Difference between abstract class and interface

Java Tutorial

Java Tutorial Java Exercises Data Structures

In this article, we will discuss the difference between Abstract Class and Interface in Java with examples. I have covered the abstract class and interface in separate tutorials of OOPs Concepts so I would recommend you to read them first, before going though the differences.

1. Abstract class in java
2. Interface in Java

But there are many differences between abstract class and interface that are given below.

Abstract classInterface
1) Abstract class can have abstract and non-abstract methods.Interface can have only abstract methods. Since Java 8, it can have default and static methods also.
2) Abstract class doesn't support multiple inheritance.Interface supports multiple inheritance.
3) Abstract class can have final, non-final, static and non-static variables.Interface has only static and final variables.
4) Abstract class can provide the implementation of interface.Interface can't provide the implementation of abstract class.
5) The abstract keyword is used to declare abstract class.The interface keyword is used to declare interface.
6) An abstract classcan extend another Java class and implement multiple Java interfaces.An interface can extend another Java interface only.
7) An abstract classcan be extended using keyword ?extends?. An interface classcan be implemented using keyword ?implements?.
8) A Javaabstract classcan have class members like private, protected, etc.Members of a Java interface are public by default.
9)Example:
public abstract class Shape{
public abstract void draw();
}		Example:
public interface Drawable{
void draw();
}
Example of abstract class and interface in Java

//Creating interface that has 4 methods  
interface A{  
void a();//bydefault, public and abstract  
void b();  
void c();  
void d();  
}  
  
//Creating abstract class that provides the implementation 
//of one method of A interface  
abstract class B implements A{  
public void c(){System.out.println("I am C");}  
}  
  
//Creating subclass of abstract class, now we need to provide 
//the implementation of rest of the methods  
class M extends B{  
public void a(){System.out.println("I am a");}  
public void b(){System.out.println("I am b");}  
public void d(){System.out.println("I am d");}  
}  
  
//Creating a test class that calls the methods of A interface  
class Test5{  
public static void main(String args[]){  
A a=new M();  
a.a();  
a.b();  
a.c();  
a.d();  
}
}


Output:

 I am a
 I am b
 I am c
 I am d
Abstract class vs interface in Java
Difference No.1: Abstract class can extend only one class or one abstract class at a time

class Example1{
   public void display1(){
      System.out.println("display1 method");
   }
}
abstract class Example2{
   public void display2(){
      System.out.println("display2 method");
   }
}
abstract class Example3 extends Example1{
   abstract void display3();
}
class Example4 extends Example3{
   public void display3(){
      System.out.println("display3 method");
   }
}
class Demo{
   public static void main(String args[]){
       Example4 obj=new Example4();
       obj.display3();
   }
}


Output:

display3 method

Interface can extend any number of interfaces at a time


//first interface
interface Example1{
    public void display1();
}
//second interface
interface Example2 {
    public void display2();
}
//This interface is extending both the above interfaces
interface Example3 extends Example1,Example2{
}
class Example4 implements Example3{
    public void display1(){
        System.out.println("display2 method");
    }
    public void display2(){
        System.out.println("display3 method");
    }
}
class Demo{
    public static void main(String args[]){
        Example4 obj=new Example4();
        obj.display1();
    }
}


Output:

display2 method
Difference No.2: Abstract class can be extended(inherited) by a class or an abstract class

class Example1{
   public void display1(){
      System.out.println("display1 method");
   }
}
abstract class Example2{
   public void display2(){
       System.out.println("display2 method");
   }
}
abstract class Example3 extends Example2{
   abstract void display3();
}
class Example4 extends Example3{
   public void display2(){
       System.out.println("display2 method");
   }
   public void display3(){
       System.out.println("display3 method");
   }
}
class Demo{
   public static void main(String args[]){
       Example4 obj=new Example4();
       obj.display2();
   }
}


Output:

display2 method
Interfaces can be extended only by interfaces. Classes has to implement them instead of extend

interface Example1{
    public void display1();
}
interface Example2 extends Example1{
}
class Example3 implements Example2{
   public void display1(){
      System.out.println("display1 method");
   }
}
class Demo{
   public static void main(String args[]){
      Example3 obj=new Example3();
      obj.display1();
   }
}


Output:


display1 method
Difference No.3: Abstract class can have both abstract and concrete methods

abstract class Example1 {
   abstract void display1();
   public void display2(){
     System.out.println("display2 method");
   }
}
class Example2 extends Example1{
   public void display1(){
      System.out.println("display1 method");
   }
}
class Demo{
   public static void main(String args[]){
     Example2 obj=new Example2();
     obj.display1();
   }
}


Interface can only have abstract methods, they cannot have concrete methods

interface Example1{
   public abstract void display1();
}
class Example2 implements Example1{
   public void display1(){
      System.out.println("display1 method");
   }
}
class Demo{
   public static void main(String args[]){
      Example2 obj=new Example2();
      obj.display1();
   }
}


Output:


display1 method
Difference No.4: In abstract class, the keyword ‘abstract’ is mandatory to declare a method as an abstract

abstract class Example1{
   public abstract void display1();
}

class Example2 extends Example1{
   public void display1(){
      System.out.println("display1 method");
   }
   public void display2(){
      System.out.println("display2 method");
   }
}
class Demo{
   public static void main(String args[]){ 
       Example2 obj=new Example2(); 
       obj.display1();
   }
}


In interfaces, the keyword ‘abstract’ is optional to declare a method as an abstract because all the methods are abstract by default

interface Example1{
    public void display1();
}
class Example2 implements Example1{
    public void display1(){
        System.out.println("display1 method");
    }
    public void display2(){
        System.out.println("display2 method");
    } 
}
class Demo{
   public static void main(String args[]){
       Example2 obj=new Example2();
       obj.display1();
   }
}


Difference No.5: Abstract class can have protected and public abstract methods

abstract class Example1{
   protected abstract void display1();
   public abstract void display2();
   public abstract void display3();
}
class Example2 extends Example1{
   public void display1(){
       System.out.println("display1 method");
   }
   public void display2(){
      System.out.println("display2 method");
   }
   public void display3(){
      System.out.println("display3 method");
   }
}
class Demo{
   public static void main(String args[]){
      Example2 obj=new Example2();
      obj.display1();
   }
}


Interface can have only public abstract methods

interface Example1{
   void display1();
}
class Example2 implements Example1{
   public void display1(){
      System.out.println("display1 method");
   }
   public void display2(){ 
      System.out.println("display2 method");
   }
}
class Demo{
   public static void main(String args[]){
       Example2 obj=new Example2();
       obj.display1();
   }
}


Difference No.6: Abstract class can have static, final or static final variables with any access specifier

abstract class Example1{
   private int numOne=10;
   protected final int numTwo=20;
   public static final int numThree=500;
   public void display1(){
      System.out.println("Num1="+numOne);
   }
}
class Example2 extends Example1{
   public void display2(){
      System.out.println("Num2="+numTwo);
      System.out.println("Num2="+numThree);
   }
}
class Demo{
   public static void main(String args[]){
      Example2 obj=new Example2(); 
      obj.display1();
      obj.display2();
   }
}


Interface can have only public static final (constant) variable

interface Example1{
   int numOne=10;
}
class Example2 implements Example1{
   public void display1(){
      System.out.println("Num1="+numOne);
   }
}
class Demo{
   public static void main(String args[]){
      Example2 obj=new Example2();
      obj.display1();
   }
}





Instagram


Java Programming