.println(”Using equals method : ” + s1.equals(s2));
System.out.println(”== comparison : ” + s3 == s4);
System.out.println(”Using equals method : ” + s3.equals(s4));
}
}
Output
== comparison : true
== comparison : true
Using equals method : true
== comparison : false
Using equals method : true
Q. What if the static modifier is removed from the signature of the main method?
Or
Q. What if I do not provide the String array as the argument to the method?
A: Program compiles. But at runtime throws an error “NoSuchMethodError”.
Q. Why oracle Type 4 driver is named as oracle thin driver?
A: Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP
version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not
require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string
uses the TCP/IP port address, not the TNSNAMES entry for the database name.
Q. What is the difference between final, finally and finalize? What do you understand
by the java final keyword?
Or
Q. What is final, finalize() and finally?
Or
Q. What is finalize() method?
Or
Q. What does it mean that a class or member is final?
A:
final – declare constant
finally – handles exception
finalize – helps in garbage collection
Variables defined in an interface are implicitly final. A final class can’t be extended i.e., final class may not be sub-classed. This is done for
security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread
safety a little easier to achieve. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.
Q. What is the Java API?
A: The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user
interface (GUI) widgets.
Q. What is the GregorianCalendar class?
A: The GregorianCalendar provides support for traditional Western calendars.
Q. What is the ResourceBundle class?
A: The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.
Q. Why there are no global variables in Java?
A: Global variables are globally accessible. Java does not support globally accessible variables due to following reasons:
The global variables breaks the referential transparency
Global variables create collisions in namespace.
Q. How to convert String to Number in java program?
A: The valueOf() function of Integer class is is used to convert string to Number. Here is the code example:
String numString = “1000″;
int id=Integer.valueOf(numString).intValue();
Q. What is the SimpleTimeZone class?
A: The SimpleTimeZone class provides support for a Gregorian calendar.
Q. What is the difference between a while statement and a do statement?
A: A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement(post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once.
Q. What is the Locale class?
A: The Locale class is used to tailor a program output to the conventions of a particular geographic, political, or cultural region.
Q. Describe the principles of OOPS.
A: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
Q. Explain the Inheritance principle.
A: Inheritance is the process by which one object acquires the properties of another object. Inheritance allows well-tested procedures to be
reused and enables changes to make once and have effect in all relevant places
Q. What is implicit casting?
Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios.
Example
int i = 1000;
long j = i; //Implicit casting
Q. Is sizeof a keyword in java?
A: The sizeof is not a keyword.
Q. What is a native method?
A: A native method is a method that is implemented in a language other than Java.
Q. In System.out.println(), what is System, out and println?
A: System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object.
Q. What are Encapsulation, Inheritance and Polymorphism
Or
Q. Explain the Polymorphism principle. Explain the different forms of Polymorphism.
A: Polymorphism in simple terms means one name many forms. Polymorphism enables one entity to be used as a general category for
different types of actions. The specific action is determined by the exact nature of the situation.
Polymorphism exists in three distinct forms in Java:
Method overloading (Static Polimorphism)
Method overriding (Dynamic or Runtime Polimorphism) through inheritance
Method overriding (Dynamic or Runtime Polimorphism) through the Java interface
Q. What is explicit casting?
A: Explicit casting in the process in which the complier are specifically informed to about transforming the object.
Example
long i = 700.20;
int j = (int) i; //Explicit casting
Q. What is the Java Virtual Machine (JVM)?
A: The Java Virtual Machine is software that can be ported onto various hardware-based platforms
Q. What do you understand by downcasting?
A: The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy
Q. What are Java Access Specifiers?
Or
Q. What is the difference between public, private, protected and default Access Specifiers?
Or
Q. What are different types of access modifiers?
A: Access specifiers are keywords that determine the type of access to the member of a class. These keywords are for allowing privileges to
parts of a program such as functions and variables. These are:
public: accessible to all classes
protected: accessible to the classes within the same package and any subclasses.
private: accessible only to the class to which they belong
default: accessible to the class to which they belong and to subclasses within the same package
Q. Which class is the superclass of every class?
A: Object.
Q. Name primitive Java types.
A: The 8 primitive types are byte, char, short, int, long, float, double, and boolean. Additional is String.
Q. What is the difference between static and non-static variables?
Or
Q. What are “class variables”?
Or
Q. What is static in java?
Or
Q. What is a static method?
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static variables i.e. there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class. These are declared outside a class and stored in static memory.
Class variables are mostly used for constants. Static variables are always called by the class name. This variable is created when the program starts and gets destroyed when the programs stops. The scope of the class variable is same an instance variable. Its initial value is same as instance variable and gets a default value when it’s not initialized corresponding to the data type. Similarly, a static method is a method that belongs to the class rather than any object of the class and doesn’t apply to an object or even require that any objects of the class have been instantiated. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are
attached to a class, not an object.
A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method with a non-static method. In other words, you can’t change a static method into an instance method in a subclass. Non-static variables take on unique values with each object instance.
Q. What is the difference between the boolean & operator and the && operator?
A: If an expression involving the boolean & operator is evaluated, both operands are evaluated, whereas the && operator is a short cut operator. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. If the first operand evaluates to false, the evaluation of the second operand is skipped.
Q. How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
Q. What if I write static public void instead of public static void?
A: Program compiles and runs properly.
Q. What is the difference between declaring a variable and defining a variable?
A: In declaration we only mention the type of the variable and its name without initializing it. Defining means declaration + initialization.
E.g. String s; is just a declaration while
String s = new String(”bob”);
Or
String s = “bob”;
are both definitions.
Q. What type of parameter passing does Java support?
A: In Java the arguments (primitives and objects) are always passed by value. With objects, the Object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.
Q. Explain the Encapsulation principle.
A: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. Objects allow procedures to be encapsulated with their data to reduce potential interference. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Q. What do you understand by a variable?
A: Variable is a named memory location that can be easily referred in the program. The variable is used to hold the data and it can be changed during the course of the execution of the program.
Q. What do you understand by numeric promotion?
A: The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integral and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
Q. What do you understand by casting in java language? What are the types of casting?
A: The process of converting one data type to another is called Casting. There are two types of casting in Java; these are implicit casting and
explicit casting.
Q. What is the first argument of the String array in main method?
A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. If we
do not provide any arguments on the command line, then the String array of main method will be empty but not null.
Q. How can one prove that the array is not null but empty?
A: Print array.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on
attempting to print array.length.
Q. Can an application have multiple classes having main method?
A: Yes. While starting the application we mention the class name to be run. The JVM will look for the main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
Q. When is static variable loaded? Is it at compile time or runtime? When exactly a static block is loaded in Java?
A: Static variable are loaded when classloader brings the class to the JVM. It is not necessary that an object has to be created. Static variables will be allocated memory space when they have been loaded. The code in a static block is loaded/executed only once i.e. when the class is first initialized. A class can have any number of static blocks. Static block is not member of a class, they do not have a return statement and they cannot be called directly. Cannot contain this or super. They are primarily used to initialize static fields.
Q. Can I have multiple main methods in the same class?
A: We can have multiple overloaded main methods but there can be only one main method with the following signature :
public static void main(String[] args) {}
No the program fails to compile. The compiler says that the main method is already defined in the class.
Q. Explain working of Java Virtual Machine (JVM)?
A: JVM is an abstract computing machine like any other real computing machine which first converts .java file into .class file by using Compiler (.class is nothing but byte code file.) and Interpreter reads byte codes.
Q. How can I swap two variables without using a third variable?
A: Add two variables and assign the value into First variable. Subtract the Second value with the result Value. and assign to Second variable.
Subtract the Result of First Variable With Result of Second Variable and Assign to First Variable. Example:
int a=5, b=10;
a=a+b;
b=a-b;
a=a-b;
You use an XOR swap (BBEST APPROACH), as in case of using above approach it may goes over/under flow. For example:
int a = 5;
int b = 10;
a = a ^ b;
b = a ^ b;
a = a ^ b;
Q. What is data encapsulation?
A: Encapsulation may be used by creating ‘get’ and ’set’ methods in a class (JAVABEAN) which are used to access the fields of the object. Typically the fields are made private while the get and set methods are public. Encapsulation can be used to validate the data that is to be stored, to do calculations on data that is stored in a field or fields, or for use in introspection (often the case when using javabeans in Struts, for instance). Wrapping of data and function into a single unit is called as data encapsulation. Encapsulation is nothing but wrapping up the data and associated methods into a single unit in such a way that data can be accessed with the help of associated methods. Encapsulation provides data security. It is nothing but data hiding.
Q. What is reflection API? How are they implemented?
A: Reflection is the process of introspecting the features and state of a class at runtime and dynamically manipulate at run time. This is supported using Reflection API with built-in classes like Class, Method, Fields, Constructors etc. Example: Using Java Reflection API we can get the class name, by using the getName method.
Q. Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why?
A: Yes, the JVM maintains a cache by itself. It creates the Objects on the HEAP, but references to those objects are on the STACK.
Q. What is phantom memory?
A: Phantom memory is false memory. Memory that does not exist in reality.
Q. Can a method be static and synchronized?
A: A static method can be synchronized. If you do so, the JVM will obtain a lock on the java.lang.Class instance associated with the object. It is similar to saying:
synchronized(XYZ.class) {}
Q. What is difference between String and StringTokenizer?
A: A StringTokenizer is utility class used to break up string.
Example:
StringTokenizer st = new StringTokenizer(”Hello World”);
while(st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
Output:
Hello
World
Q: What is transient variable?
A: Transient variable can’t be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can’t be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Note:
transient identifies a variable not to be written out when an instance is serialized (It can't be copied to remove system)
volatile indicates that the field is used by synchronized threads and that the compiler should not attempt to perform optimizations with it.
When more than one thread share a (volatile) data it is checked every time. Every thread keeps the latest value of volatile variable.
Q: Name the containers which uses Border Layout as their default layout?
A: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.
Q: What do you understand by Synchronization?
A: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object’s value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1() {
//Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction() {
synchronized(this) {
//Synchronized code here.
}
}
Java Interview Questions.
OOPS
Q. What is meant by Object Oriented Programming?
A. OOP is a method of programming in which programs are organised as cooperative collections of objects. Each object is an instance of a class and each class belong to a hierarchy.
Q. What is a Class?
A. Class is a template for a set of objects that share a common structure and a common behaviour.
Q. What is an Object?
A. Object is an instance of a class. It has state,behaviour and identity. It is also called as an instance of a class.
Q. What is an Instance?
A. An instance has state, behaviour and identity. The structure and behaviour of similar classes are defined in their common class. An instance is also called as an object.
Q. What are the core OOP's concepts?
A. Abstraction, Encapsulation,Inheritance and Polymorphism are the core OOP's concepts.
Q. What is meant by abstraction?
A. Abstraction defines the essential characteristics of an object that distinguish it from all other kinds of objects. Abstraction provides crisply-defined conceptual boundaries relative to the perspective of the viewer. Its the process of focussing on the essential characteristics of an object. Abstraction is one of the fundamental elements of the object model.
Q. What is meant by Encapsulation?
A. Encapsulation is the process of compartmentalising the elements of an abtraction that defines the structure and behaviour. Encapsulation helps to separate the contractual interface of an abstraction and implementation.
Q. What is meant by Inheritance?
A. Inheritance is a relationship among classes, wherein one class shares the structure or behaviour defined in another class. This is called Single Inheritance. If a class shares the structure or behaviour from multiple classes, then it is called Multiple Inheritance. Inheritance defines "is-a" hierarchy among classes in which one subclass inherits from one or more generalised superclasses.
Q. What is meant by Polymorphism?
A. Polymorphism literally means taking more than one form. Polymorphism is a characteristic of being able to assign a different behavior or value in a subclass, to something that was declared in a parent class.
Q. What is an Abstract Class?
A. Abstract class is a class that has no instances. An abstract class is written with the expectation that its concrete subclasses will add to its structure and behaviour, typically by implementing its abstract operations.
Q. What is an Interface?
A. Interface is an outside view of a class or object which emphaizes its abstraction while hiding its structure and secrets of its behaviour.
Q. What is a base class?
A. Base class is the most generalised class in a class structure. Most applications have such root classes. In Java, Object is the base class for all classes.
Q. What is a subclass?
A. Subclass is a class that inherits from one or more classes.
Q. What is a superclass?
A. Superclass is a class from which another class inherits.
Q. What is a constructor?
A. Constructor is an operation that creates an object and/or initialises its state.
Q. What is a destructor?
A. Destructor is an operation that frees the state of an object and/or destroys the object itself. In Java, there is no concept of destructors. Its taken care by the JVM.
Q. What is meant by Binding?
A. Binding denotes association of a name with a class
Q. What is meant by static binding?
A. Static binding is a binding in which the class association is made during compile time. This is also called as Early binding.
Q. What is meant by Dynamic binding?
A. Dynamic binding is a binding in which the class association is not made until the object is created at execution time. It is also called as Late binding.
Q. Define Modularity?
A. Modularity is the property of a system that has been decomposed into a set of cohesive and loosely coupled modules.
Q. What is meant by Persistence?
A. Persistence is the property of an object by which its existence transcends space and time.
Q. What is colloboration?
A. Colloboration is a process whereby several objects cooperate to provide some higher level behaviour.
Q. In Java, How to make an object completely encapsulated?
A. All the instance variables should be declared as private and public getter and setter methods should be provided for accessing the instance variables.
Q. How is polymorphism acheived in java?
A. Inheritance, Overloading and Overriding are used to acheive Polymorphism in java.
Core Java Interview Questions
Q. What are the most important features of Java?
A. Java is object oriented, platform independent, secure, robust, simple, etc
Q. Which one of them do you consider the best feature of Java?
A. Platform independence.
Q. What do you mean by platform independence?
A. Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).
Q. What is byte code?
A. Byte code is a set of instructions generated by the compiler. JVM executes the byte code.
Q. How does Java acheive platform independence?
A. A Java source file on compilation produces an intermediary . Class rather than a executable file. This . Class file is interpreted by the JVM. Since JVM acts as an intermediary layer.
Q. What is a JVM?
A. JVM is Java Virtual Machine which is a run time environment for the compiled java class files.
Q. Are JVM's platform independent?
A. JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor. A Windows JVM cannot be installed in Linux.
Q. Who provides the JVM?
A. Any software vendor can provide a JVM but it should comply to the Java langauge specification.
Q. What is the difference between a JDK and a JVM?
A. JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.
Q. What is a pointer and does Java support pointers?
A. Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.
Q. How is an object reference different from a pointer?
A. Both object reference and pointer point to the memory location of the object. You can manipulate pointers but not object references.
Q. Does Java support multiple inheritance?
A. Java doesn't support multiple inheritance.
Q. Why Java doesn't support multiple inheritance?
A. When a class inherits from more than class, it will lead to the diamond problem - say A is the super class of B and C & D is a subclass of both B and C. D inherits properties of A from two different inheritance paths ie via both B & C. This leads to ambiguity and related problems, so multiple inheritance is not allowed in Java.
Q. Is Java a pure object oriented language?
A. Java is a pure object oriented language. Except for the primitives everything else are objects in Java.
Q. What is the difference between Path and Classpath?
A. Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(. Exe) files and classpath is used to specify the location . Class files.
Q. Why does Java not support operator overloading?
A. Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.
Q. What are keywords?
Keywords cannot be used as identifiers.
Q. What are reserved keywords? And can you name few reserved keywords?
A. Few of the words are reserved as keywords for future usage. Compiler forces the developer not to use a reserved keyword. const and
Q. What is the overhead of introducing a new keyword?
A. When a new keyword is used in a new version of the JDK, there is high chances that has been used by developers as identifiers. This make tougher for the code base to migrate to new version since it requires code change, recompilation,testing and release cycle.
Q. Have you come across difficulties due to introduction of a new keyword?
A. Yes. enum keyword was used extensively as identifier in one of our project - we had to change the code in a lot of places to migrate it to newer v ersion.
Q. What are identifiers?
A. Identifiers are names given to a variable, method, class and interface. Identifiers must conform to the following rules:a. The identifiers can contain a to z, A to Z,0 to 9,_ and $.b. Special characters other than _ and $ cannot be used in identifiers. C. Identifiers cannot start with numbers. d. keywords cannot be used as identifiers.
