Java Interview Questions and Answers

S.K. Pandey

These 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.

What is Java?

Answer: Java, formerly known as oak, is an object-oriented programming language developed by Sun. It shares many superficial similarities with C, C++, and Objective C (for instance for loops have the same syntax in all four languages); but it is not based on any of those languages, nor have efforts been made to make it compatible with them.

Java is sometimes referred to as C++ ++ --. The language was originally created because C++ proved inadequate for certain tasks. Since the designers were not burdened with compatibility with existing languages, they were able to learn from the experience and mistakes of previous object-oriented languages. They added a few things C++ doesn't have like garbage collection and multithreading; and they threw away C++ features that had proven to be better in theory than in practice like multiple inheritance and operator overloading.

Even more importantly Java was designed from the ground up to allow for secure execution of code across a network, even when the source of that code was untrusted and possibly malicious. This required the elimination of more features of C and C++. Most notably there are no pointers in Java. Java programs cannot (at least in theory) access arbitrary addresses in memory.
Furthermore Java was designed not only to be cross-platform in source form like C, but also in compiled binary form. Since this is frankly impossible across processor architectures, Java is compiled to an intermediate byte-code which is interpreted on the fly by the Java interpreter. Thus to port Java programs to a new platform all that is needed is a port of the interpreter and a few native code libraries.

Finally Java was designed to make it a lot easier to write bug free code. Shipping C code has, on average, one bug per 55 lines of code. About half of these bugs are related to memory allocation and deallocation. Thus Java has a number of features to make bugs less common:

  • Strong Typing
  • No unsafe constructs
  • The language is small so its easy to become fluent.
  • The language is easy to read and write. Obfuscated Java isn't nearly as common as obfuscated C.
  • There are no undefined or architecture dependent constructs.
  • Java is object oriented so reuse is easy.
  • Java has concurrency.

Where did Java come from?

Answer: In the late 1970's Bill Joy thought about doing a language that would merge the best features of MESA and C. However other projects (like cofounding Sun) intervened. In the late 1980's he got Sun's engineers started on a complete revision of the UNIX operating system that involved merging SunOS4.x with AT&T's SYSVR4.

In 1989 Joy sold his Sun stock, invested heavily in Microsoft and moved out of mainstream Sun to Aspen, Colorado. By the early 90's Bill was getting tired of huge programs. He decided that he wanted to be able to write a 10,000 line program that made a difference. In late 1990 Bill wrote a paper called Further which outlined his pitch to Sun engineers that they should produce an object environment based on C++. Today Joy freely admits that C++ was too complicated and wasn't up to the job.
Around this time James Gosling (of emacs fame) had been working for several months on an SGML editor called "Imagination" using C++. The Oak language (now Java) grew out of Gosling's frustration with C++ on his "Imagination" project.

Patrick Naughton, then of Sun, now vice-president of technology at StarWave, started the Green Project on December 5th, 1990. Naughton defined the project as an effort to "do fewer things better". That December he recruited Gosling and Mike Sheridan to help start the project. Joy showed them his Further paper, and work began on graphics and user interface issues for several months in C.

In April of 1991 the Green Project (Naughton, Gosling and Sheridan) settled on smart consumer electronics as the delivery platform, and Gosling started working in earnest on Oak. Gosling wrote the original compiler in C; and Naughton, Gosling and Sheridan wrote the runtime-interpreter, also in C. Oak was running its first programs in August of 1991. Joy got his first demos of the system that winter, when Gosling and Naughton went skiing at Joy's place in Aspen.

By the fall of 1992 "*7", a cross between a PDA and a remote control, was ready This was demoed to Scott McNealy, Sun's president, in October. He was blown away. Following that the Green Project was set up as First Person Inc., a wholly owned Sun subsidiary.

In early 1993 the Green team heard about a Time-Warner request for proposal for a settop box operating system. First Person quickly shifted focus from smart consumer electronics (which was proving to be more hype than reality) to the set-top box OS market, and placed a bid with Time-Warner.

Fortuitously, Sun lost the bid. The Time-Warner project went nowhere, the same place it probably would have gone if Sun had won the bid. First Person continued work on settop boxes until early 1994, when it concluded that like smart consumer electronics settop boxes were more hype than reality.

Without a market to be seen First Person was rolled back into Sun in 1994. However around this time it was realized that the requirements for smart consumer electronics and settop box software (small, platform independent secure reliable code) were the same requirements for the nascent web.

For a third time the project was redirected, this time at the web. A prototype browser called WebRunner was written by Patrick Naughton in one weekend of inspired hacking. After additional work by Naughton and Jonathan Payne this browser became HotJava. The rest, as they say, is history.

Information in this section is primarily based on the first hand accounts of Bill Joy and Patrick Naughton .

Can a private method of a superclass be declared within a subclass?

Answer: Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.

Why Java does not support multiple inheritance ?

Answer: Java DOES support multiple inheritance via interface implementation.

Why Java does not support pointers ?   

Answer: Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C# shine.

What is the difference between final, finally and finalize?

Answer: o final - declare constant
               o finally - handles exception
               o finalize - helps in garbage collection

Where and how can you use a private constructor ?

Answer: Private constructor can be used if you do not want any other class to instanstiate the object , the instantiation is done from a static public method, this method is used when dealing with the factory method pattern when the designer wants only one controller (factory method ) to create the object.

In System.out.println(),what is System,out and println,pls explain?

