What are the key Differences between Interface and Abstract class in Java?

 

What are the key differences between Interface and Abstract class?

If you are learning Java you might know about what are the key features of java, how can we make use of basic java concepts, mechanisms like interfaces, abstraction. So, if you are looking for the key differences between java Interfaces and abstraction then this is the right place. In this article we will see what are the different aspects on which we can differentiate java Interfaces and Abstract classes.

·       Interface

1.      When we should use –

You can use Interface if you only know the specification and don’t know how to develop it or you don’t know the implementation.

Syntax:

interface <interface_name> {

        // declare the fields that are constant

        // declare abstract methods

}

e.g.

interface Emp {

        public abstract getEmpInfo();

        public abstract getSalaryInfo();

}

2.      Multiple Inheritance

By using Interfaces, we can achieve Multiple inheritance. It means one class can implement one or more interfaces.

3.      Variables

While talking about interface variables, variables inside the interface are by default public static final.

4.      Variable initialization

You should initialize the variable, whenever you are writing it inside the interface.

e.g., public static final int i = 20;

5.      Constructor

We can’t create constructor for the interfaces.

As we know all the variables inside the interface are static, we need a constructor to initialize the variables (non-static variables). So, in interface we don’t have any non-static variables hence we don’t need to have constructor for that.

6.      Object

We can not create object of interface.

·       Abstract class –

1.      When we should use –

In the case where you know the partial implementation of class at that time you use the abstract class.

Syntax:

abstract class <class_name> {

        //abstract methods

        //non abstract methods

        //data members

}

e.g.,

abstract class A {

        getSalaryInfo(){

                       System.out.println("Employee Salary Info");

      }

      public abstract getEmpInfo();

}

2.      Multiple Inheritance

Abstract class can’t extend more than one class because java does not support Multiple Inheritance.

3.      Variables

There is no such restriction on how we can declare the variables. Abstract class variable can be anything, you can make it public, private etc.

4.      Variable initialization

You may or may not initialize the variables.

e.g.,

public int i = 1;

private int j;

5.      Constructor

We can initialize the non-static variables whenever we are creating the objects, so we can create the constructor for abstract class to initialize the variables.

6.      Object

Abstract class does not have any implementation, it is partially implemented, hence it doesn’t have complete implementation, since it has partial implementation, we can’t create object of abstract class.