Q. What is meant by naming conventions? And what are the naming conventions followed in Java?
A. Naming conventions are part of coding standards which are prescribed for better readability and maintenance. The following are simple conventions followed in Java:1. Instance and local Variables should start with a lowercase and subsequent word should start with a capital letter. Examples: int quantity; double unitPrice; 2. Class level variables ie constants should be in capital letters and _ is used word seprator final static double PI = 3.14; final static int MAX_PRIORITY = 10;3. Method names should start with small case and subsequent word should be capital letter. public double getAvailableBalance(String accountNo) throws InvalidAccountException{}4. Classes and Interfaces should start with a capital letter and subsequent words should also be capital letters. public class HelloWorld{}
Q. If you dont follow coding standards, will it result in compilation error?
A. No. These are standards. Each company or fot that matter each software unit might have its own coding standards. These are not enforced by the compiler.
Q. How to make sure that all programmers are following the coding standards?
A. The best and simple way is to do peer reviews and code walkthroughs. You can use external plugins to your IDE (integrated development environment) to enforce during coding itself.
Q. What are literals?
A. Literals are source code representation of primitive data types.
Q. Name the eight data types which are available in Java?
A. boolean, byte, short, int, long, float, double and char.
Q. Are primitive data types objects in Java?
A. Primitive data types are not objects.
Q. What are all the number data types? And are they signed or unsigned?
A. Excpet boolean and char, others are number data types and they are all signed which means that they can hold both positive and negative values.
Q. What are the possible values that a boolean data type can hold?
A. The boolean is the simplest data type which can hold true or false.
Q. What are default values?
A. Values which are defaulted during object initialisation are called default values. Each data type has a default value.
Q. What are the default values of primitive data types?
A. For boolean data type, it is false. For byte,short,int and long, it is 0. For float and double, it is 0.0. For char, the default value is 'u000'.
Q. Are object references defaulted?
A. Yes. Object references are defaulted to null.
Q. Are arrays defaulted?
A. If arrays is just declared but not initialised then the array reference will be defaulted to null. This is because arrays are objects in Java. Int arr[]; // Here the arr reference is defaulted to null. If array values are not assigned, then will be defaulted to their respective default values.double priceRange[] = new double[3]; // Here all the elements in the array will be defaulted to 0.0 - the default value of double. String str[] = new String[3]; // Here all the elements will be defaulted to null - the default value for object references.
Q. Can you explain keyword , identifier and literal with an example?
A. Consider the below statement:int i = 10;Here int is the keyword which has special meaning attached Java programming langauge ie whatever is declared is an integer value. I is the identifier or the variable name.10 is the literal or the actual value.
Q. What are the 3 ways to represent an integer value?
A. The following are the 3 different ways to represent an integer value:int i = 123 // this is the usual decimal representation. Int i = 0123 // this is octal representation. Octal values start with a zero. Int i = 0XCAAD // this is hexadecimal representation. Hexadecimal values start with 0X.
Q. In how many ways a char value be represented?
A. Char value can be represented in 3 ways. They are as follows:char ch = 'A'; // represented using single quotes. Char ch = 'u0041'; // represented using unicode representation. Char ch = 41; // represented using integer value.
Q. How is it possible to represent char using a integer value?
A. char is internally represented as a unsigned 16 bit integer value ie it will accept integer values from 0 to 65536.
Q. Can char be used when an integer value is expected?
A. Yes. A fine example is switch statement will accept char value for multiway condition checking.
Q. Can char be manipulated like integers?
A. Yes possible. The below is an example. Char ch = 'A';System.out.println(ch++); The above statement will print B. ++ is a numeral operand and since char is internally represented as integer, ++ operand can be applied on char value.
Q. What is Unicode?
A. A universal, 16-bit, standard coded character set for the representation of all human scripts.
Q. What should i have to do if i have to print my name in Spanish?
A. Provide code...
Q. How will the below literal value be internally represented?
float f = 21.22;
A. It will be represented as a double value. Floating point literals are always double by default. If you want a float, you must append an F or f to the literal.
Q. Give your observation on the below statement.
A. int i = 10/0;
The statement will result in RuntimeException (DivideByZeroException). Integer values cannot be divided by zero.
Q. Give your observation on the below statement.
A. double d = 10.12/0;
This will compile and execute fine. The result will be Infinity
Q. What is a Variable?
A. a variable is a facility for storing data. The current value of the variable is the data actually stored in the variable.
Q. What are the 3 types of variables?
A. There are 3 types of variables in Java. They are :1. Local variables.2. Instance variables.3. Class variables.
Q. What are Local variables?
A. Local varaiables are those which are declared within a block of code like methods, loops, exception blocks, etc. Local variables should be initialised before accessing them. Local variables are stored on the stack, hence they are sometimes called stack variables. They are also called as method variables or block variables.
Q. When are local variables eligible for garbage collection?
A. As soon as the block is completed the variables are eligible for GC. Block could be a condition, a loop, a exception block or a methodConsider the below class:public class Test { public static void main (String str[]){ String name = str[0]; for (int i=0; i<=10; i++){ System.out.println(name + i); } }}In the above the class, the variable i is eligible for garbage collection immediately after the completion of for loop.name string variable and str[] argument are eligible for GC after the completion of main method.
Q. What are Instance variables?
A. Instance variables are those which are defined at the class level. As we know, object have identity and behaviour - the identity is provided by the instance variables. These are also called as object variables. Instance variables need not be initialized before using them. Instance variables will be initialized to their default values automatically when not initialized.
Q. Are arrays primitive data types?
A. No. Arrays are not primitives - they are objects.
Q. What are different ways to declare an array?
A. Object obj[]; // most frequently used form of array declaration.Object []obj; // nothing wrong with this. Object[] obj; // nothing wrong with this. int i[][]; // most frequently form multi-dimensional arrayint[] i[]; // nothing wrong with this. int[] i[],j; // nothing wrong with this. Important : i is double dimensional whereas j is single dimensional.
Q. What are the 3 steps in defining arrays?
A. declaration,initialization and assignmentint i[]; // array declaration. I[] = new int[2]; // array initialization. I[0] = 10; i[1] = 7; i[2] = 9; // element value assignment. What is the simplest way to defining an primitive array? The below statement merges declaration,initialization and assignment into a single step:int i [] = {10, 20, 30 40}; What is wrong with the below code segment?int i[] = new int[5]System.out.println(i.length()) length is an instance variable of array object - here it is given a method.
Q. What is wrong with the below code segment?
A. int i[5] = new int[]System.out.println(i.length)
length of an array should be given when it is initialized. Here it is given during declaration. It will result in compilation error.
Q. What will be the output of the below code segment?
A. int i[] = new int[5]System.out.println(i.length)
It will print 6. Remember array indexes start with 0.
Q. What are the frequent RuntimeException's encountered because of improper coding with respect to arrays?
A. ArrayIndexOutOfBoundsException and NullPointerException.
56. Should a main method be compulsorily declared in all java classes?
A. No not required. main method should be defined only if the source class is a java application.
Q. What is the return type of the main method?
A. Main method doesn't return anything hence declared void.
Q. Why is the main method declared static?
A. main method is the entry point for a Java application and main method is called by the JVM even before the instantiation of the class hence it is declared as static. static methods can be called even before the creation of objects.
Q. What is the argument of main method?
A. main method accepts an array of String objects as argument.
Q. How to pass an argument to main method?
A. You should pass the argument as a command line argument. Command line arguments are seprated by a space. The following is an example:java Hello Tom JerryIn the above command line Hello is the class, Tom is the first argument and Jerry is the second argument.
Q. What will happen if no argument is passed to the main method?
A. If you dont access the argument, the main method will execute without any problem. If try to access the argument, NullPointerException will be thrown.
Q. Can a main method be overloaded?
A. Yes. You can have any number of main methods with different method signature and implementation in the class.
Q. Can a main method be declared final?
A. Yes. Any inheriting class will not be able to have it's own default main method.
Q.Does the order of public and static declaration matter in main method?
A. No it doesn't matter but void should always come before main().
Q. Can a source file contain more than one Class declaration?
A. Yes. A single source file can contain any number of Class declarations but only one of the class can be declared as public.
Q. If a source file has 2 class declaration in it, on compilation how many class files will be created?
A. 2 class files will be generated by the compiler.
Q. Can the first line of the source code be a comment?
A. Yes. Comments can appear anywhere in the code. It is just a "skip this line" instruction to the conpiler.
Q. Can the source file name and the class name in the file be different?
A. If the class in the source is not of public access, then the name can be anything but should confirm to identifier rules.
Q. Explain Inheritance?
A. Inheritance is a concept where the properties and behaviour of a class is reused in another class. The class which is being reused or inherited is called the super class or the parent class. The class which is inheriting is called the subclass or the child class of the inherited class.
Q. Does Java support multiple inheritance?
A. No. Java Supports only single inheritance.
Q. Why Java doesn’t support muliple inheritance?
A. When a class inherits from more than class, it will lead to inheritance path amibiguity (this is normally calledthe diamond problem). Say A is the super class of B and C & D is a subclass of both B and C. D inherits properties of A from two different inheritance paths ie via both B & C. This leads to ambiguity and related problems, so multiple inheritance is not allowed in Java.
Q. Which keyword is used for inheriting from another class?
A. extends keyword is used for inheritance.
Q. Can a subclass be referenced for a super class object?
No. If Vehicle is super class and Car is the subclass then the following reference would be wrong – Car c = new Vehicle();
Q. Can a parent class be referenced for a subclass object?
A. Yes. The below is an example :Vehicle v = new Car();
Q. Can an interface be referenced for any object?
A. Yes. We can create a object reference for an interface. The object should have provided the implementation for the object.Runnable r = new Test(); Test class should have implemented the runnable interface and overridded the run method.
Q. What are constructors?
A. Constructors are used to initialise an object. Constructors are invoked when the new operator on the class are called.
Q. What are the decalaration of a constructor?
A. Constructor name should be the same as class name. Constructors doesn’t return anything – not even void.
Q.Does constructors throw exceptions?
A. Yes. Like methods constructors can also throw exceptions.
Q. Can constructors be overloaded?
A. Yes. Constructors can be overloaded.
80. Is it compulsory to define a constructor for a class?
A. No. If you don’t define a constructor, the compiler will provide a default constructor.
Q. What is a default constructor?
A. Default constructor is a no argument constructor which initialises the instance variables to their default values. This will be provided by the compiler at the time of compilation. Default constructor will be provided only when you don’t have any constructor defined.
Q. Explain about “this” operator?
A. “this” is used to refer the currently executing object and it’s state. “this” is also used for chaining constructors and methods.
Q. Explain about “super” operator?
A. “super” is used to refer to the parent object of the currently running object. “super” is also to invoke super class methods and classes.
Q. What is the sequence of constructor invocation?
A. When you instantiate a subclass, the object will begin with an invocation of the constructor in the base class and initialize downwards through constructors in each subclass till it reaches the final subclass constructor. Note: This top down imagery for class inheritance, rather than a upward tree-like approach, is standard in OOP but is sometimes confusing to newcomers.
Q. Can a constructor be defined for an interface?
A. No
Q. Explain Polymorphism?
A. The dictionary definition of polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class. Polymorphism is the capability of an action or method to do different things based on the object that it is acting upon. Overloading and overriding are two types of polymorphism.
Q. What is Overloading?
A. In a class, if two methods have the same name and a different signature,it is known as overloading in Object oriented concepts.
Q. When to use overloading?
A. Overloading is a powerful feature, but you should use it only as needed. Use it when you actually do need multiple methods with different parameters, but the methods do the same thing. That is, don't use overloading if the multiple methods perform different tasks. This approach will only confuse other developers who get a peek at your code.
Q. Explain Overriding?
A. Overriding means, to give a specific definition by the derived class for a method implemented in the base class.
Q. What is a package?
A. Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.
Q. Which package is imported by default?
A. java.lang package is imported by default even without a package declaration.
Q. Is package statement mandatory in a Java source file?
It's not mandatory.
Q. What will happen if there is no package for Java source file?
A. The classes will packaged into a no name default package. In practice, we always put classes into a meaningful package.
Q. What is the impact using a * during importing(for example import java. Io.*;?
A. When a * is used in a import statement, it indicates that the classes used in the current source can be available in the imported package. Using slightly increases the compilation time but has no impact on the execution time.
Q. Can a class declared as private be accessed outside it's package?
A. A class can't be declared as private. It will result in compile time error.
Q. Can a class be declared as protected?
A. A class can't be declared as protected. only methods can be declared as protected.
Q. What is the access scope of a protected method?
A. A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.
Q. What is the impact of marking a constructor as private?
A. Nobody can instantiate the class from outside that class.
Q. What is meant by default access?
A. default access means the class,method,construtor or the variable can be accessed only within the package.
Q. Is default a keyword?
A. Yes. default is a keyword but is associated with switch statement not with access specifiers.
Q.Then how to give default access to a class?
A. If you dont specify any access, then it means the class is of default access.
Q. Can i give two access specifiers to a method at the same time?
A. No its not possible. It will result in compile time error.
Q. What is the purpose of declaring a variable as final?
A. A final variable's value can't be changed. final variables should be initialized before using them.
Q. Can a final variable be declared inside a method?
A. No. Local variables cannot be declared as final.
Q. What is the impact of declaring a method as final?
A. A method declared as final can't be overridden. A sub-class doesn't have the independence to provide different implementation to the method.
Q. I don't want my class to be inherited by any other class. What should i do?
A. You should declared your class as final. A class declared as final can't be inherited by any other class.
Q. When will you declare a class as final?
A. When a class is independent and completely concrete in nature, then the class has to be marked as final.
Q. Can you give few examples of final classes defined in Java API?
A. java.lang. String,java.lang.Math are final classes.
Q. How to define a constant variable in Java?
A. The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also. Static final int PI = 3.14; is an example for constant.
Q. Can a class be declared as static?
A. No a class cannot be defined as static. Only a method,a variable or a block of code can be declared as static.
Q. When will you define a method as static?
A. When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.
Q. I want to print "Hello" even before main is executed. How will you acheive that?
A. Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main method.
Q. What is the use of a static code block?
A. static code blocks could be used for one time initialisation activities.
Q. Can’t you use the constructor for initialisation rather than static block?
A. Constructors are used for object level initialisation whereas the static block are used for class level initialisation ie to initialise constants.
Q. Will the static block be executed for each object?
A. No. It will be executed only once for each class ie at the time of loading a class.
Q. What are the restriction imposed on a static method or a static block of code?
A. A static method should not refer to instance variables without creating an instance. It cannot use "this" or "super". A static method can acces only static variables or static methods.
Q. When overriding a static method, can it be converted to a non-static method?
A. No. It should be static only.
Q. What is the importance of static variable?
A. static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.
Q. Can we declare a static variable inside a method?
A. Static varaibles are class level variables and they can't be declared inside a method. If declared, the class will not compile.
Q. What is an Abstract Class and what is it's purpose?
A. A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.
Q. What is the use of a abstract variable?
A. Variables can't be declared as abstract. only classes and methods can be declared as abstract.
Q. Can a abstract class be declared final?
A. Not possible. An abstract class without being inherited is of no use and a final class cannot be inherited. So if a abstract class is declared as final, it will result in compile time error.
Q. What is an abstract method?
A. An abstract method is a method whose implementation is deferred to a subclass.
Q. Can a abstract class be defined without any abstract methods?
A. Yes it's possible. This is basically to avoid instance creation of the class.
Q. What happens if a subclass has inherited a abstract class but has not provided implementation for all the abstract methods of the super class?
A. Then the subclass also becomes abstract. This will be enforced by the compiler.
Q. What happens if a class has implemented an interface but has not provided implementation for a method in a interface?
A. Its the same as the earlier answer. The class has to be marked as abstract. This will be enforced by the compiler.
Q. Can you create an object of an abstract class?
A. Not possible. Abstract classes are not concrete and hence can't be instantiated. If you try instantiating, you will get compilation error.
Q. Can I create a reference for a an abstract class?
A. Yes. You can create a reference for an abstract class only when the object being has provided the implementation for the abstract class - it means that the object should be of a concrete subclass of the abstract class. This applies to interfaces also. Below is an example for interface referencing an object:java. Sql. Connection con = java. Sql.DriverManager. GetConnection("");
Q. Can a class be marked as native?
A. No. Only methods can be marked as native.
Q. What is the use of native methods?
A. When a java method accesses native library written in someother programming language then the method has to be marked as native.