Answer: System is a predefined final class,out is a PrintStream object and println is a built-in overloaded method in the out object.

What is meant by "Abstract Interface"?

Answer: First, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.

Can you make an instance of an abstract class? For example - java.util.Calender is an abstract class with a method getInstance() which returns an instance of the Calender class. 
Answer: No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method.

What is the output of x<y? a:b = p*q when x=1,y=2,p=3,q=4?

Answer: When this kind of question has been asked, find the problems you think is necessary to ask before you give an answer. Ask if variables a and b have been declared or initialized. If the answer is yes. You can say that the syntax is wrong. If the statement is rewritten as: x<y? a:(b=p*q); the return value would be variable a because the x is 1 and less than y = 2; the x < y statement return true and variable a is returned.

What is the difference between Swing and AWT components?

Answer: 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.

Parsers? DOM vs SAX parser

Answer: parsers are fundamental xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema.

What is a platform?

Answer: A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000/XP, Linux, Solaris, and MacOS.

What is the main difference between Java platform and other platforms?

Answer: The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms. The Java platform has two components:
1. The Java Virtual Machine (Java VM)
2. The Java Application Programming Interface (Java API)

What is the Java Virtual Machine?

Answer: The Java Virtual Machine is a software that can be ported onto various hardware-based platforms.

What is the Java API?

Answer: The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.

What is the package?

Answer: The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.

What is native code?

Answer: The native code is code that after you compile it, the compiled code runs on a specific hardware platform.

Is Java code slower than native code?  

Answer: Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.

What is the serialization?

Answer: The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.

How to make a class or a bean serializable?

Answer: By implementing either the interface, or the interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.

How many methods in the Serializable interface?

Answer: There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable.

How many methods in the Externalizable interface?

Answer: There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal().

What is the difference between Serializalble and Externalizable interface? 

Answer: 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.

What is a transient variable?

Answer: A transient variable is a variable that may not be serialized. If you don't want some field to be serialized, you can mark that field transient or static.

Why do threads block on I/O?

Answer: Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.

Which containers use a border layout as their default layout?

Answer: The Window, Frame and Dialog classes use a border layout as their default layout.

How are Observer and Observable used?

Answer: 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.

What is synchronization and why is it important?

Answer: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often causes dirty data and leads to significant errors.

What are synchronized methods and synchronized statements?

Answer: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

How are Observer and Observable used?

Answer: 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.

What are three ways in which a thread can enter the waiting state?

Answer: A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, 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.

Can a lock be acquired on a class? 

Answer: Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

What's new with the stop(), suspend() and resume() methods in JDK 1.2?

Answer: The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

What is the preferred size of a component?

Answer: The preferred size of a component is the minimum component size that will allow the component to display normally.

What method is used to specify a container's layout?

Answer: The setLayout() method is used to specify a container's layout.

Which containers use a FlowLayout as their default layout?

Answer: The Panel and Applet classes use the FlowLayout as their default layout.

What is thread?

Answer: A thread is an independent path of execution in a system.

What is multithreading?

Answer: Multithreading means various threads that run in a system.

How does multithreading take place on a computer with a single CPU?

Answer: 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.

How to create multithread in a program?

Answer: You have two ways to do so. First, making your class "extends" Thread class. Second, making your class "implements" Runnable interface. Put jobs in a run() method and call start() method to start the thread.

Can Java object be locked down for exclusive use by a given thread?

Answer: Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it

Can each Java object keep track of all the threads that want to exclusively access to it? 

Answer: Yes

What state does a thread enter when it terminates its processing?

Answer: When a thread terminates its processing, it enters the dead state.

What invokes a thread's run() method?

Answer: 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.

What is the purpose of the wait(), notify(), and notifyAll() methods?

Answer: The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.

What are the high-level thread states?

Answer: The high-level thread states are ready, running, waiting, and dead.

What is the Collections API?
Answer: The Collections API is a set of classes and interfaces that support operations on collections of objects.

What is the List interface? 

Answer: The List interface provides support for ordered collections of objects.

How does Java handle integer overflows and underflows?

Answer: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

What is the Vector class?

Answer: The Vector class provides the capability to implement a growable array of objects .

What modifiers may be used with an inner class that is a member of an outer class?

Answer: A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

If a method is declared as protected, where may the method be accessed?

Answer: 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.

What is an Iterator interface?

Answer:  The Iterator interface is used to step through the elements of a Collection.

How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?

Answer: 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.

What is the difference between yielding and sleeping?

Answer: 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.

Is sizeof a keyword? 

Answer:  The sizeof operator is not a keyword.

What are wrapped classes?

Answer: Wrapped classes are classes that allow primitive types to be accessed as objects. Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:






















 Does garbage collection guarantee that a program will not run out of memory?

Answer: No, it doesn't. 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

What is the difference between preemptive scheduling and time slicing?

Answer: 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.

Name Component subclasses that support painting.

Answer: The Canvas, Frame, Panel, and Applet classes support painting.

What is a native method?

Answer: A native method is a method that is implemented in a language other than Java.

How can you write a loop indefinitely?

Answer: for(;;)--for loop; while(true)--always true, etc.

Can an anonymous class be declared as implementing an interface and extending a class?

Answer: An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

What is the purpose of finalization?

Answer: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Which class is the superclass for every class ?

Answer: Object.

What is the difference between the Boolean & operator and the && operator? 

Answer: If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. 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. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above.

What is the GregorianCalendar class?

