Using an Interface as a Type

When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. If you define a reference variable whose type is an interface, any object you assign to itmust be an instance of a class that implements the interface.
As an example, here is a method for finding the largest object in a pair of objects, for any objects that are instantiated from a class that implements  Relatable :

public Object findLargest(Object object1, Object object2) {

   Relatable obj1 = (Relatable)object1;

   Relatable obj2 = (Relatable)object2;

   if ((obj1).isLargerThan(obj2) > 0)

      return object1;

   else 

      return object2;

}

By casting  object1  to a  Relatable  type, it can invoke the  isLargerThan  method.
If you make a point of implementing  Relatable  in a wide variety of classes, the objects instantiated from any of those classes can be compared with the  findLargest()  method—provided that both objects are of the same class. Similarly, they can all be compared with the following methods:

public Object findSmallest(Object object1, Object object2) {

   Relatable obj1 = (Relatable)object1;

   Relatable obj2 = (Relatable)object2;

   if ((obj1).isLargerThan(obj2) < 0)

      return object1;

   else 

      return object2;

}



public boolean isEqual(Object object1, Object object2) {

   Relatable obj1 = (Relatable)object1;

   Relatable obj2 = (Relatable)object2;

   if ( (obj1).isLargerThan(obj2) == 0)

      return true;

   else 

      return false;

}

These methods work for any "relatable"objects, no matter what their class inheritance is. When they implement  Relatable , they can be of both their own class (or superclass) type and a  Relatable  type. This gives them some of the advantages of multiple inheritance, where they can have behavior from both a superclass and an interface.