Q. What is the disadvantage of native methods?
A. By using native methods, the java program loses platform independence - the accessed platform might be tightly coupled with a operating system hence java program also loses OS independence.
Q. What is the purpose of transient modifier?
A. Only variables can be marked as transient. Variables marked as transient will not be persisted during object persistence.
Q. What is the purpose of volatile modifier?
A. Only variables can be marked as volatile. Volatile variables might be modified asynchronously.
Q. What is an Interface?
A. Interfaces say what a class must do but does not say how a class must do it. Interfaces are 100% abstract.
Q. Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?
A. No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.
Q. Can a method inside a Interface be declared as final?
A. No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.
Q. Can an Interface implement another Interface?
A. Intefaces doesn't provide implementation hence a interface cannot implement another interface.
Q. Can an Interface extend another Interface?
A. Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
Q. Can a Class extend more than one Class?
A. Not possible. A Class can extend only one class but can implement any number of Interfaces.
Q. Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?
A. Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.
Q. Can an Interface be final?
A. Not possible. Doing so so will result in compilation error.
Q. Can a class be defined inside an Interface?
A. Yes it's possible.
Q. Can an Interface be defined inside a class?
A. Yes it's possible.
Q. What is a Marker Interface?
A. An Interface which doesn't have any declaration inside but still enforces a mechanism.
Q. Can we define private and protected modifiers for variables in interfaces?
A. No. Always all variables declared inside a interface are of public access.
Q. What modifiers are allowed for methods in an Interface?
A. Only public and abstract modifiers are allowed for methods in interfaces.
Q. When can an object reference be cast to an interface reference?
A. An object reference can be cast to an interface reference when the object implements the referenced interface.
Q. What are Inner Classes?
A. Inner classes are classes which are defined inside another class.
Q. What are Nested Classes?
A. Static Inner classes are called sometimes referred as nested classes because these classes can exist without any relationship with the containing class.
Q. What is the super class of all Inner Classes?
A. Inner class is just a concept and can be applied to any class, hence there is no common super class for inner classes.
Q. What are the disadvantages of Inner classes?
A. 1. Inner classes are not reusable hence defeats one of the fundamental feature of Java.2. Highly confusing and difficult syntax which leads poor code maintainability.
Q.Name the different types of Inner Classes?
A. The following are the different types of Inner classes:Regular Inner ClassMethod Local Inner ClassStatic Inner ClassAnonymous Inner Class
Q. What are Regular Inner Classes?
A. A Regular inner class is declared inside the curly braces of another class outside any method or other code block. This is the simplest form of inner classes.
Q. Can a regular inner class access a private member of the enclosing class?
A. Yes. Since inner classes are treated as a member of the outer class they can access private members of the outer class.
Q. How will you instantiate a regular inner class from outside the enclosing class?
A. Outer out=new Outer(); Outer.Inner in=out.new Inner();
Q. What are Local Inner Classes or Method Local Inner Classes?
A. A method-local inner class is defined within a method of the enclosing class.
Q. What are the constraints on Method Local Inner Classes?
A. The following are the restrictions for Method Inner Classes:Method Local Inner classes cannot acccess local variables but can access final variables.Only abstract and final modifiers can be applied to Method Local Inner classesMethod Local Inner classes can be instantiated only within the method in which it is contained that too after the class definition.
Q. What are Anonymous Inner Classes? Name the various forms of Anonymous Inner Classes.
A. Anonymous Inner Classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface. The following are the different forms of inner classes:Anonymous subclass(i. E. extends a class)Anonymous implementer (i. E. implements an interface)Argument-Defined Anonymous Inner Classes.
Q. How many classes can an Anonymous Inner classes inherit from?
A. One.
Q. How many Interfaces can an Anonymous Inner classes implement?
A. One. Normal classes and other inner classes can implement more than one interface whereas anonymous inner classes can either implement a single interface or extend a single class.
Q. What are Static Inner Classes?
A. Static Inner Classes are inner classes which marked with a static modifier. These classes need not have any relationship with the outer class. These can be instantiated even without the existence of the outer class object.
Q. Can you instantiate the static Inner Class without the existence of the outer class object?
A. If Yes, Write a sample statement. Yes. It can be instantiated as follows by referencing the Outer class.Outer. Inner in = new Outer. Inner();
Q. What are the constraints on Static Inner Classes?
A. It cannot access non-static members of the outer class. It cannot use this reference to the outer class.
Q. How many class files are produced for source file having one Outer class and one Inner class?
A. Two class files will be produced as follows:Outer. ClassOuter$Inner. Class
Java Exception Handling Interview Questions
Q. Which package contains exception handling related classes?
A. java.lang
Q. What are the two types of Exceptions?
A. Checked Exceptions and Unchecked Exceptions.
Q. What is the base class of all exceptions?
A. java.lang.Throwable
Q. What is the difference between Exception and Error in java?
A. Exception and Error are the subclasses of the Throwable class. Exception class is used for exceptional conditions that user program should catch. Error defines exceptions that are not excepted to be caught by the user program. Example is Stack Overflow.
Q. What is the difference between throw and throws?
A. throw is used to explicitly raise a exception within the program, the statement would be throw new Exception(); throws clause is used to indicate the exceptions that are not handled by the method. It must specify this behavior so the callers of the method can guard against the exceptions. throws is specified in the method signature. If multiple exceptions are not handled, then they are separated by a comma. the statement would be as follows: public void doSomething() throws IOException,MyException{}
Q. Differentiate between Checked Exceptions and Unchecked Exceptions?
A. Checked Exceptions are those exceptions which should be explicitly handled by the calling method. Unhandled checked exceptions results in compilation error.Unchecked Exceptions are those which occur at runtime and need not be explicitly handled. RuntimeException and it's subclasses, Error and it's subclasses fall under unchecked exceptions.
Q. What are User defined Exceptions?
A. Apart from the exceptions already defined in Java package libraries, user can define his own exception classes by extending Exception class.
Q. What is the importance of finally block in exception handling?
A. Finally block will be executed whether or not an exception is thrown. If an exception is thrown, the finally block will execute even if no catch statement match the exception. Any time a method is about to return to the caller from inside try/catch block, via an uncaught exception or an explicit return statement, the finally block will be executed. Finally is used to free up resources like database connections, IO handles, etc.
Q. Can a catch block exist without a try block?
A. No. A catch block should always go with a try block.
Q. Can a finally block exist with a try block but without a catch?
A. Yes. The following are the combinations try/catch or try/catch/finally or try/finally.
Q. What will happen to the Exception object after exception handling?
A. Exception object will be garbage collected.
Q. The subclass exception should precede the base class exception when used within the catch clause. True/False?
A. True.
Q. Exceptions can be caught or rethrown to a calling method. True/False?
A. True.
Q. The statements following the throw keyword in a program are not executed. True/False?
A. True.
Q. How does finally block differ from finalize() method?
A. Finally block will be executed whether or not an exception is thrown. So it is used to free resoources. finalize() is a protected method in the Object class which is called by the JVM just before an object is garbage collected.
Q. What are the constraints imposed by overriding on exception handling?
A. An overriding method in a subclass may only throw exceptions declared in the parent class or children of the exceptions declared in the parent class.
Threading/Multithreading.
Q. What is synchronization in respect to multi-threading in Java?
A. With respect to multi-threading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one Java thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to erroneous behavior or program.
Q. Explain different way of using thread?
A. A Java thread could be implemented by using Runnable interface or by extending the Thread class. The Runnable is more advantageous, when you are going for multiple inheritance.
Q. What is the difference between Thread.start() & Thread.run() method?
A.Thread.start() method (native method) of Thread class actually does the job of running the Thread.run() method in a thread. If we directly call Thread.run() method it will executed in same thread, so does not solve the purpose of creating a new thread.
Q. Why do we need run() & start() method both. Can we achieve it with only run method?
A. We need run() & start() method both because JVM needs to create a separate thread which can not be differentiated from a normal method call. So this job is done by start method native implementation which has to be explicitly called. Another advantage of having these two methods is we can have any object run as a thread if it implements Runnable interface. This is to avoid Java’s multiple inheritance problems which will make it difficult to inherit another class with Thread.
Q. What is ThreadLocal class? How can it be used?
A. Below are some key points about ThreadLocal variables
· A thread-local variable effectively provides a separate copy of its value for each thread that uses it.
· ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread
· In case when multiple threads access a ThreadLocal instance, separate copy of Threadlocal variable is maintained for each thread.
· Common use is seen in DAO pattern where the DAO class can be singleton but the Database connection can be maintained separately for each thread. (Per Thread Singleton)
ThreadLocal variable are difficult to understand and I have found below reference links very useful in getting better understanding on them
Q. When InvalidMonitorStateException is thrown? Why?
A. This exception is thrown when you try to call wait()/notify()/notifyAll() any of these methods for an Object from a point in your program where u are NOT having a lock on that object.(i.e. u r not executing any synchronized block/method of that object and still trying to call wait()/notify()/notifyAll()) wait(), notify() and notifyAll() all throw IllegalMonitorStateException. since This exception is a subclass of RuntimeException so we r not bound to catch it (although u may if u want to). and being a RuntimeException this exception is not mentioned in the signature of wait(), notify(), notifyAll() methods.
Q. What is the difference between sleep(), suspend() and wait() ?
A. Thread.sleep() takes the current thread to a "Not Runnable" state for specified amount of time. The thread holds the monitors it has acquired. For example, if a thread is running a synchronized block or method and sleep method is called then no other thread will be able to enter this block or method. The sleeping thread can wake up when some other thread calls t.interrupt on it. Note that sleep is a static method, that means it always affects the current thread (the one executing sleep method). A common mistake is trying to call t2.sleep() where t2 is a different thread; even then, it is the current thread that will sleep, not the t2 thread. thread.suspend() is deprecated method. Its possible to send other threads into suspended state by making a suspend method call. In suspended state a thread keeps all its monitors and can not be interrupted. This may cause deadlocks therefore it has been deprecated. object.wait() call also takes the current thread into a "Not Runnable" state, just like sleep(), but with a slight change. Wait method is invoked on a lock object, not thread.
Here is the sequence of operations you can think
Here is the sequence of operations you can think
· A thread T1 is already running a synchronized block with a lock on object - lets say "lockObject"
· Another thread T2 comes to execute the synchronized block and find that its already acquired.
· Now T2 calls lockObject.wait() method for waiting on lock to be release by T1 thread.
· T1 thread finishes all its synchronized block work.
· T1 thread calls lockObject.notifyAll() to notify all waiting threads that its done using the lock.
· Since T2 thread is first in the queue of waiting it acquires the lock and starts processing.
A. Synchronized static methods have a lock on the class "Class", so when a thread enters a synchronized static method, the class itself gets locked by the thread monitor and no other thread can enter any static synchronized methods on that class. This is unlike instance methods, as multiple threads can access "same synchronized instance methods" at same time for different instances.
Q. Can a thread call a non-synchronized instance method of an Object when a synchronized method is being executed ?
A. Yes, a Non synchronized method can always be called without any problem. In fact Java does not do any check for a non-synchronized method. The Lock object check is performed only for synchronized methods/blocks. In case the method is not declared synchronized Jave will call even if you are playing with shared data. So you have to be careful while doing such thing. The decision of declaring a method as synchronized has to be based on critical section access. If your method does not access a critical section (shared resource or data structure) it need not be declared synchronized. Below is the example which demonstrates this, The Common class has two methods synchronizedMethod1() and method1() MyThread class is calling both the methods in separate threads.
public classCommon {
public synchronized void synchronizedMethod1() {
System.out.println("synchronizedMethod1 called");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("synchronizedMethod1 done");
}
public void method1() {
System.out.println("Method 1 called");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Method 1 done");
}
}
public class MyThread extends Thread {
private int id = 0;
privateCommon common;
publicMyThread(String name, int no, Common object) {
super(name);
common = object;
id = no;
}
public void run() {
System.out.println("Running Thread" + this.getName());
try {
if (id == 0) {
common.synchronizedMethod1();
} else {
common.method1();
}
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Common c = new Common();
MyThread t1 = new MyThread("MyThread-1", 0, c);
MyThread t2 = new MyThread("MyThread-2", 1, c);
t1.start();
t2.start();
}
}
Here is the output of the program
view plainprint?
Running ThreadMyThread-1
synchronizedMethod1 called
Running ThreadMyThread-2
Method 1 called
synchronizedMethod1 done
Method 1 done
Q. Can two threads call two different synchronized instance methods of an Object?
A. No. If a object has synchronized instance methods then the Object itself is used a lock object for controlling the synchronization. Therefore all other instance methods need to wait until previous method call is completed. See the below sample code which demonstrate it very clearly. The Class Common has 2 methods called synchronizedMethod1() and synchronizedMethod2() MyThread class is calling both the methods.
public class Common {
public synchronized void synchronizedMethod1() {
System.out.println("synchronizedMethod1 called");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("synchronizedMethod1 done");
}
public synchronized void synchronizedMethod2() {
System.out.println("synchronizedMethod2 called");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("synchronizedMethod2 done");
}
}
public class MyThread extends Thread {
private int id = 0;
privateCommon common;
publicMyThread(String name, int no, Common object) {
super(name);
common = object;
id = no;
}
public void run() {
System.out.println("Running Thread" + this.getName());
try {
if (id == 0) {
common.synchronizedMethod1();
} else {
common.synchronizedMethod2();
}
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Common c = new Common();
MyThread t1 = new MyThread("MyThread-1", 0, c);
MyThread t2 = new MyThread("MyThread-2", 1, c);
t1.start();
t2.start();
}
}
Q. What is a deadlock?
A. Deadlock is a situation where two or more threads are blocked forever, waiting for each other. This may occur when two threads, each having a lock on one resource, attempt to acquire a lock on the other's resource. Each thread would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. In terms of Java API, thread deadlock can occur in following conditions:
· When two threads call Thread.join() on each other.
· When two threads use nested synchronized blocks to lock two objects and the blocks lock the same objects in different order.
Q. What is Starvation? and What is a Livelock?
A. Starvation and livelock are much less common a problem than deadlock, but are still problems that every designer of concurrent software is likely to encounter.
LiveLock
Livelock occurs when all threads are blocked, or are otherwise unable to proceed due to unavailability of required resources, and the non-existence of any unblocked thread to make those resources available. In terms of Java API, thread livelock can occur in following conditions:
· When all the threads in a program execute Object.wait(0) on an object with zero parameter. The program is live-locked and cannot proceed until one or more threads call Object.notify() or Object.notifyAll() on the relevant objects. Because all the threads are blocked, neither call can be made.
· When all the threads in a program are stuck in infinite loops.
Starvation
Starvation describes a situation where a thread is unable to gain regular access to shared resources and is unable to make progress. This happens when shared resources are made unavailable for long periods by "greedy" threads. For example, suppose an object provides a synchronized method that often takes a long time to return. If one thread invokes this method frequently, other threads that also need frequent synchronized access to the same object will often be blocked. Starvation occurs when one thread cannot access the CPU because one or more other threads are monopolizing the CPU. In Java, thread starvation can be caused by setting thread priorities inappropriately. A lower-priority thread can be starved by higher-priority threads if the higher-priority threads do not yield control of the CPU from time to time.Q. How to find a deadlock has occurred in Java? How to detect a Deadlock in Java?
A. Earlier versions of Java had no mechanism to handle/detect deadlock. Since JDK 1.5 there are some powerful methods added in the java.lang.management package to diagnose and detect deadlocks. The java.lang.management.ThreadMXBean interface is management interface for the thread system of the Java virtual machine. It has two methods which can leveraged to detect deadlock in a Java application.
· findMonitorDeadlockedThreads() - This method can be used to detect cycles of threads that are in deadlock waiting to acquire object monitors. It returns an array of thread IDs that are deadlocked waiting on monitor.
· findDeadlockedThreads() - It returns an array of thread IDs that are deadlocked waiting on monitor or ownable synchronizers.
Q. What is immutable object? How does it help in writing concurrent application?
A. An object is considered immutable if its state cannot change after it is constructed. Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state. Examples of immutable objects from the JDK include String and Integer. Immutable objects greatly simplify your multi threaded program, since they are
· Simple to construct, test, and use.
· Automatically thread-safe and have no synchronization issues.