Answer: The GregorianCalendar provides support for traditional Western calendars.

What is the SimpleTimeZone class?

Answer: The SimpleTimeZone class provides support for a Gregorian calendar.

Which Container method is used to cause a container to be laid out and redisplayed?

Answer: validate()

What is the Properties class?

Answer: The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

What is the purpose of the Runtime class?

Answer: The purpose of the Runtime class is to provide access to the Java runtime system.

What is the purpose of the System class? 

Answer: The purpose of the System class is to provide access to system resources.

What is the purpose of the finally clause of a try-catch-finally statement?

Answer: The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

What is the Locale class?

Answer: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

What must a class do to implement an interface?

Answer: It must provide all of the methods in the interface and identify the interface in its implements clause.

What is an abstract method?

Answer: An abstract method is a method whose implementation is deferred to a subclass. Or, a method that has no implementation (an interface of a method).

What is a static method? 

Answer: 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.

What is a protected method?

Answer: A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its class.

What is the difference between a static and a non-static inner class?

Answer: A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

What is an object's lock and which object's have locks?

Answer:  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. All objects and classes have locks. A class's lock is acquired on the class's Class object.

When can an object reference be cast to an interface reference?

Answer: An object reference be cast to an interface reference when the object implements the referenced interface.


What is the difference between a Window and a Frame?

Answer: The Frame class extends Window to define a main application window that can have a menu bar.

What do heavy weight components mean?

Answer: Heavy weight components like Abstract Window Toolkit (AWT), 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. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight.

Which package has light weight components?

Answer:  javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.

What are peerless components?

Answer: The peerless components are called light weight components.

What is the difference between the Font and FontMetrics classes?

Answer: The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

What happens when a thread cannot acquire a lock on an object?

Answer: If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

Answer: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

What classes of exceptions may be caught by a catch clause?

Answer: A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

What is the difference between throw and throws keywords? 

Answer: The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that designates that exceptions may come out of the method, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw.

If a class is declared without any access modifiers, where may the class be accessed?

Answer: A class that is declared without any access modifiers is said to have package or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

What is the Map interface?

Answer: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Does a class inherit the constructors of its superclass?

Answer: A class does not inherit constructors from any of its superclasses .

Name primitive Java types.

Answer: The primitive types are byte, char, short, int, long, float, double, and boolean.

Which class should you use to obtain design information about an object?

Answer: The Class class is used to obtain information about an object's design.

How can a GUI component handle its own events?

Answer: A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

How are the elements of a GridBagLayout organized?

Answer: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

What advantage do Java's layout managers provide over traditional windowing systems? 
Answer: Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

What are the problems faced by Java programmers who don't use layout managers?
Answer: Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

What is the difference between static and non-static variables?
Answer: 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.

What is the difference between the paint() and repaint() methods?
Answer:  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.

What is the purpose of the File class?
Answer: The File class is used to create objects that provide access to the files and directories of a local file system.

What restrictions are placed on method overloading?
Answer: Two methods may not have the same name and argument list but different return types.

What restrictions are placed on method overriding?
Answer: 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. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

What is casting?
Answer: There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Name Container classes.
Answer: Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane .

What class allows you to read objects directly from a stream?
Answer: The ObjectInputStream class supports the reading of objects from input streams.

How are this() and super() used with constructors?
Answer: this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

How is it possible for two String objects with identical values not to be equal under the == operator?
Answer: The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

What an I/O filter?
Answer: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

What is the Set interface?
Answer: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

What is the List interface?
Answer: The List interface provides support for ordered collections of objects.

What is the purpose of the enableEvents() method? 
Answer: The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

What is the difference between the File and RandomAccessFile classes? 
Answer: The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

What interface must an object implement before it can be written to a stream as an object?
Answer: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

 What is the ResourceBundle class?
Answer: 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.

What is the difference between a Scrollbar and a ScrollPane?
Answer: A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

What is a Java package and how is it used?
Answer:  A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

What are the Object and Class classes used for?
Answer: The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

What is Serialization and deserialization? 
Answer: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

What is tunnelling?

Answer: Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data.

Does the code in finally block get executed if there is an exception and a return statement in a catch block?

Answer:  If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.

How you restrict a user to cut and paste from the html page?

Answer: Using javaScript to lock keyboard keys. It is one of solutions.

Is Java a super set of JavaScript?

Answer: No. They are completely different. Some syntax may be similar.

What is a Container in a GUI?

Answer: A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.

How the object oriented approach helps us keep complexity of software development under control?

We can discuss such issue from the following aspects:

  • Objects allow procedures to be encapsulated with their data to reduce potential interference.
  • Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places.
  • The well-defined separations of interface and implementation allows constraints to be imposed on inheriting classes while still allowing the flexibility of overriding and overloading.

What is polymorphism?

Answer: Polymorphism allows methods to be written that needn't be concerned about the specifics of the objects they will be applied to. That is, the method can be specified at a higher level of abstraction and can be counted on to work even on objects of yet unconceived classes.

What is design by contract?

Answer: The design by contract specifies the obligations of a method to any other methods that may use its services and also theirs to it. For example, the preconditions specify what the method required to be true when the method is called. Hence making sure that preconditions are.Similarly, postconditions specify what must be true when the  method is finished, thus the called method has the responsibility of satisfying the post conditions.

In Java, the exception handling facilities support the use of design by contract, especially in the case of checked exceptions. The assert keyword can be used to make such contracts.


