Implementing Interfaces

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.

Example

/* 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 will produce the following result −

Output

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, in a similar way as a class can extend another class.