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Learning the Java Language
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Interfaces and Inheritance
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 it
must 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.
