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If a class implements an interface, it must provide a concrete definition for all the methods declared by that interface, and all the methods inherited by that interface. Otherwise, the class must be declared abstract and the definitions must be provided by a class that extends the abstract class.
The cardinal rule regarding class inheritance
A similar rule exists for defining classes that inherit abstract methods from other classes:
If a class inherits one or more abstract methods from its superclasses, it must provide concrete definitions for all the inherited abstract methods. Otherwise, the class must be declared abstract and theconcrete definitions must be provided by a class that extends the abstract class.
What does that mean in this case?
In this case, this means that the class named B must provide concrete definitions for the methods named p and q , because:
As in method overriding, the signature of the concrete method in the defining class must match the signature of the method as it is declared in the interface.
Class B satisfies the cardinal rule
As you can see from Listing 3 , the class named B does provide concrete (but empty) definitions of the methods named p and q .
(As mentioned earlier, I made the methods empty in this program for simplicity. However, it is not uncommon to define empty methods in classes that implement interfaces that declare a large number of methods, such as the MouseListener interface. See my tutorials on event-driven programming here for examples.)
The class named C
Listing 4 defines a class named C , which extends Object , and also implements I2 . As in the case of the class named B , this class must, and does, provide concrete (but empty) definitions for the methods named p and q .
| Listing 4 . Definition of the class named C. |
|---|
class C extends Object implements I2{
public void p(){}//end p()
//---------------------------------//public void q(){}//end q();
//---------------------------------//}//end class B |
A driver class
Finally, the driver class named Poly05 shown in Listing 5 defines an empty main method.
| Listing 5 . The driver class named Poly05. |
|---|
public class Poly05{
public static void main(String[]args){
}//end main}//end class Poly05 |
Doesn't do anything
As mentioned earlier, the purpose of this program is solely to illustrate an inheritance and interface structure. This program can be compiled and executed, but it doesn't do anything useful.
A new relationship
At this point, it might be useful for you to sketch out the structure in a simple hierarchy diagram.
If you do, you will see that implementation of the interface named I2 by the classes named B and C , has created a relationship between those two classes that is totally independent of the normal class hierarchical relationship.
What is the new relationship?
By declaring that both of these classes implement the same interface named I2 , we are guaranteeing that an object of either class will contain concrete definitions of the two methods declared in the interfaces named I2 and I1 .
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