Java Interview Questions

1.    What is the difference between procedural and object-oriented programs?

       a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code.

        b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.

2.       What are Encapsulation, Inheritance and Polymorphism?

        - Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse.

        - Inheritance is the process by which one object acquires the properties of another object. 

        - Polymorphism is the feature that allows one interface to be used for general class actions.

3.       What is the difference between Assignment and Initialization?

        - Assignment can be done as many times as desired whereas initialization can be done only once.

4.       What is OOPs?

         - Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.

5.        What are Class, Constructor and Primitive data types?

        - Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform.

        - Constructor is a special kind of method that determines how an object is initialized when created.

        - Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.

6.       What is an Object and how do you allocate memory to it?

        - Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.

7.        What is the difference between constructor and method?

        - Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.

8.       What are methods and how are they defined?

        - Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. 

       -  Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. 

        - A method’s signature is a combination of the first three parts mentioned above.

9.       What is the use of bin and lib in JDK?

        - Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

10.     What is casting?

          - Casting is used to convert the value of one type to another.

11.      How many ways can an argument be passed to a subroutine and explain them?

        - An argument can be passed in two ways. They are passing by value and passing by reference.

         Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. 

        Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.

12.     What is the difference between an argument and a parameter?

        - While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

13.     What are different types of access modifiers?

         public: Any thing declared as public can be accessed from anywhere. 

         private: Any thing declared as private can’t be seen outside of its class. 

         protected: Any thing declared as protected can be accessed by classes in the same package and 

         sub classes  in the other packages. 

         default modifier : Can be accessed only to classes in the same package.

14.     What is final, finalize() and finally?

        final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value.

        finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection.

        finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. 

        For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.

15.     What is UNICODE?

         - Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.