To create a object immutable You need to make the class final and all its member final so that once objects gets crated no one can modify its state. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.
Q. How will you take thread dump in Java? How will you analyze Thread dump?
A. A Thread Dump is a complete list of active threads. A java thread dump is a way of finding out what each thread in the JVM is doing at a particular point of time. This is especially useful when your java application seems to have some performance issues. Thread dump will help you to find out which thread is causing this. There are several ways to take thread dumps from a JVM. It is highly recommended to take more than 1 thread dump and analyze the results based on it. Follow below steps to take thread dump of a java process
· Step 1
On UNIX, Linux and Mac OSX Environment run below command:
ps -el | grep java
On Windows:
Press Ctrl+Shift+Esc to open the task manager and find the PID of the java process.
On UNIX, Linux and Mac OSX Environment run below command:
ps -el | grep java
On Windows:
Press Ctrl+Shift+Esc to open the task manager and find the PID of the java process.
Step 2:
Use jstack command to print the Java stack traces for a given Java process PID
jstack [PID]
Use jstack command to print the Java stack traces for a given Java process PID
jstack [PID]
Q. What is a thread leak? What does it mean in Java?
A. Thread leak is when a application does not release references to a thread object properly. Due to this some Threads do not get garbage collected and the number of unused threads grow with time. Thread leak can often cause serious issues on a Java application since over a period of time too many threads will be created but not released and may cause applications to respond slow or hang.
Q. How can I trace whether the application has a thread leak?
A. If an application has thread leak then with time it will have too many unused threads. Try to find out what type of threads is leaking out. This can be done using following ways
· Give unique and descriptive names to the threads created in application. - Add log entry in all thread at various entry and exit points in threads.
· Change debugging config levels (debug, info, error etc) and analyze log messages.
· When you find the class that is leaking out threads check how new threads are instantiated and how they're closed.
· Make sure the thread is Guaranteed to close properly by doing following - Handling all Exceptions properly.
· Make sure the thread is Guaranteed to close properly by doing following
· Handling all Exceptions properly.
· releasing all resources (e.g. connections, files etc) before it closes.
Q. What is thread pool? Why should we use thread pools?
A. A thread pool is a collection of threads on which task can be scheduled. Instead of creating a new thread for each task, you can have one of the threads from the thread pool pulled out of the pool and assigned to the task. When the thread is finished with the task, it adds itself back to the pool and waits for another assignment. One common type of thread pool is the fixed thread pool. This type of pool always has a specified number of threads running; if a thread is somehow terminated while it is still in use, it is automatically replaced with a new thread. Below are key reasons to use a Thread Pool
· Using thread pools minimizes the JVM overhead due to thread creation. Thread objects use a significant amount of memory, and in a large-scale application, allocating and de-allocating many thread objects creates a significant memory management overhead.
· You have control over the maximum number of tasks that are being processed in parallel (= number of threads in the pool).
Most of the executor implementations in java.util.concurrent use thread pools, which consist of worker threads. This kind of thread exists separately from the Runnable and Callable tasks it executes and is often used to execute multiple tasks. Q. Can we synchronize the run method? If yes then what will be the behavior?
A. Yes, the run method of a runnable class can be synchronized. If you make run method synchronized then the lock on runnable object will be occupied before executing the run method. In case we start multiple threads using the same runnable object in the constructor of the Thread then it would work. But until the 1st thread ends the 2nd thread cannot start and until the 2nd thread ends the next cannot start as all the threads depend on lock on same object.
Q. Can we synchronize the constructor of a Java Class?
A. As per Java Language Specification, constructors cannot be synchronized because other threads cannot see the object being created before the thread creating it has finished it. There is no practical need of a Java Objects constructor to be synchronized, since it would lock the object being constructed, which is normally not available to other threads until all constructors of the object finish.
Q. Define Serialization? What do you mean by Serialization in Java?
Q. Why is Serialization required? What is the need to Serialize?
Q. What is the Difference between Externalizable and Serializable Interfaces?
Q. When will you use Serializable or Externalizable interface? and why?
Q. What are the ways to speed up Object Serialization? How to improve Serialization performance?
Q. What is a Serial Version UID (serialVersionUID) and why should I use it? How to generate one?
Q. What would happen if the SerialVersionUID of an object is not defined?
Q. Does setting the serialVersionUID class field improve Java serialization performance?
Q. What are the alternatives to Serialization? If Serialization is not used, is it possible to persist or transfer an object using any other approach?
Q. What are transient variables? What role do they play in Serialization process?
Q. Why does serialization NOT save the value of static class attributes? Why static variables are not serialized?
Q. How to Serialize a collection in java? How to serialize a ArrayList, Hashmap or Hashset object in Java?
Q. Is it possible to customize the serialization process? How can we customize the Serialization process?
Q. How can a sub-class of Serializable super class avoid serialization? If serializable interface is implemented by the super class of a class, how can the serialization of the class be avoided?
Q. What changes are compatible and incompatible to the mechanism of java Serialization?
Java serialization is one of the most commonly misunderstood areas. Many developers still think its only used for saving objects on the file system.
Q. Why Java uses Classpath parameter or environment variables?
Q. When does Java read values of Classpath environment variable?
Q. How to set Java Classpath on Windows, Unix, Linux and Mac?
A. Setting CLASSPATH on Windows XP
Setting CLASSPATH on Windows 7
Setting CLASSPATH on Unix, Linux and Mac
Use export command to set the CLASSPATH environment variable in your system.
export CLASSPATH=/path/to/dir1:/path/to/dir2:path/to/abc.jar
Q. How do I check the CLASSPATH variable is set in my machine?
A. Checking CLASSPATH on Windows
Checking CLASSPATH on Unix, Linux or Mac
If CLASSPATH variable is not set you will see CLASSPATH: Undefined variable error on Unix/Linux/Mac systems.
Setting Multiple Jars in Classpath on Windows
Setting Multiple Jars in Classpath on Unix, Linux or Mac
Older version of Java
Q. What is the difference between NoClassDefFoundError and ClassNotFoundException? When NoClassDefFoundError and ClassNotFoundException are thrown?
ClassNotFoundException
NoClassDefFoundError
Key Differences
Some scenario when ClassNotFoundException may occur
Scenario 1 (Use of reflection) - You will see ClassNotFoundException when you are using reflection to load classes at runtime, and the class you are trying to load does not exist or not in classpath.
Scenario 2 (Multiple Classloaders being used) - You will see ClassNotFoundException when a class is being loaded from another class which was already loaded in a parent classloader and the class from the child classloader is not visible.Q. How can we include Jar within a Jar in java classpath?
Use Jar class loader library
Explode and combine into one jar
Q. How to read a file from CLASSPATH in java?
Using ClassLoader.getResourceAsStream
Using Class.getResourceAsStream
Q. How to find which jar file is being used by Java run-time?
Q. How to find the load location of a Java class file at run-time?
Using Classloader
Using Protection Domain
Q. How Java handles Two classes with same name in classpath?
A. If I have two classes with same name say MyClass.java in two different jar in my classpath which one will be picked up by JVM , is there anyway I can suggest JVM to pick a specific one ?
Java interpreter loads classes based on the order they are specified in the CLASSPATH variable.
For example, lets say this is your classpath variable value
The Java interpreter will first look for MyClass class in the directory C:\Project\Dir1. Only if it doesn't find it in that directory it will look in the C:\Project\Dir2 directory.
Q. How to Add A Jar File To Java Load Path At Run Time
You can run this code by below command. (Make sure to use forward slash "/" as directory separator.)Q. Why calling System.setProperty() does not affect the classpath at run-time?
Q. How to Add A Jar File To Java System Classpath At Run-time?
Running this java class prints the number of bytes available. This indicates the file is available for further processing.
Q. How Generics works in Java ? What is type erasure ?
Q. What is Bounded and Unbounded wildcards in Generics ?
Q. What is difference between List<? extends T> and List <? super T> ?
Q. How to write a generic method which accepts generic argument and return Generic Type?
Q. Write a program to implement LRU cache using Generics ?
This is not an exhaustive list of questions and I am sure I have missed many important questions from Multi-threading area. Can you think of a question which should be included in this list? Please feel free to share any question/suggestion in the comments section.
Java Serialization Interview QuestionsQ. Define Serialization? What do you mean by Serialization in Java?
A. Serialization is a mechanism by which you can save or transfer the state of an object by converting it to a byte stream. This can be done in java by implementing Serialiazable interface. Serializable is defined as a marker interface which needs to be implemented for transferring an object over a network or persistence of its state to a file. Since its a marker interface, it does not contain any methods. Implementation of this interface enables the conversion of object into byte stream and thus can be transferred. The object conversion is done by the JVM using its default serialization mechanism.
Q. Why is Serialization required? What is the need to Serialize?
A. Serialization is required for a variety of reasons. It is required to send across the state of an object over a network by means of a socket. One can also store an object’s state in a file. Additionally, manipulation of the state of an object as streams of bytes is required. The core of Java Serialization is the Serializable interface. When Serializable interface is implemented by your class it provides an indication to the compiler that java Serialization mechanism needs to be used to serialize the object.
Q. What is the Difference between Externalizable and Serializable Interfaces?
A. This is one of top serialization questions that is asked in many big companies to test your in-depth understanding of serialization. Serializable is a marker interface therefore you are not forced to implement any methods, however Externalizable contains two methods readExternal() and writeExternal() which must be implemented. Serializable interface provides a inbuilt serialization mechanism to you which can be in-efficient at times. However Externilizable interface is designed to give you greater control over the serialization mechanism. The two methods provide you immense opportunity to enhance the performance of specific object serialization based on application needs. Serializable interface provides a default serialization mechanism, on the other hand, Externalizable interface instead of relying on default Java Serialization provides flexibility to control this mechanism. One can drastically improve the application performance by implementing the Externalizable interface correctly. However there is also a chance that you may not write the best implementation, so if you are not really sure about the best way to serialize, I would suggest your stick to the default implementation using Serializable interface.
Q. When will you use Serializable or Externalizable interface? and why?
A. Most of the times when you want to do a selective attribute serialization you can use Serializable interface with transient modifier for variables not to be serialized. However, use of Externalizable interface can be really effective in cases when you have to serialize only some dynamically selected attributes of a large object. Lets take an example, Some times when you have a big Java object with hundreds of attributes and you want to serialize only a dozen dynamically selected attributes to keep the state of the object you should use Externalizable interface writeExternal method to selectively serialize the chosen attributes. In case you have small objects and you know that most or all attributes are required to be serialized then you should be fine with using Serializable interface and use of transient variable as appropriate.
Q. What are the ways to speed up Object Serialization? How to improve Serialization performance?
A. The default Java Serialization mechanism is really useful, however it can have a really bad performance based on your application and business requirements. The serialization process performance heavily depends on the number and size of attributes you are going to serialize for an object. Below are some tips you can use for speeding up the marshaling and un-marshaling of objects during Java serialization process.
· Mark the unwanted or non Serializable attributes as transient. This is a straight forward benefit since your attributes for serialization are clearly marked and can be easily achieved using Serialzable interface itself.
· Save only the state of the object, not the derived attributes. Sometimes we keep the derived attributes as part of the object however serializing them can be costly. Therefore consider calcualting them during de-serialization process.
· Serialize attributes only with NON-default values. For examples, serializing a int variable with value zero is just going to take extra space however, choosing not to serialize it would save you a lot of performance. This approach can avoid some types of attributes taking unwanted space. This will require use of Externalizable interface since attribute serialization is determined at runtime based on the value of each attribute.
· Use Externalizable interface and implement the readExternal and writeExternal methods to dynamically identify the attributes to be serialized. Sometimes there can be a custom logic used for serialization of various attributes.
Q. What is a Serial Version UID (serialVersionUID) and why should I use it? How to generate one?
A. The serialVersionUID represents your class version, and you should change it if the current version of your class is not backwards compatible with its earlier versions. This is extract from Java API Documentation
The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization.
Most of the times, we probably do not use serialization directly. In such cases, I would suggest to generate a default serializable uid by clicking the quick fix option in eclipse.
Q. What would happen if the SerialVersionUID of an object is not defined?
A. If you don't define serialVersionUID in your serilizable class, Java compiler will make one by creating a hash code using most of your class attributes and features. When an object gets serialized, this hash code is stamped on the object which is known as the SerialVersionUID of that object. This ID is required for the version control of an object. SerialVersionUID can be specified in the class file also. In case, this ID is not specified by you, then Java compiler will regenerate a SerialVersionUID based on updated class and it will not be possible for the already serialized class to recover when a class field is added or modified. Its recommended that you always declare a serialVersionUID in your Serializable classes.
Q. Does setting the serialVersionUID class field improve Java serialization performance?
A. Declaring an explicit serialVersionUID field in your classes saves some CPU time only the first time the JVM process serializes a given Class. However the gain is not significant, In case when you have not declared the serialVersionUID its value is computed by JVM once and subsequently kept in a soft cache for future use.
Q. What are the alternatives to Serialization? If Serialization is not used, is it possible to persist or transfer an object using any other approach?
A. In case, Serialization is not used, Java objects can be serialized by many ways, some of the popular methods are listed below:
· Saving object state to database, this is most common technique used by most applications. You can use ORM tools (e.g. hibernate) to save the objects in a database and read them from the database.
· Xml based data transfer is another popular mechanism, and a lot of XML based web services use this mechanism to transfer data over network. Also a lot of tools save XML files to persist data/configurations.
· JSON Data Transfer - is recently popular data transfer format. A lot of web services are being developed in JSON due to its small footprint and inherent integration with web browser due to JavaScript format.
Q. What are transient variables? What role do they play in Serialization process?
A. The transient keyword in Java is used to indicate that a field should not be serialized. Once the process of de-serialization is carried out, the transient variables do not undergo a change and retain their default value. Marking unwanted fields as transient can help you boost the serialization performance. Below is a simple example where you can see the use of transient keyword.
class MyVideo implements Serializable {
private Video video;
private transient Image thumbnailVideo;
private void generateThumbnail() {
// Generate thumbnail.
}
private void readObject(ObjectInputStream inputStream) throws IOException, ClassNotFoundException {
inputStream.defaultReadObject();
generateThumbnail();
}
}
Q. Why does serialization NOT save the value of static class attributes? Why static variables are not serialized?
A. The Java variables declared as static are not considered part of the state of an object since they are shared by all instances of that class. Saving static variables with each serialized object would have following problems
· It will make redundant copy of same variable in multiple objects which makes it in-efficient.
· The static variable can be modified by any object and a serialized copy would be stale or not in sync with current value.
Q. How to Serialize a collection in java? How to serialize a ArrayList, Hashmap or Hashset object in Java?
A. All standard implementations of collections List, Set and Map interface already implement java.io.Serializable. All the commonly used collection classes like java.util.ArrayList, java.util.Vector, java.util.Hashmap, java.util.Hashtable, java.util.HashSet, java.util.TreeSet do implement Serializable. This means you do not really need to write anything specific to serialize collection objects. However you should keep following things in mind before you serialize a collection object - Make sure all the objects added in collection are Serializable. - Serializing the collection can be costly therefore make sure you serialize only required data isntead of serializing the whole collection. - In case you are using a custom implementation of Collection interface then you may need to implement serialization for it.
Q. Is it possible to customize the serialization process? How can we customize the Serialization process?
A. Yes, the serialization process can be customized. When an object is serialized, objectOutputStream.writeObject (to save this object) is invoked and when an object is read, ObjectInputStream.readObject () is invoked. What most people do not know is that Java Virtual Machine provides you with an option to define these methods as per your needs. Once this is done, these two methods will be invoked by the JVM instead of the application of the default serialization process. Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:
private void writeObject(java.io.ObjectOutputStream out) throws IOException
private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException;
private void readObjectNoData() throws ObjectStreamException;
Q. How can a sub-class of Serializable super class avoid serialization? If serializable interface is implemented by the super class of a class, how can the serialization of the class be avoided?
