• Interface can have only abstract methods. Since Java 8, it can have default and static methods.

• All methods are by-default public.

• Interface has only public, static and final variables.

• Interface can't have constructor.

• Following modifiers can't be applied to the methods declared in an interface :-

  private, protected, final, synchronized.

• Use interface keyword to declare interface.

Exampl :

interface Animal
{
   public void talk();
}
class Dog implements Animal
{
 public void talk(){
   System.out.println("bark");
 }
}
class Cat implements Animal
{
 public void talk(){
  System.out.println("mew");
 }
}
class MainClass
{
 public static void main(String[] ar){
    Animal ob = new Dog();
    ob.talk();
    Animal ob1 = new Cat();
    ob1.talk();
 }
}           

bark
mew         

• An interface can extends one or more interface.

  interface A{
  }
  interface B extends A{
  }
  interface C extends A, B
  {}                  

• One or more interfaces can be implemented in a class.

  interface A{
  }
  interface B{
  }
  class Test implements A, B{
  }                  

• A class can extends one class and implements one ore more interfaces at the same time, but class will extends first and then implements interface.

  interface A{
  }
  class B{
  }
  class Test extends B implements A{
  }