Chapter 9.3.2 : Implementing Interfaces

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.
A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */

public class MammalInt implements Animal{

   public void eat(){

      System.out.println("Mammal eats");

   }

   public void travel(){

      System.out.println("Mammal travels");

   } 

   public int noOfLegs(){

      return 0;

   }

   public static void main(String args[]){

      MammalInt m = new MammalInt();

      m.eat();

      m.travel();

   }

} 

This would produce the following result:

Mammal eats

Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

·         Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

·         The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

·         An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

·         A class can implement more than one interface at a time.

·         A class can extend only one class, but implement many interfaces.

·         An interface can extend another interface, similarly to the way that a class can extend another class.