A. In Java, if the super class of a class is implementing Serializable interface, it means that it is already serializable. Since, an interface cannot be unimplemented, it is not possible to make a class non-serializable. However, the serialization of a new class can be avoided. For this, writeObject () and readObject() methods should be implemented in your class so that a Not Serializable Exception can be thrown by these methods. And, this can be done by customizing the Java Serialization process. Below the code that demonstrates it.
private void writeObject(java.io.ObjectOutputStream out) throws IOException {
throw new NotSerializableException(“Can not serialize this class”);
}
private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
throw new NotSerializableException(“Can not serialize this class”);
}
private void readObjectNoData() throws ObjectStreamException; {
throw new NotSerializableException(“Can not serialize this class”);
}
Q. What changes are compatible and incompatible to the mechanism of java Serialization?
A. This is one of a difficult and tricky questions and answering this correctly would mean you are an expert in Java Serialization concept. In an already serialized object, the most challenging task is to change the structure of a class when a new field is added or removed. As per the specifications of Java Serialization, addition of any method or field is considered to be a compatible change whereas changing of class hierarchy or non-implementation of Serializable interface is considered to be a non-compatible change. You can go through the Java serialization specification for the extensive list of compatible and non-compatible changes. If a serialized object need to be compatible with an older version, it is necessary that the newer version follows some rules for compatible and incompatible changes. A compatible change to the implementing class is one that can be applied to a new version of the class, which still keeps the object stream compatible with older version of same class. Some Simple Examples of compatible changes are:
· Addition of a new field or class will not affect serialization, since any new data in the stream is simply ignored by older versions. the newly added field will be set to its default values when the object of an older version of the class is un marshaled.
· The access modifiers change (like private, public, protected or default) is compatible since they are not reflected in the serialized object stream.
· Changing a transient field to a non-transient field is compatible change since it is similar to adding a field.
· Changing a static field to a non-static field is compatible change since it is also similar to adding a field.
Some Simple Examples of incompatible changes are:
· Changing implementation from Serializable to Externalizable interface can not be done since this will result in the creation of an incompatible object stream.
· Deleting a existing Serializable fields will cause a problem.
· Changing a non-transient field to a transient field is incompatible change since it is similar to deleting a field.
· Changing a non-static field to a static field is incompatible change since it is also similar to deleting a field.
· Changing the type of a attribute within a class would be incompatible, since this would cause a failure when attempting to read and convert the original field into the new field.
· Changing the package of class is incompatible. Since the fully-qualified class name is written as part of the object byte stream.
Java serialization is one of the most commonly misunderstood areas. Many developers still think its only used for saving objects on the file system.
Classpath Interview Questions
Q. Why Java uses Classpath parameter or environment variables?
A. In a Java class import statements are used to access other classes. You also do a wild card import like org.fromdev.* on your java file.
In such cases, It will become very impractical/slow for the Java Virtual Machine to search for classes in every file/folder on a machine, therefore you can provide the Java Virtual Machine with a list of places to look. This is done by putting folder and jar files on your classpath.
Environment variables in general are a set of dynamic name value pair that can be used by processes to decide the behavior based on a system. These are supported by all modern operating systems.
The CLASSPATH variable is a Java way to tell the possible locations of user classes or jar files for a Java application. Since each user/computer may choose to have classes in different location its best to have custom locations configured in Classpath variable.
In case you have not set the classpath environment variable, the default value is used as the "." (current directory). That means, the current directory is searched.
In such cases, It will become very impractical/slow for the Java Virtual Machine to search for classes in every file/folder on a machine, therefore you can provide the Java Virtual Machine with a list of places to look. This is done by putting folder and jar files on your classpath.
Environment variables in general are a set of dynamic name value pair that can be used by processes to decide the behavior based on a system. These are supported by all modern operating systems.
The CLASSPATH variable is a Java way to tell the possible locations of user classes or jar files for a Java application. Since each user/computer may choose to have classes in different location its best to have custom locations configured in Classpath variable.
In case you have not set the classpath environment variable, the default value is used as the "." (current directory). That means, the current directory is searched.
Q. When does Java read values of Classpath environment variable?
A. Java uses the CLASSPATH environment variable to read the classes and libraries from file system. This variable is used by all JDK Tools and Extension including Java Compiler (javac) and JRE(java) use this variable to locate the dependent user classes and jar files to perform specific tasks.
Java Compiler uses it to locate the dependent user classes and jar files to compile Java source files.
Java Run-time Environment (JRE) uses the classpath variable to identify the location of files to be loaded for the run-time dependencies (e.g. classes and jar files) of java program.
Java Compiler uses it to locate the dependent user classes and jar files to compile Java source files.
Java Run-time Environment (JRE) uses the classpath variable to identify the location of files to be loaded for the run-time dependencies (e.g. classes and jar files) of java program.
Q. How to set Java Classpath on Windows, Unix, Linux and Mac?
A. Setting CLASSPATH on Windows XP
Follow below steps on Windows XP to set Java CLASSPATH
· Right-click My Computer, and then click Properties.
· Click the Advanced tab.
· Click Environment variables.
· Click one the following options, for either a user or a system variable
· Click New to add a new variable name and value.
· Enter Variable name as CLASSPATH
· enter all directories and jar files separated by semicolon. (e.g. c:\dir1;c:\dir2;c:\dir3\abc.jar)
Setting CLASSPATH on Windows 7
· Click Start
· Then right-click on Computer,
· select Properties
· click Select Advanced System Settings tab.
· click the Environment Variables button.
Setting CLASSPATH on Unix, Linux and Mac
Use export command to set the CLASSPATH environment variable in your system.
export CLASSPATH=/path/to/dir1:/path/to/dir2:path/to/abc.jar
Q. How do I check the CLASSPATH variable is set in my machine?
A. Checking CLASSPATH on Windows
To check CLASSPATH variable is set on Microsoft Windows, run following command on command prompt
C:> echo %CLASSPATH%
If CLASSPATH variable is not set you will see %CLASSPATH% on windows system.
If CLASSPATH variable is not set you will see %CLASSPATH% on windows system.
Checking CLASSPATH on Unix, Linux or Mac
To check CLASSPATH variable is set on Unix/Linux/Mac run following command on shell
$ echo $CLASSPATH
If CLASSPATH variable is not set you will see CLASSPATH: Undefined variable error on Unix/Linux/Mac systems.
Q. How to set Multiple Jar Files in Java Classpath
A. Java versions Older then Java 6 does not support wildcard characters. Setting Multiple jars using wildcard in Java classpath are allowed in Java 6 and later versions.
For example, to specify all jar files in a directory "lib" the classpath entry should look like this
lib/*
The wildcard entry (*) in classpath value will match only jar files NOT class files. To match both class files and JAR files in a same directory lib, you need to specify both values as shown below
For example, to specify all jar files in a directory "lib" the classpath entry should look like this
lib/*
The wildcard entry (*) in classpath value will match only jar files NOT class files. To match both class files and JAR files in a same directory lib, you need to specify both values as shown below
Setting Multiple Jars in Classpath on Windows
Windows environment variable values are separated by semicolon, therefore you classpath entry would look like this
lib/*;lib
lib/*;lib
Setting Multiple Jars in Classpath on Unix, Linux or Mac
Unix environment variable values are separated by colon, therefore you classpath entry would look like this
lib/*:lib
lib/*:lib
Older version of Java
In older version of Java(older than Java 6), each jar file needs to be specified in the classpath. It can be a tedious and erroneous task if you are using many third party libraries.
Q. What is the difference between NoClassDefFoundError and ClassNotFoundException? When NoClassDefFoundError and ClassNotFoundException are thrown?
A. NoClassDefFoundError and ClassNotFoundException are very closely related and often confused with each other by many developers. Below is the description of each from the Java API Specifications
ClassNotFoundException
Thrown when an application tries to load in a class through its string name using:
· The forName method in class Class.
· The findSystemClass method in class ClassLoader.
· The loadClass method in class ClassLoader.
but the definition of the class with the specified name could not be found due to following reasons
· The specified name class does not exist.
· The specified name class is not in the classpath
· The specified name class is not visible to the classloader.
NoClassDefFoundError
Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.
The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.
The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.
Key Differences
· The NoClassDefFoundError is thrown when the source was successfully compiled, but during runtime, the required class files were not found. This may be a case when some dependency jar files were not included or not in classpath.
A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader or not visible to the Classloader.
A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader or not visible to the Classloader.
· Another important distinction between these two is, NoClassDefFoundError is a sub class of java.lang.Error and the ClassNotFoundException is a subclass of java.lang.Exception.
· NoClassDefFoundError is a critical error for JVM since its having problems finding a class it expected to find.
On the other hand, the ClassNotFoundException is an Exception. Use of reflection api can be error-prone and there is no compile-time check to validate reflection call is loading right classes, so there can be situations when some classes may not be found.
On the other hand, the ClassNotFoundException is an Exception. Use of reflection api can be error-prone and there is no compile-time check to validate reflection call is loading right classes, so there can be situations when some classes may not be found.
Some scenario when ClassNotFoundException may occur
Scenario 1 (Use of reflection) - You will see ClassNotFoundException when you are using reflection to load classes at runtime, and the class you are trying to load does not exist or not in classpath.
Scenario 2 (Multiple Classloaders being used) - You will see ClassNotFoundException when a class is being loaded from another class which was already loaded in a parent classloader and the class from the child classloader is not visible.
Q. How can we include Jar within a Jar in java classpath?
A. There is no easy way to do this in current java versions. There are 2 alternatives to deal with this problem using third party libraries
Use Jar class loader library
The JarClassLoader library provides you the feature of loading resources from a top JAR and from JARs inside the top JAR.
Explode and combine into one jar
Instead of trying to bundle jar file inside jar you can explode all required jars and re-bundle them as one jar using following two libraries
· The first is One-Jar, which uses a special classloader to allow the nesting of jars.
· The second is UberJar, (or Maven Shade Plugin), which explodes the included libraries and puts all the classes in the top-level jar.
Q. How to read a file from CLASSPATH in java?
A. This can be done in two simple ways
Using ClassLoader.getResourceAsStream
This method can be used to load any file from CLASSPATH
InputStream in =this.getClass().getClassLoader().getResourceAsStream("MyFile.txt");
Using Class.getResourceAsStream
This method can be used to load files using relative path to the package of the class
InputStream in =this.getClass().getResourceAsStream("SomeTextFile.txt");
This method can also be used to load any files from CLASSPATH by prefixing a "/"
Q. How to find which jar file is being used by Java run-time?
A. On Windows
You need use below windows program Process Explorer that lets you see which files are open for a particular process or program
On Unix, Linux or Mac
It can be done using lsof command. lsof is one of my favorite and useful java debugging commands on Unix. Below is the syntax for using this command –
lsof -p [pid]
Q. How to find the load location of a Java class file at run-time?
A. There are two ways to find it
Using Classloader
Below code snippet can be used to find the location of java class com.fromdev.MyClass
this.getClass().getClassLoader().getResource("com/fromdev/MyClass.class")
this.getClass().getClassLoader().getResource("com/fromdev/MyClass.class")
Using Protection Domain
We can use this to find the exact location a jar file containing the class JVM is using
clazz.getProtectionDomain().getCodeSource().getLocation()
clazz.getProtectionDomain().getCodeSource().getLocation()
Q. How Java handles Two classes with same name in classpath?
A. If I have two classes with same name say MyClass.java in two different jar in my classpath which one will be picked up by JVM , is there anyway I can suggest JVM to pick a specific one ?
Java interpreter loads classes based on the order they are specified in the CLASSPATH variable.
For example, lets say this is your classpath variable value
C:\Project\Dir1;C:\Project\Dir2
The Java interpreter will first look for MyClass class in the directory C:\Project\Dir1. Only if it doesn't find it in that directory it will look in the C:\Project\Dir2 directory.
Q. How to Add A Jar File To Java Load Path At Run Time
A. This can done by use of URLClassloader. A sample implementation of this code is shown below.
import java.net.URL;
import java.net.URLClassLoader;
public class SimpleJarLoader {
public static void main(String args[]) {
if (args.length < 2) {
System.out.println("Usage: [Class name] [Jar Path]");
return;
}
try {
System.out.println("Trying to load the class...");
Class.forName(args[0]);
} catch (Exception ex) {
System.out.println("Not able to load class..." + args[0]);
}
try {
String url = "jar:file:/" + args[1] + "!/";
URL urls[] = { new URL(url) };
URLClassLoader cl = new URLClassLoader(urls,
SimpleJarLoader.class.getClassLoader());
System.out.println("Looking into jar... " + url);
cl.loadClass(args[0]);
System.out.println("Woohoo....I found it");
} catch (Exception ex) {
System.out.println("Oops...Still cant find the jar");
ex.printStackTrace();
}
}
}
You can run this code by below command. (Make sure to use forward slash "/" as directory separator.)
java SimpleJarLoader org.springframework.core.SpringVersion C:/spring.jar
The output is like this
Trying to load the class...
Not able to load class...org.springframework.core.SpringVersion
Looking into jar... jar:file:/C:/spring.jar!/
Woohoo....I found it
Q. Why calling System.setProperty() does not affect the classpath at run-time?
A. You can easily set any system properties in java using System.setPropoerty method, However it may not have any effect in case of CLASSPATH property. This is mainly because the Java system class loader is initialized very early in the JVM startup sequence. The class loader copies the classpath into its own data structures, and the classpath property is not read again. Therefore changing it after its already copied does not affect anything. There are mainly two reasons for this - First most important reason is security. You do not want a malicious code change the classpath at runtime and load some unwanted classes. Second reason is performance, since reading the classpath every-time its needed may not be efficient.
Q. How to Add A Jar File To Java System Classpath At Run-time?
A. This can be done by using a simple reflection API hack as demonstrated in below sample code. This example assumes you have a file "c:/Sample.txt" that is not already in class path and at run-time c:/ is added the System classpath and then Sample.txt is made available.
import java.io.File;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
public class HackJavaClasspath {
public static void addURL(URL url) throws Exception {
URLClassLoader cl = (URLClassLoader) ClassLoader.getSystemClassLoader();
Class clazz = URLClassLoader.class;
Method method = clazz.getDeclaredMethod("addURL", new Class[] { URL.class });
method.setAccessible(true);
method.invoke(cl, new Object[] { url });
}
public static void main(String[] args) throws Exception {
//Add c: to the classpath
addURL(new File("c:/").toURI().toURL());
//Now load the file from new location
InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream("Sample.txt");
System.out.println(in.available());
}
}
Running this java class prints the number of bytes available. This indicates the file is available for further processing.
Q. How to get a list of resources from a directory in Java classpath?
Java Generics Interview Questions
Generic interview questions in Java interviews are getting more and more common with Java 5 around there for considerable time and many application either moving to Java 5 and almost all new java development happening on Tiger(code name of Java 5). Importance of Generics and Java 5 features like Enum, Collection utilities are getting more and more popular on Java interviews. Generic interview question can get real tricky if you are not familiar with bounded and unbounded generic wildcards, How generics works internally, type erasure and familiarity with writing parametrized generics classes and methods in Java. Best way to prepare for Generics interview is to try simple program best on various features of generics. Anyway In this Java interview article we will see some popular interview questions and there answers around generics in Java.
Q. What is generics in Java ? What are advantages of using Generics?
A. This is one of the first interview questions asked on generics in any Java interview, mostly at beginners and intermediate level. Those who are coming from prior to Java 5 background knows that how inconvenient it was to store object in Collection and then cast it back to correct Type before using it. Generics prevents from those. it provides compile time type-safety and ensures that you only insert correct Type in collection and avoids ClassCastException in runtime.
Q. How Generics works in Java ? What is type erasure ?
A. This is one of better interview question in Generics. Generics is implemented using Type erasure, compiler erases all type related information during compile time and no type related information is available during runtime. for example List<String> is represented by only List at runtime. This was done to ensure binary compatibility with the libraries which were developed prior to Java 5. you don’t have access to Type argument at runtime and Generic type is translated to Raw type by compiler during runtime. you can get lot of follow up question based on this Generic interview question based upon your response e.g. Why Generics is implemented using Type erasure or presenting some invalid generic code which results in compiler error. read my post How Generics work in Java for more details
Q. What is Bounded and Unbounded wildcards in Generics ?
A. This is another very popular Java Interview Questions on Generics. Bounded Wildcards are those which impose bound on Type. there are two kinds of Bounded wildcards <? extends T> which impose an upper bound by ensuring that type must be sub class of T and <? super T> where its imposing lower bound by ensuring Type must be super class of T. This Generic Type must be instantiated with Type within bound otherwise it will result in compilation error. On the other hand <?> represent and unbounded type because <?> can be replace with any Type. See more on my post
Q. What is difference between List<? extends T> and List <? super T> ?
A. This is related to previous generics interview questions, some time instead of asking what is bounded and unbounded wildcards interviewer present this question to gauge your understanding of generics. Both of List declaration is example of bounded wildcards, List<? extends T> will accept any List with Type extending T while List<? super T> will accept any List with type super class of T. for Example List<? extends Number> can accept List<Integer> or List<Float> . see more on above link.
