What if we want to make a class such as Circle (defined earlier) extend Comparable? Circle already extends Shape and Java does not permit multiple inheritance.
Notice, however, that Comparable is a special kind of abstract class, one that has no concrete components. In Java, this kind of abstract class can be renamed an interface--the word interface is derived from its English meaning and refers to the ``visible boundary'' of a class that implements all the methods specified in the interface.
We can redefine Comparable as follows:
interface Comparable{
public abstract int cmp(Comparable s);
// return -1 if this < s, 0 if this == 0, +1 if this > s
}
A class that extends an interface is said to ``implement'' it:
class Myclass implements Comparable{
...
public int cmp(Comparable s) { ... }
...
}
In Java, a class may extend only one other class, but may implement any number of interfaces. Thus, one can have
class Circle extends Shape implements Comparable{
...
public double perimeter(){...}
...
public int cmp(Comparable s){...}
...
}
As we mentioned earlier, extending multiple classes might create a conflict between concrete definitions in the multiple parent classes. However, since interfaces have no concrete components, no such contradiction can occur if a class implements multiple interfaces. At most, a class may implement two interfaces that both define abstract methods with the same signature. Since the signature does not indicate anything about the ``meaning'' of the method, any implementation of the method with that signature will be automatically consistent with both its abstract definitions.