What are use cases?

Answer: A use case describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance. It is part of the analysis of a program. The collection of use cases should, ideally, anticipate all the standard circumstances and many of the extraordinary circumstances possible so that the program will be robust.

What is the difference between interface and abstract class?


  • interface contains methods that must be abstract; abstract class may contain concrete methods.
  • interface contains variables that must be static and final; abstract class may contain non-final and final variables.
  • members in an interface are public by default, abstract class may contain non-public members.
  • interface is used to "implements"; whereas abstract class is used to "extends".
  • interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance.
  • interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces.
  • interface is absolutely abstract; abstract class can be invoked if a main() exists.
  • interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces.
  • If given a choice, use interface instead of abstract class.

What is transient variable?

Answer: 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.
Name the containers which uses Border Layout as their default layout?
Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.

What do you understand by Synchronization?

Answer: 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.

What is Collection API?

Answer: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. 
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.  

 Is Iterator a Class or Interface? What is its use?

Answer: Iterator is an interface which is used to step through the elements of a Collection. 

What is similarities/difference between an Abstract class and Interface?

Differences are as follows:

  • Interfaces provide a form of multiple inheritance. A class can extend only one other class.
  • Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
  • A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
  • Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast. 


  • Neither Abstract classes or Interface can be instantiated.

How to define an Abstract class?

Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated. 
Example of Abstract class:
abstract class testAbstractClass { 
    protected String myString; 
    public String getMyString() { 
        return myString; 
    public abstract string anyAbstractFunction();

How to define an Interface?

Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
    public void functionOne();

    public long CONSTANT_ONE = 1000;

Explain the user defined Exceptions?

Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
class myCustomException extends Exception {
     // The class simply has to exist to be an exception

Explain the new Features of JDBC 2.0 Core API?

Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:

  • Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
  • JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
  • Java applications can now use the ResultSet.updateXXX methods.
  • New data types - interfaces mapping the SQL3 data types
  • Custom  mapping of user-defined types (UTDs)
  • Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values. 

Explain garbage collection?

Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(),  JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. 
How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.  
What is OOPS?
Answer: OOP is the common abbreviation for Object-Oriented Programming.  

Describe the principles of OOPS.
Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.   

Explain the Encapsulation principle.
Answer: 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. 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.  

Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties of another object.   

Explain the Polymorphism principle.
Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".   

Explain the different forms of Polymorphism.
Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

  • Method overloading
  • Method overriding through inheritance
  • Method overriding through the Java interface

What are Access Specifiers available in Java?

Answer: Access specifiers are keywords that determines the type of access to the       member of a class. These are: Public ,Protected , Private , Defaults

Describe the wrapper classes in Java.
Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

What do you understand by JSP Actions?

Answer: JSP actions are XML tags that direct the server to use existing components or control the behavior of the JSP engine. JSP Actions consist of a typical (XML-based) prefix of "jsp" followed by a colon, followed by the action name followed by one or more attribute parameters.
There are six JSP Actions:

What is the difference between <jsp:include page = ... > and 

<%@ include file = ... >?.
Answer: Both the tag includes the information from one page in another. The differences are as follows:
<jsp:include page = ... >: This is like a function call from one jsp to another jsp. It is executed ( the included page is executed  and the generated html content is included in the content of calling jsp) each time the client page is accessed by the client. This approach is useful to for modularizing the web application. If the included file changed then the new content will be included in the output. 

<%@ include file = ... >: In this case the content of the included file is textually embedded in the page that have <%@ include file=".."> directive. In this case in the included file changes, the changed content will not included in the output. This approach is used when the code from one jsp file required to include in multiple jsp files.

What is the difference between <jsp:forward page = ... > and

Answer: The <jsp:forward> element forwards the request object containing the client request information from one JSP file to another file. The target file can be an HTML file, another JSP file, or a servlet, as long as it is in the same application context as the forwarding JSP file. 
sendRedirect sends HTTP temporary redirect response to the browser, and browser creates a new request to go the redirected page. The  response.sendRedirect kills the session variables.

What are implicit Objects available to the JSP Page?

Answer: Implicit objects are the objects available to the JSP page. These objects are created by Web container and contain information related to a particular request, page, or application.

What are all the different scope values for the <jsp:useBean> tag?

Answer:<jsp:useBean> tag is used to use any java object in the jsp page. Here are the scope values for <jsp:useBean> tag:
a) page
b) request
c) session and
d) application

What is JSP Output Comments?

Answer: JSP Output Comments are the comments that can be viewed in the HTML source file.
<!-- This file displays the user login screen -->
<!-- This page was loaded on
<%= (new java.util.Date()).toLocaleString() %> -->

What is expression in JSP?

Answer: Expression tag is used to insert Java values directly into the output. Syntax for the Expression tag is: 
<%= expression %>
An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. The following expression tag displays time on the output:
<%=new java.util.Date()%>

What types of comments are available in the JSP?

Answer: There are two types of comments are allowed in the JSP. These are hidden and output comments. A hidden comments does not appear in the generated output in the html, while output comments appear in the generated output.
Example of hidden comment:
<%-- This is hidden comment --%>
Example of output comment:
<!-- This is output comment -->

What is JSP declaration?