Q. How to write a generic method which accepts generic argument and return Generic Type?
A. Writing generic method is not difficult, instead of using raw type you need to use Generic Type like T, E or K,V which are well known placeholders for Type, Element and Key, Value. Look on Java Collection framework for examples of generics methods. In simplest form a generic method would look like:
public V put(K key, V value) {
return cache.put(key, value);
}
Q. How to write parametrized class in Java using Generics ?
A. This is an extension of previous Java generics interview question. Instead of asking to write Generic method Interviewer may ask to write a type safe class using generics. again key is instead of using raw types you need to used generic types and always use standard place holder used in JDK.
Q. Write a program to implement LRU cache using Generics ?
A. This is an exercise for anyone who like Coding in Java. One hint is that LinkedHashMap can be used implement fixed size LRU cache where one needs to remove eldest entry when Cache is full. LinkedHashMap provides a method called removeEldestEntry() which is called by put() and putAll() and can be used to instruct to remove eldest entry. you are free to come up with your own implementation as long as you have a written a working version along with Unit test.
Java String interview Question
Q. What is String in Java ? Is String is data type?
A. String in Java is not a primitive data type like int, long or double. String is a class or in more simple term a user defined type. This is confusing for some one who comes from C background. String is defined in java.lang package and wrappers its content in a character array. String provides equals() method to compare two String and provides various other method to operate on String like toUpperCase() to convert String into upper case, replace() to replace String contents, substring() to get substring, split() to split long String into multiple String.
Q. Why String is final in Java
A. String is final by design in Java, some of the points which makes sense why String is final is Security, optimization and to maintain pool of String in Java. for details on each of this point see Why String is final in Java.
Q. What is Difference between String and StringBuffer in Java
A. This is probably the most common question on String I have seen in Java interviews. Though String and Stringbuffer are two different class they are used in context of concatenating two Strings, Since String is immutable in Java every operation which changes String produces new String, which can be avoided by using Stringbuffer. See String vs StringBuffer for more details.
Q. What is difference in String on C and Java
A. If you have mentioned C in your resume, then you are likely to face this String interview question. Well C String and Java String are completely different to each other, C String is a null terminated character array while String in Java is an Object. Also String is more feature rich in Java than C.
Q. Why char array is better than String for storing password?
A. This String interview question is debatable and you might not agree with interviewer but this is also a chance to show that how deep and differently you can think of. One of the reason which people give Why you should store password in char array over String is related to immutability, since its not possible to remove erase contents of String but you can erase contents of char array. See Why char array preferred over String for password for complete discussion.
Q. How do you compare two String in Java ?
A. This is another common String interview question which appears on fresher level interviews. There are multiple ways to compare two String like equals() method, equalsIgnoreCase() etc, You can also see 4 ways to compare String in Java for more examples. Main thing which interviewer checks is that whether candidate mentioned equality operator or not "==", comparing String with equality operator is common mistake which works in some case and doesn't work in other. next String interview question is follow-up up of this.
Q. Can we compare String using == operator? What is risk?
A. As discussed in previous String question, You can compare String using equality operator but that is not suggested or advised because equality operator is used to compare primitives and equals() method should be used to compare objects. As we have seen in pitfall of autoboxing in Java that how equality operator can cause subtle issue while comparing primitive to Object, any way String is free from that issue because it doesn't have corresponding primitive type and not participate in autoboxing. Almost all the time comparing String means comparing contents of String i.e. characters and equals() method is used to perform character based comparison. equals() return true if two String points to same object or two String has same contents while == operator returns true if two String object points to same object but return false if two different String object contains same contents. That explains why sometime it works and sometime it doesn't. In short always use equals method in Java to check equality of two String object.
Q. How substring method work in Java
A. This is one of the tricky Java question relate to String and until you are familiar with internals of String class, its difficult to answer. Substring shares same character array as original String which can create memory leak if original String is quite big and not required to retain in memory but unintentionally retained by substring which is very small in size and prevents large array from begin claimed during Garbage collection in Java. See How Substring works in Java for more details.
Q. What is String pool in Java
A. Another tough Java question asked in String interview. String pool is a special storage area in Java heap, mostly located on PerGen space, to store String literals like "abc". When Java program creates a new String using String literal, JVM checks for that String in pool and if String literal is already present in pool than same object is returned instead of creating a whole new object. String pool check is only performed when you create String as literal, if you create String using new() operator, a new String object will be created even if String with same content is available in pool.
Q. What does intern() method do in Java
A. As discussed in previous String interview question, String object crated by new() operator is by default not added in String pool as opposed to String literal. intern() method allows to put an String object into pool.
Q. Does String is thread-safe in Java
A. If you are familiar with the concept of immutability and thread-safety you can easily answer this String interview question in Java. Since String is immutable, it is thread-safe and it can be shared between multiple thread without external synchronization.
That's all on Java String interview question. In Summary there are lot of specifics about String which needs to be know for any one who has started Java programming and these String question will not just help to perform better on Java Interviews but also opens new door of learning about String. I didn't know many String related concepts until I come across these question which motivated to research and learn more about String in Java.
How to Convert Date to String in Java with Example?
Sometimes we need to convert java.util.Date to string in java may for displaying purpose I need it that while working on displaytag then I thought about this article to list down various way of converting this in various ways, after some reading I found that SimpleDateFormat makes this quite easy. To get the feel of Date API in java and to familiarize ourselves with these classes we will see different examples of converting date to String in our application. Both DateFormat and SimpleDateFormat class belongs java.text package and they are very powerful and you can use them for conversion. it also good for parsing string into date and can be used to show in various locale also.
1) First step is to create a date format using SimpleDateFormat class
2) Call format() method of SimpleDateFormat by passing Date object this will return String representation of date into specified date format.
Now let’s see few examples of converting date to String in java:
//Creating Date in java with today's date.
Date dateNow = new Date();
//change date into string yyyyMMdd format example "20110914"
SimpleDateFormat dateformatyyyyMMdd = new SimpleDateFormat("yyyyMMdd");
String date_to_string = dateformatyyyyMMdd.format(dateNow);
System.out.println("date into yyyyMMdd format: " + date_to_string);
//converting date into ddMMyyyy format example "14092011"
SimpleDateFormat dateformatddMMyyyy = new SimpleDateFormat("ddMMyyyy");
date_to_string = dateformatddMMyyyy.format(dateNow);
System.out.println("Today's date into ddMMyyyy format: " + date_to_string);
//change date to string on dd-MM-yyyy format e.g. "14-09-2011"
SimpleDateFormat dateformatJava = new SimpleDateFormat("dd-MM-yyyy");
date_to_string = dateformatJava.format(dateNow);
System.out.println("Today's date into dd-MM-yyyy format: " + date_to_string);
//converting date to string dd/MM/yyyy format for example "14/09/2011"
SimpleDateFormat formatDateJava = new SimpleDateFormat("dd/MM/yyyy");
date_to_string = formatDateJava.format(dateNow);
System.out.println("Today's date into dd/MM/yyyy format: " + date_to_string);
//date to dd-MMM-yy format e.g. "14-Sep-11"
SimpleDateFormat ddMMMyyFormat = new SimpleDateFormat("dd-MMM-yy");
date_to_string = ddMMMyyFormat.format(dateNow);
System.out.println("Today's date into dd-MMM-yy format: " + date_to_string);
//convert date to dd-MMMM-yy format e.g. "14-September-11"
SimpleDateFormat ddMMMMyyFormat = new SimpleDateFormat("dd-MMMM-yy");
date_to_string = ddMMMMyyFormat.format(dateNow);
System.out.println("date into dd-MMMM-yy format: " + date_to_string);
For complete details on available symbols for date conversion you can check java doc of DateFormat and SimpleDateFormat class. Here are some important points about SimpleDateFormat which is worth remembering
1) Common point of confusion between “m” and “M” , small case “m” represent minutes while “M” represent Month Also “d” represent date in month while “D” represent Day of week. This is most common cause of error while converting String to date and back date to string. In shot ddMMyy is not equal toDDmmyy.
2) It’s also worth noting that SimpleDateFormat are not thread-safe. They are not synchronized so its better you create separate DateFormat for each thread to avoid any race condition while parsing date in java.
It’s very important for any java developer be it senior or junior to get familiarize himself with Date, Time and Calendar API. SimpleDateFormat is an excellent utility for converting String to Date and then Date to String but you just need to be little careful with format and thread-safety .
Q. Difference between StringBuilder and StringBuffer in Java
A. StringBuffer is very good with mutable String but it has one disadvantage all its public methods are synchronized which makes it thread-safe but same time slow. In JDK 5 they provided similar class called StringBuilder in Java which is a copy of StringBuffer but without synchronization. Try to use StringBuilder whenever possible it performs better in most of cases than StringBuffer class. You can also use "+" for concatenating two string because "+" operation is internal implemented using either StringBuffer or StringBuilder in Java. If you see StringBuilder vs StringBuffer you will find that they are exactly similar and all API methods applicable to StringBuffer are also applicable to StringBuilder in Java. On the other hand String vs StringBuffer is completely different and there API is also completely different, same is true for StringBuilder vs String.
Summary
In summary here are list of difference between StringBuffer, String and StringBuilder in Java :
1) String is immutable while StringBuffer and StringBuilder is mutable object.
2) StringBuffer is synchroonized while StringBuilder is not which makes StringBuilder faster than StringBuffer.
3) Concatenation operator "+" is internal implemented using either StringBuffer or StringBuilder.
4) Use String if you require immutability use Stringbuffer in java if you need mutable + thread safty and use StringBuilder in Java if you require mutable + without thread-safety.
That's all on famous String vs StringBuffer or StringBuffer vs StringBuilder discussion. All these differences helps to avoid common coding mistake of using String in place of StringBuffer in many places. from Java 5 onwards either use + operator of StringBuilder for concatenating String in Java.
Q. How to convert Enum to String in Java
A. Enum classes by default provides valueOf (String value) method which takes a String parameter and converts it into enum. String name should match with text used to declare Enum in Java file. Here is a complete code example of String to Enum in Java
Code Example String to Enum:
public class EnumTest {
private enum LOAN {
HOME_LOAN {
@Override
public String toString() {
return "Always look for cheaper Home loan";
}
},
AUTO_LOAN {
@Override
public String toString() {
return "Cheaper Auto Loan is better";
}
},
PEROSNAL_LOAN{
@Override
public String toString() {
return "Personal loan is not cheaper any more";
}
}
}
public static void main(String[] args) {
//Exmaple of String to Enum in Java
LOAN homeLoan = LOAN.valueOf("HOME_LOAN");
System.out.println(homeLoan);
LOAN autoLoan = LOAN.valueOf("AUTO_LOAN");
System.out.println(autoLoan);
LOAN personalLoan = LOAN.valueOf("PEROSNAL_LOAN");
System.out.println(personalLoan);
}
Output:
Always look for cheaper Home loan
Cheaper Auto Loan is better
Personal loan is not cheaper any more
Q. How to create comma separated String from Collection in Java
A.I am big fan on Spring framework and we use Spring in most of our projects and we have already discussed few examples of Spring framework in Java before e.g. Using Spring to calculate execution time and Spring security to control Concurrent Sessions in Java. In this Java tutorial I will share you an example of converting List to delimited String, Set to delimit String or any other Collection class into delimited String by using Spring framework's StringUtils class. Spring provides two convenient method collectionToCommaDelimitedString and collectionToDelimitedString which can be used here. This example is general and you can convert any Collection into comma separated or any delimiter separated String by using this technique. Order of individual String in delimited String is the order on which they are stored in Collection e.g. List or Set This particular Java program shows How to convert a List into comma, colon and pipe separated String.
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;
import org.springframework.util.StringUtils;
/**
* Simple Java program to demonstrate How to convert List or Set into String in Java.
* Spring framework's StringUtils class provide methods like
* collectionToCommaDelimitedString and collectionToDelimitedString
* which can convert any Java collection class like ArrayList or HashSet.
* into comma delimited String.
*
* @author Prabhat
*/
public class CollectionToString{
public static void main(String args[]) {
//List with multiple Strings for testing
List<String> frameworks = Arrays.asList("Spring MVC", "Struts 2.0", "Velocity", "Wicket");
//let's convert this list into comma separated String
String commaDelimitedString = StringUtils.collectionToCommaDelimitedString(frameworks);
System.out.println(commaDelimitedString);
//list to colon delimited String
String colonDelimitedString = StringUtils.collectionToDelimitedString(frameworks, ":");
System.out.println(colonDelimitedString);
//List to pipe delimited String
String pipeDelimitedString = StringUtils.collectionToDelimitedString(frameworks, "|");
System.out.println(pipeDelimitedString);
//Now let's convert Set into String in Java
HashSet<String> frameworkSet = new HashSet(frameworks);
//HashSet to comma separated String
commaDelimitedString = StringUtils.collectionToCommaDelimitedString(frameworkSet);
System.out.println(commaDelimitedString);
//Set to delimiter separated String using Spring
colonDelimitedString = StringUtils.collectionToDelimitedString(frameworkSet, ":");
System.out.println(colonDelimitedString);
//Set to pipe delimited String using Spring framework StringUtils
pipeDelimitedString = StringUtils.collectionToDelimitedString(frameworkSet, "|");
System.out.println(pipeDelimitedString);
}
}
Output
Spring MVC,Struts 2.0,Velocity,Wicket
Spring MVC:Struts 2.0:Velocity:Wicket
Spring MVC|Struts 2.0|Velocity|Wicket
Struts 2.0,Spring MVC,Velocity,Wicket
Struts 2.0:Spring MVC:Velocity:Wicket
Struts 2.0|Spring MVC|Velocity|Wicket
Q. How to convert String to Double in Java
A. As with above examples, there are multiple ways to convert a Double object into String. In this example of converting double to String we will see atleast four ways of doing same. this is rather much easier than opposite.
1) First way to convert Double to string is using concatantion operator "+" which produce a new string. This is by far simplest way of converting double object to string.
Double toBeString = 400.40;
String fromDouble = "" + toBeString;
2) Second way of double to String conversion is by using String.valueOf(double d) method , which takes a double value as argument and returns it in a form of String literal. here is an example of converting double to String using valueOf() method.
String strDouble = String.valueOf(toBeString);
3) Third way to convert double into String is by using toString() method of Double Class, which is essentially the same way used in first way because concatanation oeprator internally calls toString() method of object to get its string value.
String stringDouble = toBeString.toString();
4) Fourth way is rather more flexible way of getting String from Double. it uses String.format() method and returns a formatted string so you can control the precision level and get an String upto two decimal point or three decimal point based on your requirement.
convertedString = String.format("%.3f", toBeString);
This convertedString contains double value upto 3 decimal point. "f" is used to format floating point numbers. As you may have noticed we are passing Double object to methods which are expecting double premitive value and that is only possible due to autoboxing. if you are running below Java 5 use intValue() doubleValue() methods to get value in primitive format.
These are some basic ways to change any String into Double wrapper Class and vice versa. Please let us know if you are familiar with any other utility or method or doing same thing with less hassle may be overriding equals using EqualsBuilder and HashCodeBuilder.
