Interfaces in Java

• An interface in Java is a blueprint of a class, It contains static constants and abstract methods.
• Interfaces are mainly used to achieve abstraction and multiple inheritances
• Interfaces don't have a method body
• An interface can contain constants, default methods and static methods

Difference between class and Interface

• An interface doesn't have the constructor
• All methods in Interface are abstract
• Instantiation of an interface is not possible
• It is implemented by class not extended

Syntax

interface keyword is used to create an interface

 interface Example {
	final int roll = 100;
	int display();
}

Classes implementing interfaces

Interfaces contain abstract methods, so the definition isn't allowed inside interfaces. Classes perform the behavior of interfaces. Classes use the keyword implements to implement an interface.

Example

interface A {
  void read(int p);
}


class Example implements A {
  int a;
  public void read (int x){
    a = x;
  }
}

class Test {
  public static void main (String[] args){
    Example ob = new Example();
    ob.read(10);
    System.out.println(ob.a);
  }
}

Extending Interfaces

An interface can extend another interface in the same way that classes do inheritance

Example

interface A {
  void read(int p);
}

interface B extends A {
  void show();
}

class Example implements B {
  int a;
  public void read (int x){
    a = x;
  }
  public void show () {
    System.out.println(a);
  }
}

class Test {
  public static void main (String[] args){
    Example ob = new Example();
    ob.read(10);
    ob.show();
  }
}

check result here