Answer: JSP Decleratives are the JSP tag used to declare variables. Declaratives are enclosed in the <%! %> tag and ends in semi-colon. You declare variables and functions in the declaration tag and can use anywhere in the JSP. Here is the example of declaratives:
<%@page contentType="text/html" %>
int cnt=0;
private int getCount(){
//increment cnt and return the value
return cnt;
<p>Values of Cnt are:</p>

What is JSP Scriptlet?

Answer: JSP Scriptlet is jsp tag which is used to enclose java code in the JSP pages. Scriptlets begins with <% tag and ends with %> tag. Java code written inside scriptlet executes every time the JSP is invoked.
  //java codes
   String userName=null;

What are the life-cycle methods of JSP?

Answer: Life-cycle methods of the JSP are:
a) jspInit(): The container calls the jspInit() to initialize the servlet instance. It is called before any other method, and is called only once for a servlet instance.
b)_jspService(): The container calls the _jspservice() for each request and it passes the request and the response objects. _jspService() method cann't be overridden. 
c) jspDestroy(): The container calls this when its instance is about to destroyed.
The jspInit() and jspDestroy() methods can be overridden within a JSP page.

What is J2EE?

Answer: J2EE Stands for Java 2 Enterprise Edition. J2EE is an environment for developing and deploying enterprise applications. J2EE specification is defined by Sun Microsystems Inc. The J2EE platform is one of the best platform for the development and deployment of enterprise applications. The J2EE platform is consists of a set of services, application programming interfaces (APIs), and protocols, which provides the functionality necessary for developing multi-tiered, web-based applications. You can download the J2EE SDK and development tools from

What do you understand by a J2EE module?

Answer: A J2EE module is a software unit that consists of one or more J2EE components of the same container type along with one deployment descriptor of that type. J2EE specification defines four types of modules:
a) EJB
b) Web
c) application client and
d) resource adapter
In the J2EE applications modules can be deployed as stand-alone units. Modules can also be assembled into J2EE applications. 

Tell me something about J2EE component?

Answer: J2EE component is a self-contained functional software unit supported by a container and  configurable at deployment time. The J2EE specification defines the following J2EE components:

 Application clients and applets are components that run on the client.
Java servlet and JavaServer Pages (JSP) technology components areWeb Components that run on the server.
Enterprise JavaBeans (EJB) components) are business components that run on the server.
J2EE components are written in the Java programming language and are compiled in the same way as any program in the language. The difference between J2EE components and "standard" Java classes is that J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE server or client container.

What are the contents of web module?

Answer: A web module may contain:
a) JSP files
b) Java classes
c) gif and html files and 
d) web component deployment descriptors

Differentiate between .ear,  .jar and .war files.

Answer: These files are simply zipped file using java jar tool. These files are created for different purposes. Here is the description of these files:

.jar files: These files are with the .jar extenstion. The .jar files contains the libraries, resources and accessories files like property files.

.war files: These files are with the .war extension. The war file contains the web application that can be deployed on the any servlet/jsp container. The .war file contains jsp, html, javascript and other files for necessary for the development of web applications.

.ear files: The .ear file contains the EJB modules of the application.

What is the difference between Session Bean and Entity Bean?

Session Bean: Session is one of the EJBs and it represents a single client inside the Application Server. Stateless session is easy to develop and its efficient. As compare to entity beans session beans require few server resources.

A session bean is similar to an interactive session and is not shared; it can have only one client, in the same way that an interactive session can have only one user. A session bean is not persistent and it is destroyed once the session terminates. 
Entity Bean: An entity bean represents persistent global data from the database. Entity beans data are stored into database.

Why J2EE is suitable for the development distributed multi-tiered enterprise applications?

Answer: The J2EE platform consists of multi-tiered distributed application model. J2EE applications allows the developers to design and implement the business logic into components according to business requirement. J2EE architecture allows the development of multi-tired applications and the developed applications can be installed on different machines depending on the tier in the multi-tiered J2EE environment . The J2EE application parts are:

a) Client-tier components run on the client machine.
b) Web-tier components run on the J2EE server.
c) Business-tier components run on the J2EE server and the
d) Enterprise information system (EIS)-tier software runs on the EIS servers

Why do understand by a container?

Answer: Normally, thin-client multi-tiered applications are hard to write because they involve many lines of intricate code to handle transaction and state management, multithreading, resource pooling, and other complex low-level details. The component-based and platform-independent J2EE architecture makes J2EE applications easy to write because business logic is organized into reusable components. In addition, the J2EE server provides underlying services in the form of a container for every component type.

In short containers are the interface between a component and the low-level platform specific functionality that supports the component. The application like Web, enterprise bean, or application client component must be assembled and deployed on the J2EE container before executing.

What are the services provided by a container?

Answer: The services provided by container are as follows:
a) Transaction management for the bean
b) Security for the bean
c) Persistence of the bean
d) Remote access to the bean
e) Lifecycle management of the bean
f) Database-connection pooling
g) Instance pooling for the bean 

What are types of J2EE clients?

Answer: J2EE clients are the software that access the services components installed on the J2EE container. Following are the J2EE clients:
a) Applets
b) Java-Web Start clients
c) Wireless clients
d) Web applications

What is Deployment Descriptor?

Answer: A deployment descriptor is simply an XML(Extensible Markup Language) file with the extension of .xml. Deployment descriptor describes the component deployment settings. Application servers reads the deployment descriptor to deploy the components contained in the deployment unit. For example ejb-jar.xml file is used to describe the setting of the EJBs.

What do you understand by JTA and JTS?