Q. How to reverse String in Java with recursion or String Reverse Example using Iteration and Recursion in Java?
A. Here is a code example String reverse using iterative and recursive function written in Java. Recursive solution is just for demonstrative and education purpose, don’t use recursive solution in production code as it may result in StackOverFlowError if String to be reversed is very long String or if you have any bug in your reverse function, anyway its good test to make yourself comfortable with recursive functions in java.
import java.io.FileNotFoundException;
import java.io.IOException;
/**
*
*@author Prabhat
/
public class StringReverseExample {
public static void main(String args[]) throws FileNotFoundException, IOException {
//original string
String str = "Sony is going to introduce Internet TV soon";
System.out.println("Original String: " + str);
//reversed string using Stringbuffer
String reverseStr = new StringBuffer(str).reverse().toString();
System.out.println("Reverse String in Java using StringBuffer: " + reverseStr);
//iterative method to reverse String in Java
reverseStr = reverse(str);
System.out.println("Reverse String in Java using Iteration: " + reverseStr);
//recursive method to reverse String in Java
reverseStr = reverseRecursively(str);
System.out.println("Reverse String in Java using Recursion: " + reverseStr);
}
public static String reverse(String str) {
StringBuilder strBuilder = new StringBuilder();
char[] strChars = str.toCharArray();
for (int i = strChars.length - 1; i >= 0; i--) {
strBuilder.append(strChars[i]);
}
return strBuilder.toString();
}
public static String reverseRecursively(String str) {
//base case to handle one char string and empty string
if (str.length() < 2) {
return str;
}
return reverseRecursively(str.substring(1)) + str.charAt(0);
}
}
Q. How to format String in Java?
A. String.format() and System.out.printf() both works similarly and if you see the signature of both method they also accept variable arguments . Both take minimum two parameters, first of them is formatting instruction and other was actual String or anything which needs to be formatted. Java formatting instructions are both powerful and flexible and allows you to generate formatted String on many different format. Its worth to understand format of "formatting instruction" to take full benefit of String.format() method because this is the only tricky part of String formatting specially if you have not used printf() in past. I have seen developer struggle to understand the formatting behavior because of lack of knowledge of different formatting options available in Java.This is how we specify formatting instruction in Java :
String.format "%[argument number] [flags] [width] [.precision] type"
Now let's see what is meaning of each part of formatting instruction. "%" is a special character in formatted String and it denotes start of formatting instruction. String.format() can support multiple formatting instruction with multiple occurrence of "%" character in formatting instruction.
"argument number" is used to specify correct argument in case multiple arguments are available for formatting. "flags" is another special formatting instruction which is used to print String in some specific format for example you can use flag as "," to print comma on output. "width" formatting option denotes minimum number or character will be used in output but in case if number is larger than width then full number will be displayed but if its smaller in length then it will be be padded with zero. "precision" is using for print floating point formatted String, by using precision you can specify till how many decimal a floating point number will be displayed in formatted String. "type" is the only mandatory formatting option and must always comes last in format String also input String which needs to be formatted must be with same type specified in "type" parameter. for example you can not input a floating point number if you have specified "type" as decimal integer "%d", that will result in error. Now let's see an example of String format() method to understand these formatting option better:
format ( "%,6.2f", 124.000)
In above example of String.format() method flag is comma "," , width is 6 and precision is upto 2 decimal point and type is float.
String format Example in Java
In this section we will see different examples to format String in Java. We will see how we can format numbers and dates. Introduce decimal points and aligning number left or right etc. One of the common application of format() is to print leading zero in a number as shown in this Java program example:
/**
* Java program to demonstrate How to format String in Java by using
* format() method of String class and printf() method of OutputStream in Java.
* String.format() is very powerful and not only can format String but numbers
* and Date in Java
*
* @author Prabhat
*/
public class StringFormatExample{
public static void main(String args[]){
//simple example of formatted string in Java
//%d is used to format decimals like integers
//position of argument is the order in which they appear in source String
e.g here 40021 will replace first %d and 3000 will replace second %d.
String formattedString = String.format("Order with OrdId : %d and Amount: %d is missing", 40021, 3000);
System.out.println(formattedString);
System.out.printf("Order with OrdId : %d and Amount: %d is missing \n", 40021, 3000);
//%s is used to denote String arguments in formatted String
String str = String.format("Hello %s", "Raj");
System.out.println(str);
//if argument is not convertible into specified data type than
//Formatter will throw following java.util.IllegalFormatConversionException
//e.g. specifying %d and passing 3.0
//str = String.format("Number %d", 3.0);
// Exception in thread "main" java.util.IllegalFormatConversionException: d != java.lang.Double
// at java.util.Formatter$FormatSpecifier.failConversion(Formatter.java:3999)
// at java.util.Formatter$FormatSpecifier.printInteger(Formatter.java:2709)
// at java.util.Formatter$FormatSpecifier.print(Formatter.java:2661)
// at java.util.Formatter.format(Formatter.java:2433)
// at java.util.Formatter.format(Formatter.java:2367)
// at java.lang.String.format(String.java:2769)
//common meta characters used in String.format() and
//System.out.printf() method: %s - String , %d - decimal integer
// %f - float %tD - date as MM/dd/yy while %td is day %tm is month
// and %ty is 2 digit year while %tY is four digit year
//Formatting date in String format method - date in MM/dd/yy
str = String.format("Today is %tD", new Date());
System.out.println(str);
Date today = new Date();
System.out.printf("Date in dd/mm/yy format %td/%tm/%ty %n", today,today,today );
// date as July 25, 2012, difference between %td and %te is that
// %td use leading zero while %te doesn't
System.out.printf("Today is %tB %te, %tY %n", today,today,today,today);
//adding leading zero in numbers using String format,
//%d is for decimal, 8 specify formatted number should be 8 digit and 0 specify use
//leading zero, default is space, so if you don't specify leading
// character space will be used.
System.out.printf("Amount : %08d %n" , 221);
//printing positive and negative number using String format
//+ sign for positive, - for negative and %n is for new line
System.out.printf("positive number : +%d %n", 1534632142);
System.out.printf("negative number : -%d %n", 989899);
//printing floating point number with System.format()
System.out.printf("%f %n", Math.E);
//3 digit after decimal point
System.out.printf("%.3f %n", Math.E);
//8 charcter in width and 3 digit after decimal point
System.out.printf("%8.3f %n", Math.E);
//adding comma into long numbers
System.out.printf("Total %,d messages processed today", 10000000);
}
}
Output:
Order with OrdId : 40021 and Amount: 3000 is missing
Order with OrdId : 40021 and Amount: 3000 is missing
Hello Raj
Today is 07/25/12
Date in dd/mm/yy format 25/07/12
Today is July 25, 2012
Amount : 00000221
positive number : +1534632142
negative number : -989899n
2.718282
2.718
2.718
Total 10,000,000 messages processed today
Top 10 Tricky Java interview questions
Tricky Java interview questions are those question which has some surprise element on it and if you answer tricky Java question with common sense, you are likely to fail because they are tricky, they require something special to know. Most of the tricky Java questions comes from method overloading and overriding, Checked and Unchecked Exception and subtle Java programming details like Integer overflow. Most important thing to answer tricky Java question is attitude and analytical thinking , which helps even if you don't know the answer. Anyway in this Java article we will see some Java questions which is tricky and requires more than average knowledge of Java programming language to answer. As per my experience there are always one or two tricky or tough Java interview question on any core Java or J2EE interviews, so its good to prepare tricky questions from Java in advance.
Here is my list of some tricky Java interview questions, Though I have prepared and shared lot of difficult core java interview question and answers, But I have chosen them as Top 10 tricky questions because you can’t guess answers of this tricky java questions easily, you need some subtle details of Java programming language to answer these questions.
Q. What will happen if you call return statement or System.exit on try or catch block ? will finally block execute?
A. This is a very popular tricky Java question and its tricky because many programmer think that finally block always executed. This question challenge that concept by putting return statement in try or catch block or calling System.exit from try or catch block. Answer of this tricky question in Java is that finally block will execute even if you put return statement in try block or catch block but finally block won't run if you call System.exit form try or catch.
Q. Can you override private or static method in Java ?
A. Another popular Java tricky question, As I said method overriding is a good topic to ask trick questions in Java. Anyway, you can’t override private or static method in Java, if you create similar method with same return type and same method arguments that's called method hiding. See Can you override private method in Java or more details.
Q. Does Java support multiple inheritance ?
A. This is the trickiest question in Java, if C++ can support direct multiple inheritance than why not Java is the argument Interviewer often give. See Why multiple inheritance is not supported in Java to answer this tricky Java question.
Q. What will happen if we put a key object in a HashMap which is already there ?
A. This tricky Java questions is part of How HashMap works in Java, which is also a popular topic to create confusing and tricky question in Java. well if you put the same key again than it will replace the old mapping because HashMap doesn't allow duplicate keys. See How HashMap works in Java for more tricky Java questions from HashMap.
Q. If a method throws NullPointerException in super class, can we override it with a method which throws RuntimeException?
A. One more tricky Java questions from overloading and overriding concept. Answer is you can very well throw super class of RuntimeException in overridden method but you can not do same if its checked Exception. See Rules of method overriding in Javafor more details.
Q. What is the issue with following implementation of compareTo() method in Java
public int compareTo(Object o){
Employee emp = (Employee) emp;
return this.id - o.id;
}
Where id is an integer number ?
A: Well there is nothing wrong in this Java question until you guarantee that id is always positive. This Java question becomes tricky when you can not guaranteed id is positive or negative. If id is negative than subtraction may overflow and produce incorrect result. See How to override compareTo method in Java for complete answer of this Java tricky question for experienced programmer.
Q. How do you ensure that N thread can access N resources without deadlock?
A. If you are not well versed in writing multi-threading code then this is real tricky question for you. This Java question can be tricky even for experienced and senior programmer, who are not really exposed to deadlock and race conditions. Key point here is order, if you acquire resources in a particular order and release resources in reverse order you can prevent deadlock. See how to avoid deadlock in Java for a sample code example.
Q. What is difference between CyclicBarrier and CountDownLatch in Java?
A. Relatively newer Java tricky question, only been introduced form Java 5. Main difference between both of them is that you can reuseCyclicBarrier even if Barrier is broken but you can not reuse CountDownLatch in Java. See CyclicBarrier vs CountDownLatch in Java for more differences.
Q. What is difference between StringBuffer and StringBuilder in Java ?
A. Classic Java questions which some people thing tricky and some consider very easy. StringBuilder in Java is introduced in Java 5 and only difference between both of them is that Stringbuffer methods are synchronized while StringBuilder is non synchronized. See StringBuilder vs StringBuffer for more differences.
Q. Can you access non static variable in static context?
A. Another tricky Java question from Java fundamentals. No you can not access static variable in non static context in Java. Read why you can not access non-static variable from static method to learn more about this tricky Java questions.
This was my list of 10 most common tricky question in Java . It's not a bad idea to prepare tricky Java question before appearing for any core Java or J2EE interview. One or two open ended or tricky question is quite common in Java interviews.
13 Singleton Pattern Interview question in Java
Q. What is Singleton class? Have you used Singleton before?
A. Singleton is a class which has only one instance in whole application and provides a getInstance() method to access the singleton instance. There are many classes in JDK which is implemented using Singleton pattern like java.lang.Runtime which provides getRuntime() method to get access of it and used to get free memory and total memory in Java.
Q. Which classes are candidates of Singleton? Which kind of class do you make Singleton in Java?
A. Here they will check whether candidate has enough experience on usage of singleton or not. Does he is familiar of advantage/disadvantage or alternatives available for singleton in Java or not.
Answer: Any class which you want to be available to whole application and whole only one instance is viable is candidate of becoming Singleton. One example of this is Runtime class , since on whole java application only one runtime environment can be possible making Runtime Singleton is right decision. Another example is a utility classes like Popup in GUI application, if you want to show popup with message you can have one PopUp class on whole GUI application and anytime just get its instance, and call show() with message.
Q. Can you write code for getInstance() method of a Singleton class in Java?
A. Most of the java programmer fail here if they have mugged up the singleton code because you can ask lots of follow-up question based upon the code they have written. I have seen many programmer write Singleton getInstance() method with double checked locking but they are not really familiar with the caveat associated with double checking of singleton prior to Java 5.
Answer: Until asked don’t write code using double checked locking as it is more complex and chances of errors are more but if you have deep knowledge of double checked locking, volatile variable and lazy loading than this is your chance to shine. I have shared code examples of writing singleton classes using enum, using static factory and with double checked locking in my recent post Why Enum Singletons are better in Java, please see there.
Q. Is it better to make whole getInstance() method synchronized or just critical section is enough? Which one you will prefer?
A. This is really nice question and I mostly asked to just quickly check whether candidate is aware of performance trade off of unnecessary locking or not. Since locking only make sense when we need to create instance and rest of the time its just read only access so locking of critical section is always better option. read more about synchronization on How Synchronization works in Java
Answer: This is again related to double checked locking pattern, well synchronization is costly and when you apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton, It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.
Q. What is lazy and early loading of Singleton and how will you implement it?
A. This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.
As there are many ways to implement Singleton like using double checked locking or Singleton class with static final instance initialized during class loading. Former is called lazy loading because Singleton instance is created only when client calls getInstance() method while later is called early loading because Singleton instance is created when class is loaded into memory.
Q. Example of Singleton in standard Java Development Kit?
A. This is open question to all, please share which classes are Singleton in JDK. Answer to this question is java.lang.Runtime
There are many classes in Java Development Kit which is written using singleton pattern, here are few of them:
• Java.lang.Runtime with getRuntime() method
• Java.awt.Toolkit with getDefaultToolkit()
• Java.awt.Desktop with getDesktop()
Q. What is double checked locking in Singleton?
A. One of the most hyped question on Singleton pattern and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.
Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.
public Singleton getInstance(){
if(_INSTANCE == null){
synchronized(Singleton.class){
//double checked locking - because second check of Singleton instance with lock
if(_INSTANCE == null){
_INSTANCE = new Singleton();
}
}
}
return _INSTANCE;
}
Double checked locking should only be used when you have requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.
Q. How do you prevent for creating another instance of Singleton using clone() method?
A. This type of questions generally comes some time by asking how to break singleton or when Singleton is not Singleton in Java.
Preferred way is not to implement Clonnable interface as why should one wants to create clone() of Singleto and if you do just throw Exception from clone() method as “Can not create clone of Singleton class”.
Q. How do you prevent for creating another instance of Singleton using reflection?
A. Open to all. In my opinion throwing exception from constructor is an option.
This is similar to previous interview question. Since constructor of Singleton class is supposed to be private it prevents creating instance of Singleton from outside but Reflection can access private fields and methods, which opens a threat of another instance. This can be avoided by throwing Exception from constructor as “Singleton already initialized”
Q. How do you prevent for creating another instance of Singleton during serialization?
A. Another great question which requires knowledge of Serialization in Java and how to use it for persisting Singleton classes. This is open to you all but in my opinion use of readResolve() method can sort this out for you.
You can prevent this by using readResolve() method, since during serialization readObject() is used to create instance and it return new instance every time but by using readResolve you can replace it with original Singleton instance. I have shared code on how to do it in my post Enum as Singleton in Java. This is also one of the reason I have said that use Enum to create Singleton because serialization of enum is taken care by JVM and it provides guaranted of that.
Q. When is Singleton not a Singleton in Java?
A. There is a very good article present in Sun's Java site which discusses various scenarios when a Singleton is not really remains Singleton and multiple instance of Singleton is possible. Here is the link of that article http://java.sun.com/developer/technicalArticles/Programming/singletons/
Q. Why you should avoid the singleton anti-pattern at all and replace it with DI?
A. Singleton Dependency Injection: every class that needs access to a singleton gets the object through its constructors or with a DI-container.
Singleton Anti-Pattern: with more and more classes calling getInstance the code gets more and more tightly coupled, monolithic, not testable and hard to change and hard to reuse because of not configurable, hidden dependencies. Also, there would be no need for this clumsy double checked locking if you call getInstance less often (i.e. once).
Q. How many ways you can write Singleton Class in Java?
A. I know at-least four ways to implement Singleton pattern in Java
1) Singleton by synchronizing getInstance() method
2) Singleton with public static final field initialized during class loading.
3) Singleton generated by static nested class, also referred as Singleton holder pattern.
4) From Java 5 on-wards using Enums
Q. How to write thread-safe Singleton in Java?
A. Thread-Safe Singleton usually refers to write thread safe code which creates one and only one instance of Singleton if called by multiple thread at same time. There are many ways to achieve this like by using double checked locking technique as shown above and by using Enum or Singleton initialized by classloader.
At last few more questions for your practice, contributed by Mansi:
Singleton vs Static Class?
When to choose Singleton over Static Class?
Can you replace Singleton with Static Class in Java?
Difference between Singleton and Static Class in java?
Advantage of Singleton over Static Class?
Java Interview Question for Senior Developers
Here are my list of questions which is worth preparing if you are going for core java interview for senior position, these questions are mostly based upon Garbage collection , Concurrency, Collections, design, Coding and testing.
