Interview Question On Java for Fresher !!!

1.      What is the difference between a constructor and a method?

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.

A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

             2.      What is Polymorphism?

The Polymorphism can be referred as one name many forms. It is the ability of methods to behave differently, depending upon the object who is calling it. The key features of Polymorphism are:

Allows using one interface for multiple implementations.

Supports Method Overloading: Multiple methods with same name, but different formal argument.

Supports Method Overridden: Multiple methods have the same name, same return type, and same formal argument list.

            3.      Explain garbage collection.

The Java uses the garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program. Step involve in cleaning up the garbage collection:

Garbage Object Collection: first step is to collection and group all those object which are no more reference with any of the program. We can use the different methods to collect the garbage object like using runtime.gc() or system.gc().

Run Finalize method: To free up those object which is collected by the garbage collector java must execute the Finalize method to delete all those dynamically created object

            4.      What is an immutable object?

An immutable object is one that we cannot change once it is created. Steps involved in creation of an immutable object are:

Make all of its data fields private.

Methods which can perform changes in any of the data fields after the construction of object must be avoided.

           5.      How are this() and super() used with constructors?

this() Constructors: is used to pointing the current class instance.

Can be used with variables or methods.

Used to call constructer of same class.

Private variable cannot be accessed using this().

super() Constructer: is used to call constructor of parent class.

Must be the first statement in the body of constructor.

Using this we can access private variables in the super class.

           6.      What are Access Specifiers available in Java?

           Java offers four access specifiers, described below:

Public: public classes, methods, and fields can be accessed by every class.

Protected: protected methods and fields can only be accessed within the same class to which the methods and fields belong.

Default (no specifier): when we do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs.

Private: private methods and fields can only be accessed within the same class to which the methods and fields belong. Private methods and fields are not inherited by subclasses.

           7.      What is Constructor?

           A constructor is used to initialize a newly created object.

It is called just after the memory is allocated for the object.

It can be used to initialize the objects.

It is not mandatory to write a constructor for the class.

Name of constructor is same as the class name.

Cannot be inherited.

Constructor is invoked whenever an object of its associated class is created.

             8.      What are the List interface and its main implementation?

            The List helps in collections of objects. Lists may contain duplicate elements. The main                                   implementations of the List interface are as follows:

ArrayList: Resizable-array implementation of the List interface.

Vector: Synchronized resizable-array implementation of the List.

LinkedList: Doubly-linked list implementation of the List interface. Better performance than the ArrayList implementation when elements are inserted or deleted timely.

         9.      Explain the user defined Exceptions.

         User Defined Exceptions are exceptions defined by the user for specific purposed. This allows custom           exceptions to be generated and caught in the same way as normal exceptions. While defining a User            Defined Exception, we need to take care of the following aspects:

It should be extend from Exception class.

Use toString() method to display information about the exception.

            10.  Describe life cycle of thread.

          Threads follow the single flow of control. A thread is execution in a program. The life cycles of threads             are listed below:

Newborn state: After the creations of Thread instance the thread is in this state but before the start() method invocation. Thread is considered not alive in this phase.

Runnable state: A thread starts its life from Runnable state. After the invoking of start() method thread enters Runnable state.

Running state: A thread first enters Runnable state.

Blocked state: A thread can enter in this state because of waiting the resources that are hold by another thread.

Dead state: A thread can be considered dead when its run() method completes. If any thread comes on this state that means it cannot ever run again.

               11.  What is an Applets?

          Applets: These are small java programs.

They can send from one computer to another computer over the internet using the Applet Viewer that supports java.

Applets can runs in a Web browser as it is a java program. It can be a fully functional Java application because it has the entire Java API at its disposal.

Applets follow the security rules given by the Web browser.

Applet security is also known as sandbox security.

             12.  What is the Set interface?

        A Set interface is collection of element which cannot be duplicated.

The Set interface contains methods inherited from collection.

It provides methods to access the elements of a finite mathematical set.

Two Set objects are equal if they contain the same elements.

It models the mathematical set abstraction.

              13.  What is a HashSet and TreeSet?

           The HashSet is an unsorted, unordered Set.

It is Collection set that restrict duplicate elements and also repositioning of elements.

It implements the Set interface and extends AbstractSet.

            Uses hash code of the object being inserted. 

          The TreeSet is a Set implemented when we want elements in a sorted order.

Sorting of element is done according to the natural order of elements or by the help of comparator provided at creation time.

14.  What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused.

A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

15.  Describe synchronization in respect to multithreading.

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources.

Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

16.  What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie. you may not call its constructor), abstract class may contain static data.

17.  Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

18.  What is the difference between an Interface and an Abstract class?

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. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

19.  Explain different way of using thread?

The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance, the only interface can help.

           

            20.  When should I use abstract classes and when should I use interfaces?

Use Interface, when:

• Design changing frequently or when various implementations only share method signatures.
• We need some classes to use some methods which we do not want to be included in the class.

Use Abstract Class, when:

• Various implementations are of the same kind and use common behavior.
• Enabling with generalized form of abstraction and leave the implementation task with the inheriting subclass.
• creating planned inheritance hierarchies