Answer: JTA stands for Java Transaction API and JTS stands for Java Transaction Service. JTA provides a standard interface which allows the developers to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK uses the JTA transaction manager to implement the  transaction. The code developed by developers does not calls the JTS methods directly, but only invokes the JTA methods. Then JTA internally invokes the JTS routines. JTA is a high level transaction interface used by the application code to control the transaction.

What is JAXP?

Answer: The Java API for XML Processing (JAXP) enables applications to parse and transform XML documents independent of a particular XML processing implementation. JAXP or Java API for XML Parsing is an optional API provided by Javasoft. It provides basic functionality for reading, manipulating, and generating XML documents through pure Java APIs. It is a thin and lightweight API that provides a standard way to seamlessly integrate any XML-compliant parser with a Java application.

What is J2EE Connector architecture?

Answer: J2EE Connector Architecture (JCA) is a Java-based technology solution for connecting application servers and enterprise information systems (EIS) as part of enterprise application integration (EAI) solutions. While JDBC is specifically used to connect Java EE applications to databases, JCA is a more generic architecture for connection to legacy systems (including databases). JCA was developed under the Java Community Process as JSR 16 (JCA 1.0) and JSR 112 (JCA 1.5). As of 2006, the current version of JCA is version 1.5. The J2EE Connector API is used by J2EE tools developers and system integrators to create resource adapters.   

What is difference between Java Bean and Enterprise Java Bean?

Answer: Java Bean as is a plain java class with member variables and getter setter methods. Java Beans are defined under JavaBeans specification as Java-Based software component model which includes the features like introspection, customization,  events,  properties and  persistence. 

Enterprise JavaBeans or EJBs for short are Java-based software components that comply with Java's  EJB specification. EJBs are delpoyed on the EJB container and executes in the EJB container. EJB is not that simple,  it is used for building distributed applications. Examples of EJB are Session Bean, Entity Bean and Message Driven Bean. EJB is used for server side programming whereas java bean is a client side. Bean is only development but the EJB is developed and then deploy on EJB Container.   

What is the difference between JTS and JTA?

Answer: In any J2EE application transaction management is one of the most crucial requirements of the application. Given the complexity of today's business requirements, transaction processing occupies one of the most complex segments of enterprise level distributed applications to build, deploy and maintain.  JTS specifies the implementation of a Java transaction manager. JTS specifies the implementation of a Transaction Manager which supports the Java Transaction API (JTA) 1.0 This transaction manager supports the JTA, using which application servers can be built to support transactional Java applications. Internally the JTS implements the Java mapping of the OMG OTS 1.1 specifications. The Java mapping is specified in two packages: org.omg.CosTransactions and org.omg.CosTSPortability. The JTS thus provides a new architecture for transactional application servers and applications, while complying to the OMG OTS 1.1 interfaces internally. This allows the JTA compliant applications to interoperate with other OTS 1.1 complaint applications through the standard IIOP. Java-based applications and Java-based application servers access transaction management functionality via the JTA interfaces. The JTA interacts with a transaction management implementation via JTS. Similarly, the JTS can access resources via the JTA XA interfaces or can access OTS-enabled non-XA resources. JTS implementations can interoperate via CORBA OTS interfaces.

The JTA specifies an architecture for building transactional application servers and defines a set of interfaces for various components of this architecture. The components are: the application, resource managers, and the application server. The JTA specifies standard interfaces for Java-based applications and application servers to interact with transactions, transaction managers, and resource managers JTA transaction management provides a set of interfaces utilized by an application server to manage the beginning and completion of transactions. Transaction synchronization and propagation services are also provided under the domain of transaction management.

In the Java transaction model, the Java application components can conduct transactional operations on JTA compliant resources via the JTS. The JTS acts as a layer over the OTS. The applications can therefore initiate global transactions to include other OTS transaction managers, or participate in global transactions initiated by other OTS compliant transaction managers.

Can Entity Beans have no create() methods?

Answer: Entity Beans can have no create() methods. Entity Beans have no create() method, when entity bean is not used to store the data in the database. In this case entity bean is used to retrieve the data from database. 

What are the call back methods in Session bean?

Answer: Callback methods are called by the container to notify the important events to the beans in its life cycle.  The callback methods are defined in the javax.ejb.EntityBean interface.The callback methods example are ejbCreate(), ejbPassivate(), and ejbActivate().  

What is bean managed transaction?

Answer: In EJB transactions can be maintained by the container or developer can write own code to maintain the transaction. If a developer doesn’t want a container to manage transactions, developer can write own code to maintain the database transaction.   

What are transaction isolation levels in EJB?

Answer: Thre are four levels of transaction isolation are:
* Uncommitted Read
* Committed Read
* Repeatable Read
* Serializable
The four transaction isolation levels and the corresponding behaviors are described below:

Isolation Level

Dirty Read

Non-Repeatable Read

Phantom Read

Read Uncommitted




Read Committed

Not possible



Repeatable Read

Not possible

Not possible



Not possible

Not possible

Not possible

How  can one prove that the array is not null but empty?

Answer: Print args.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 args.length.

What is meant by Endianness ?

Answer:  Endianness describes how multiple data types such as short , int and long are stored in memory.If it takes two bytes to represent a short, then to predict if the most significant or the least significant comes first.If the most significant byte is first, followed by the least significant one then the machine is said to be big endian. Machines such as the SPARC and Power PC are big-endian, while the Intel x86 series is little-endian.

How many types of literals are there in JAVA?

