Difference Between Abstract Class and Interface in Java

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

Each of the above mentioned points are explained with an example below:

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();
   }
}

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();
    }
}

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("Example4-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

Example4-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 'abstrac' 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();
   }
}