Q. Can we compare String using equality operator (==) operator?
A. We can compare String using equality operator. But we mostly use equals() method because of following reasons:
• The equality operator is used to compare primitives values only where as equals method() should be used to compare objects.
• The equality operator can invoke subtle issue while comparing primitive to Object. Where as equals() method is used to perform character based comparison.
• equals() return true if two String represent to the same object or when both strings contents are exactly same but equality() operator returns true if two String object represents to same object but return false if two different String object contains same contents
Q. What is intern() method in Java?
A. The intern() method is of String class. The intern() method is supposed to return the String from the String pool if the String is found in String pool, otherwise a new string object will be added in String pool and the reference of this String is returned.
Example:
String str1 = "hello";
String str2 = "hello";
String str3 = "hello".intern();
if ( s1 == s2 ){
System.out.println("str1 and str2 are same");
}
if ( str1 == str3 ){
System.out.println("str1 and str3 are same" );
}
We are assuming that str1 and str3 are same will be printed as str3 is interned, and str1 and str2 are same will not be printed. Actual output is: both lines are printed. Which make clear that by default String constants are interned?
Q. When is class garbage collected?
A. Java uses the garbage collector to free memory which is occupied by those objects which is no more reference by any of the program. An object becomes eligible for Garbage Collection when no live thread can access it. There are many ways to make a class reachable and thus prevent it from being eligible for Garbage Collection:
• Objects of that class are still reachable.
• Class object representing the class is still reachable.
• ClassLoader that loaded the class is still reachable.
• Other classes loaded by the ClassLoader are still reachable.
When all of the above are false, then the ClassLoader together with all classes it loaded are eligible for Garbage Collection.
Q. What is the difference between a Choice and a List?
A. The between Choice and list are following:
• Choice class presents a pop-up menu of choices whereas List is a collection of different visible item.
• Choice is displayed in a compact form, to see the list of available choices we need to scroll down whereas list displays the entire available items.
• Choice allows selecting only one items but List supports the selection of one or more List items.
• Lists typically allow duplicate elements selection. List allow pairs of elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they allow null elements at all.
Q. What is JCA in java?
A. Java Cryptography Architecture term from Sun for implementing security functions for the Java platform. It provides a platform and gives architecture and APIs for encryption and decryption. JCA is used by the developer to combine the application with the security measure. A programmer uses the JCA to meet the security measure. It helps in performing the third partly security rules. It uses the hash table, encryption message digest, etc to implement the security.
Q. What is JPA in java?
A. The Java Persistence API is enabling us to create the persistence layer for desktop and web applications. Java Persistence deals in following:
• Java Persistence API
• Query language
• Java Persistence Criteria API
• Object mapping metadata
Q. What is difference between eager and lazy loading?
A. The difference between eager and loading are:
• Eager loading means to load the data before the requirement whereas lazy loading mean that load the data only when required.
• Eager loading fetch the data in one query whereas lazy loading fetch the data when needed by triggering the sub query.
Q. What is JMS in Java?
A. Java Message Service (JMS) is used for creating the communication interface between two clients by using the message passing services. This helps the application to interact with other components irrespective of components location whether they rely on same system or connect to the main system through LAN or internet.
Q. What is shallow cloning and deep cloning?
A. Shallow copy: in this object is copied without its contained objects. Shallow clone only copies the top level structure of the object not the lower levels. It is an exact bit copy of all the attributes.
Deep copy: In this object is copied along with the objects it refers to. Deep clone copies all the levels of the object from top to the bottom recursively.
Q. What is the difference between applications and applets?
A. The differences between an Applet and an application are as follows:
• Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading.
• Application starts execution with its main method whereas applet starts execution with its init method.
• Application must be run on local machine whereas applet needs no explicit installation on local machine.
• Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser.
• Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
Q. What are Class loaders?
A. Class loader enables the program to load the class at run time. This is located in the java.lang package.
Using Class Loader we can also load the customize classes which is required for the application execution. The class loaders in Java are organized in a tree. When JVM is started three class loaders are used:
• Bootstrap class loader: the core java libraries. It is written in native code.
• Extensions class loader: loads the code in the extension directories. It is implemented by ExtClassLoader class.
• System class loader: code found on the java.class.path which map to the system class path variables. It is implemented by AppClassLoader class. All user classes by default are load by the system class loader.
Q. What is the Comparable interface?
A. The Comparable interface is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered. All classes implementing the Comparable interface must implement the compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:
int i = object1.compareTo(object2)
• If object1 < object2: The value of i returned will be negative.
• If object1 > object2: The value of i returned will be positive.
• If object1 = object2: The value of i returned will be zero.
Q. What is Single Threaded Model in Servlets?
A. Single Thread Model is a marker interface which contains no methods and variable in it.
• When using Single thread model with Servlet, it make sure that that only one thread can be executed at a time. It avoids running of two or more thead simultaneously.
• If we wants to make single threaded we can implement this interface in the following fashion.
public class SingleThreadedTest implements SingleThreadModel {
//some code
}
Q. What are the different Authentication Options available in Servets?
A. Authentication options available in Servlets: There are four different options for authentication in servlet:
1. Basic Authentication: Username and password provided by the client to authenticate the user.
2. Form-based authentication- In this the login form is made by the programmer by using HTML.
3. Digest Authentication- It is similar to basic authentication but in this the passwords are encrypted using Hash formula. This makes digest more secured.
4. Client certificate Authentication- It requires that each client accessing the resource has a certificate that it send to authenticate itself. This requires SSL protocol.
Q. What is the disadvantage of garbage collector?
A. Garbage Collector runs in its own thread which affects the performance of the system. It increases the workload of JVM because it constantly monitor the object which is not referenced.. The two main disadvantages of garbage collector are:
• TIME: to collect all those no referenced object JVM spends a considerable amount of time by scanning the entire heap.
• Mark and sweep: some time it is difficult to implement mark and sweep in the application.
Q. Does java support global variable?
A. No, java does not support global variable because of the following reasons:
• Globally accessible: global variables are globally accessible.
• Referential transparency:global variable breaks the referential transparency and also a global variable generate problem in the namespace.
• Object oriented: As JAVA is object oriented language so where each variable is declared inside the class. To use this variable, object should be initialized.
Q. Explain different layout manager in Java.
A. There are following types of layouts are used to organize or to arrange objects:
• Border Layout: Have five areas for holding components: north, east, west, south and center.
• Flow Layout: Default layout manager, lays out the components from left to right
• Card Layout: Different components at different times are laid out, Controlled by a combo box.
• Grid Layout: Group of components are laid out I equal size and displays them in certain rows and columns.
• Grid Bag Layout: Flexible layout for placing components within a grid of cells.
Q. What is chained exceptions in java?
A. When in a program the first exception causes another exception that is termed as Chained Exception. Chained exceptions helps in finding the root cause of the exception that occurs during application’s execution. The constructors that support chained exceptions in Throwable classes are:
• Throwable initCause(Throwable)
• Throwable(Throwable)
• Throwable(String, Throwable)
• Throwable getCause()
Q. When is the main thread stop in java?
A. When we execute the java program, it call the main() method because main() method is the first thread in the program. This main() method or thread invokes the other thread which is required for the complete execution of the program. The main thread should be the last thread in the program to end. so to make main() method to be last we make sure when we called to another function by giving execution control to that must return the control back to the main() method.
Q. What are the ways to create child threads?
A. There are two ways to create java threads:
• Implementing the Runnable interface: this overcomes the limitation of inheriting from only one parent class Thread. Using Runnable interface, lays a path to ground work of a class that utilizes threads
• Extending Thread class: It inherits the methods and data members, fields from the class tread. In this process only one class can be inherited from the parent class Thread.
The advantage is a class can extend Thread class and also implements the Runnable interface, if required. The Runnable interface is set for implementing a thread and the class that implements the interface performs all the work.
Q. Why byte code is important to Java?
A. The compiled Java source code is known as byte code. We need bytecode due to following reasons:
• Is independent of the input language.
• Plays an important role in the execution speed of the application.
• Can run on any platform irrespective of system architecture.
• Can be used for internet applications where security is important
• Enable us to load classes which are required for the execution of the application.
• allows the web applications to run on various platforms, on various browsers on different infrastructures.
Q. What is an Iterator and explain traversing through a collector using Iterator?
A. We can access each element in the Collection by using Iterators irrespective of how they are organized in the collector. Iterator can be implemented a different way for every Collection. To use an iterator to traverse through the contents of a collection we do:
• Obtain an iterator by calling the collections iterator() method to the start of the collection.
• Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext()returns true.
• Within the loop, obtain each element by calling next().
• remove() method is used to remove the current element in the iteration.
Q. What is Race condition?
A. Race Condition: it is the situation when two threads raise the request for the same resource allocation, but the manner in which resources are allocated are significant, is called race conditions.
• Race condition is created in program in order to run the parallel execution of program by using multiple threads in a same period.
• A race condition occurs when two threads operate on same object without proper synchronization and there operation interleaves on each other.
• The risk of Race condition is higher in Java.
Q. What is difference between Java and JavaScript?
A.The difference between java and java scripts are:
• Java is an Object Oriented Programming Language and capable of running on multiple operating systems with the help of interpreter whereas Java Script is the object oriented scripting language and it is embedded in HTML and runs directly on the browser.
• JVM is used to executed java program on different program whereas Java Script code is not compiled they are directly run on the browser.
• Java language is used to develop the software whereas java script is used providing interactivity to the simple HTML pages.
Q. What is the difference between factory and abstract factory pattern?
A. The differences between factory and abstract factory are following:
• Factory pattern is a single method but abstract factory is an object.
• The Abstract Factory pattern is one level of abstraction higher than the factory pattern.
• Factory pattern generally returns the common parent class or method whereas the abstract factory pattern returns the one of the several factories.
Q. What is Singleton?
A. Singleton in Java is a class with just one instance in whole Java application.
• This make sure that only one instance of a class is created.
• getInstance() method is used to get single instance of the class.
• It creates a global point to access all object
Q. What is the difference between JAR and WAR files?
A. The differences between JAR and WAR files are:
• JAR files (Java Archive) allows combining many files into one whereas WAR files (Web Application Archive) stores XML, java classes, and JavaServer pages for Web Application purposes.
• JAR is used to hold Java classes in a library whereas in WAR files are stored in lib directory of the application.
• In this EJB module which contains enterprise java beans class files and EJB deployment descriptor are packed as JAR files with .jar extension whereas in WAR web modules which contains Servlet class files, JSP Files, GIF and HTML files are packaged as JAR file with .war extension.
Explain the difference between StringBuilder and StringBuffer class.
Q. How are Observer and Observable used?
A. The observable class represents an observable object.
The object to be observed can be represented by sub-classing observable class.
When there is a change in an observable instance, an application calling the Observable's notifyObservers method causes all of its observers to be notified of the change by a call to their update method.
Q. Difference between Swing and Awt.
A. AWT are heavy-weight components. Swings are light-weight components. Thus, swing works faster than AWT.
Q. Define inner class in Java
A. Class that is nested within a class is called as inner class. The inner class can access private members of the outer class. It is mainly used to implement data structure and some time called as a helper class.
Q. Differences between constructors and methods.
A. A constructor is used to create objects of a class. A method is an ordinary member in a class.
Constructor does not have a return type. A method should have a return type.
Constructor name is the name of the class. A method name should not be the name of the class
Constructor is invoked at the time of creation of the class. Method needs to be invoked in another method by using the dot operator.
Constructor can not have ‘return’ statement. All methods that return non-void return type should have ‘return’ statement.
Q. Define Method overriding. Explain its uses.
A. Method overriding is the process of writing functionality for methods with same signature and return type, both in super class and subclass The uses of method overriding:
Time to invest method signature is reduced
Different functionality in both super class and sub class by sharing same signature
The functionality can be enhanced
The behavior can be replaced in the sub class
More advanced java interview questions - senior level java interview
Q. What is the purpose of the File class?
A. File class provides access to the files and directories of a local file system.
Q. Can you explain Native methods in Java?
A. • The Java native method is used to merge the power of C or C++ programming into Java.
• To enhance to high performance language, when efficient native Java compilers are not fully implemented, use of native method boosts the performance to at least the speed of C compiled code.
Java applications can call code written in C, C++, or assembler. This is sometimes done for performance and sometimes to access the underlying host operating system or GUI API using the JNI.
Q. Explain class loaders in Java with an example.
A. • The class loader describes the behavior of converting a named class into the bits responsible for implementing that class.
• Class loaders eradicate the JREs need to know anything about files and file systems when running Java programs.
• A class loader creates a flat name space of class bodies that are referenced by a string name and are written as:
Class r = loadClass(String className, boolean resolveIt);
Q. What is Reflection API in Java?
A. • The Reflection API allows Java code to examine classes and objects at run time.
• The new reflection classes allow you to call another class's methods dynamically at run time.
• With the reflection classes, you can also examine an instance's fields and change the fields' contents.
• It is also possible to instantiate new objects, invoke methods and get/set field values using reflection.
• The Reflection API consists of the java.lang.Class class and the java.lang.reflect classes: Field, Method, Constructor, Array, and Modifier.
Q. Static class loading vs. dynamic class loading.
A. • The static class loading is done through the new operator.
• Dynamic class loading is achieved through Run time type identification. Also called as reflection
This is done with the help of the following methods:
getClass(); getName(); getDeclaredFields();
Instance can also be created using forName() method. It loads the class into the current class memory.
Q. Can you explain shallow cloning and deep cloning?
A. Cloning of objects can be very useful if you use the prototype pattern or if you want to store an internal copy of an object inside an aggregation class for example.
• Deep cloning - You clone the object and their constituent parts.
• It should be used when it is inappropriate to separate the parts; the object is formed of, from it.
• Shallow cloning - You clone only the object, not their parts. You add references to their parts.
• It should be used when it is adequate to have the references added to the cloned object
Q. Explain the purpose of Comparator Interface.
A. • Comparators can be used to control the order of certain data structures and collection of objects too.
• The interface can be found in java.util.Comparator
• A Comparator must define a compare function which takes two Objects and returns a -1, 0, or 1
• Sorting can be done implicitly by using data structures of by implementing sort methods explicitly.
Q. What is the impact of private constructor?
A. • Private Constructors can't be access from any derived classes neither from another class.
• So you have to provide a public function that calls the private constructor if the object has not been initialized, or you have to return an instance to the object, if it was initialized.
This can be useful for objects that can't be instantiated.
Q. Can you explain static Initializers in Java?
A. A static initializer block resembles a method with no name, no arguments, and no return type. There is no need to refer to it from outside the class definition.
Syntax:
Static {
//CODE
}
The code in a static initializer block is executed by the virtual machine when the class is loaded.
Because it is executed automatically when the class is loaded, parameters don't make any sense, so a static initializer block doesn't have an argument list.
Q. Define Externalizable Interface and explain its purpose.
A. • The Externizable interface extends the serializable interface.
• When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject()two methods to control more complex object serailization process.
• When you use Externalizable interface, you have a complete control over your class's serialization process. The two methods to be implemented are : void readExternal(ObjectInput)
• The object implements the readExternal method to restore its contents by calling the methods of DataInput for primitive types and readObject for objects, strings and arrays.
void writeExternal(ObjectOutput)
• The object implements the writeExternal method to save its contents by calling the methods of DataOutput for its primitive values or calling the writeObject method of ObjectOutput for objects, strings, and arrays.
Q: How are Observer and Observable used?
A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
Q: What is the difference between the >> and >>> operators?
A: The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
Q: Which method of the Component class is used to set the position and size of a component?
A: setBounds() method is used to set the position and size of a component.
Q: Which java.util classes and interfaces support event handling?
A: The EventObject class and the EventListener interface support event processing.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
Q: What restrictions are placed on the location of a package statement within a source code file?
A: package statement must appear as the first line in a source code file (excluding blank lines and comments).
Q: Can an object's finalize() method be invoked while it is reachable?
A: An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.
Q: What is the immediate superclass of the Applet class?
A: Panel.
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks.
The scheduler then determines which task should execute next, based on priority and other factors.
Nice post...covers all the important topics in a simple manner!
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Thanks for your feedback @Neeraj.
DeleteCan you post on Design patterns ? Btw very nice posts
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