Answer:  There are four types of literals they are Integer literals, Floating point literals, Boolean literals and character literals.

A note on compiling & Executing a JAVA pgm
Answer:  (i) The name of the source file is called in terms of .java
(ii) A source file is called a compilation unit. This has one or more class definitions.
(iii) The name of the class should be same as that of the file.
(iv) Once compiled the .java file creates a .class file. This is done by the compiler javac
(v) This classfile contains the bytecode version of the program.


Answer:   (i) All Java applications begin execution by calling main ()
(ii) When a class member is defined as public. Then that member may be accessed by code outside the class in which it is declared.
(iii) The opposite of public is private which prevents a member from being used by code defined outside of its class.
(iv) The keyword static allows main() to be called without having to instantiate a particular instance of the class. This is mandatory because main () is called by the Java interpreter before any objects are made.
(v) CASE SENSITIVE : Main () is different from main(). It is important to know that that Main() would be compiled. But the Java interpreter would report an error if it would not find main().

What is meant by Garbage collection ?

Answer:   The technique that automatically destroys the dynamically created objects is called garbage collection. When no reference to an object exists, that object is assumed to be no longer needed , and memory occupied by that object can be reclaimed.

What are the access modifiers?

Answer:   There are three types of access modifiers.

  • Private - Makes a method or a variable accessible only from within its own class.
  • (ii) Protected - Makes a method or a variable accessible only to classes in the same package or subclasses of the class.
  • (iii) Public - Makes a class , method or variable accessible from any other class.

A note on keywords for Error handling.

Answer:    (i) Catch - Declares the block of code used to handle an exception.
(ii) Finally - Block of code , usually following a typecatch statement, which is executed no matter what program flow occurs when dealing with an exception. (iii) Throw - Used to pass an exception up to the method that calls this method.

  • Throws - Indicates the method will pass an exception to the method that called it.
  • Try - Block of code that will be tried but which may cause an exception.
  • Assert - Evaluates a conditional _expression to verify the programmer's assumption.

How many ways can you represent integer numbers in JAVA?

Answer:  There are three ways , you can represent integer numbers in JAVA. They are decimal (base 10) , octal (base 8) , and hexadecimal (base 16).

Difference between a process and a thread?

Answer:  I remember having read this answer somewhere. "Threads are like tiny ropes. A Process would denote a rope made out from these threads." Fun apart, a process can contain multiple threads. Also a process gets its own memory address space while a thread doesn't.

What are checked and unchecked exceptions?

Answer:  Checked exceptions are the ones which you expect beforehand to be raised when an exceptional condition occurs and so write your code in a try-catch block to handle that sufficiently. For example: InsuffucientBalanceException which might be raised when money is being withdrawn from a bank account and the account has insufficient balance. Checked exceptions are sub classes of Exception.

Unchecked exceptions are the ones which cannot be handled in the code. These are rather unexpected exceptions like NullPointerException, OutOfMemoryError, DivideByZeroException, typically, programming errors. Unchecked exceptions are subclasses of RunTimeExceptions.

What is singleton class? where is it used?

Answer:  Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

Garbage collection thread belongs to which priority..min?normal?max?
Answer:  The Java garbage collection is implemented as a low priority thread.

What is meant by time slicing?
Answer:  Its a task scheduling method. With time slicing, or "Round-Robin Systems", several processes are executed sequentially to completion. Each executable task is assigned a fixed-time quantum called a time slice in which to execute.

What is a compilation unit?
Answer:  The smallest unit of source code that can be compiled, i.e. a .java file.

Is string a wrapper class?
Answer:  String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and vice-versa.

how can you retrieve warning in jdbc?
Answer:  Write my JDBC code in a try-catch block and catch the SQLExcpetions

Is there any tool in java that can create reports?
Answer:  Yes there are third party tools available.

What is JTS? Where is it used?
Answer:  JTS specifies the implementation of a transaction manager which supports the Java Transaction API (JTA) and implements the Java mapping of the Object Management Group (OMG) Object Transaction Service (OTS) specification (at the level below the API).

Why java does not have multiple inheritance?

Answer: The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object oriented, and familiar" goal. As a simple language, Java's creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple). In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

Why java is not a 100% oops?

Answer:  In a 100% Object oriented language, everything is an Object. That is not the case with Java. Java uses primitive types such as int, char, double. Java primitive types ARE NOT OBJECTS.  All the rest are objects. Keep in mind that int, char and other primitive types CAN NOT BE STORED in collections (e.g. Vector, Hashtable) to store these types in a collections, you need to Use wrapper classes (e.g. Integer, Double).

What is a resource bundle?
Answer:  In its simplest form, a resource bundle is represented by a text file containing keys and a text value for each key.

What is meant by resource leak?

Answer:  Resource leakage generally refers to memory leakage but can refer to any type of system resource that isn't managed correctly.

Memory leakage refers to memory that is no longer used but is not freed and therefore is not available for the system to reuse. Java is supposed to remove most of the memory leakage issues of other languages but it is still possible to create situations that waste memory such as java.util.Map that you never remove information from.

Resource leakage in a more general sence can refer to any system or external application finite resource. The java.awt.Graphics context is supposed to be like this because in java it is a wrapper on a system resource. If you don't close it, it can't be cleaned up by the system. Database resources are another example. Poor Databse code can prevent connections from being recycled by the DBMS and represent a drag on the database.

A note on defining floating point literal ?
Answer:  A floating point literal is defined as float g = 3576.2115F.

A note on arrays of object references?

Answer:   If the array type is CLASS then one can put objects of any subclass of the declared type into the array. The following example on sports explains the above concept :
class sports { }
class football extends sports { }
class hockey extends sports { }
class baseball extends sports { }
sports [ ] mysports = { new football (),
new hockey (),
new baseball ()};

What is meant by "instanceof" comparison?
Answer:  It is used for object reference variables only.You can use it to check wether an object is of a particular type.

when is a method said to be overloaded?
Answer:  Two or more methods are defined within the same class that share the same name and their parameter declarations are different then the methods are said to be overloaded.

What is meant by Recursion?

Answer:  It is the process of defining something in terms of itself. Interms of JAVA it is the attribute that allows a method to call itself.The following example of calculating a factorial gives an example of recursion.
class Factorial {
int fact (int n) {
int result;
if (n= 1) return 1;
result = fact(n -1) * n;
return result;
class Recursion {
Public static void main (string args[ ]) {
Factorial f = new Factorial ();
system.out.println ("Factorial of 10 is " + f.fact(10));

A cool example to explain the concept of METHOD in JAVA.

Answer:  Let us say you are in Mcdonalds and you order for #7 for here with medium coke. The cashier takes your order and punches it on the computer. The folks in the kitchen get the order and they get the crispy chicken and pass it on to the guy who puts a medium fries and finally a medium coke is filled and the order is served to you. In other terms if all this was supposed to be done by a robot then it could have been programmed the following way.
void #7forherewithmediumcoke( )
Get (crispy chicken, lattice, butter, fries, coke);
make (sandwich);
fill (coke, fries);

Java network programming

Answer:  Java is a great language for network programming. IT includes UDP and TCP socket API that is arguably simpler then C/C++/PERL Socket API in Windows (Winsock) or Unix (Berklay Socket API). On the other hand it provides RMI API that could be used for higher level implementations. With RMI network operations look like method calls.

Java also had API for handling Http / SMTP (email) / FTP and CORBA. Many third party APIs are also available for accessing other network services. Apache foundation has released Http client for Java, that can replace Built in Http API in Java and supports cookies, redirects, http 1.1 and many more features.

What two protocols are used in Java RMI technology?
Answer:  Java Object Serialization and HTTP. The Object Serialization protocol is used to marshal call and return data. The HTTP protocol is used to "POST" a remote method invocation and obtain return data when circumstances warrant.

What is difference between Swing and JSF?
Answer:  The key difference is that JSF runs on the server in a standard Java servlet container like Tomcat or WebLogic and generates html output that is viewed on Web browsers (This client). Swing is used for creating Thick clients.

What is JSF?

Answer:  JSF stands for Java Server Faces, or simply Faces. It is a framework for building Web-based user interfaces in Java. Like Swing, it provides a set of standard widgets such as buttons, hyperlinks, checkboxes, and so on.

What is the difference between URL instance and URLConnection instance?

Answer:  A URL instance represents the location of a resource, and a URLConnection instance represents a link for accessing or communicating with the resource at the location.

How do I make a connection to URL?

Answer:  You obtain a URL instance and then invoke openConnection on it. URLConnection is an abstract class, which means you can't directly create instances of it using a constructor. We have to invoke openConnection method on a URL instance, to get the right kind of connection for your URL. Eg. URL url;
URLConnection connection;
try{ url = new URL("...");
connection = url.openConnection();
}catch (MalFormedURLException e) { }

What Is a Socket?

Answer:  A socket is one end-point of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent. Socket classes are used to represent the connection between a client program and a server program. The package provides two classes--Socket and ServerSocket--which implement the client side of the connection and the server side of the connection, respectively.

What information is needed to create a TCP Socket?
Answer:  The Local System’s IP Address and Port Number. And the Remote System's IPAddress and Port Number.

What are the two important TCP Socket classes?
Answer:  Socket and ServerSocket. ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.

When MalformedURLException and UnknownHostException throws?
Answer:  When the specified URL is not connected then the URL throw MalformedURLException and If InetAddress? methods getByName and getLocalHost are unable to resolve the host name they throw an UnknownHostException.

What does RMI stand for?
Answer:  It stands for Remote Method Invocation.

What is RMI?
Answer:  RMI is a set of APIs that allows to build distributed applications. RMI uses interfaces to define remote objects to turn local method invocations into remote method invocations.

What is JNDI
Answer:  Abbreviate of Java Naming and Directory Interface.

What class is the top of the AWT event hierarchy?
Answer:  The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

What is CDATA ?
Answer:  A predefined XML tag for character data that means "don't interpret these characters," as opposed to parsed character data (PCDATA), in which the normal rules of XML syntax apply. CDATA sections are typically used to show examples of XML syntax.

What is certificate authority ?
Answer:  A trusted organization that issues public key certificates and provides identification to the bearer.

What is client-certificate authentication  ?
Answer:  An authentication mechanism that uses HTTP over SSL, in which the server and, optionally, the client authenticate each other  with a public key certificate that conforms to a standard that is defined by X.509 Public Key Infrastructure.

What is CORBA ?
Answer:  Common Object Request Broker Architecture. A language-independent distributed object model specified by the OMG.

What is CTS ?
Answer:  Compatibility test suite. A suite of compatibility tests for verifying that a J2EE product complies with the J2EE platform  specification.