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In this case, the only subclass-superclass relationship between the classes named A and B was that they were both subclasses of the same superclass. Even that relationship was established forconvenience, and was not a requirement.
Different behavior of interface methods
The methods having the same signature, (declared in the common interface, and defined in the classes) , need not have any similarity in terms of behavior.
A new interface relationship
The fact that both classes implemented the interface named X created a new relationship among the classes, which is not based onclass inheritance.
C. Base A-intfcMethod
Illustrates the use of an interface as a type
The program defines a class named Base , and a class named A , which extends Base , and implements an interface named X , as shown below.
class Base{
public void inherMethod(){System.out.print("Base ");
}//end inherMethod()}//end class Base
class A extends Base implements X{public void inherMethod(){
System.out.print(" A-inherMethod ");
}//end inherMethod()public void intfcMethod(){System.out.print("A-intfcMethod ");
}//end intfcMethod()}//end class A
interface X{public void intfcMethod();
}//end X Implementing interfaces
A class may implement none, one, or more interfaces.
The cardinal rule on interfaces
If a class implements one or more interfaces, that class must either be declared abstract, or it must provide concrete definitions of all methodsdeclared in and inherited into all of the interfaces that it implements. If the class is declared abstract, its subclasses must provide concrete definitions ofthe interface methods.
A concrete definition of an interface method
The interface named X in this program declares a method named intfcMethod . The class named A provides a concrete definition of that method.
(The minimum requirement for a concrete definition is a method that matches the method signature and has an empty body.)
Storing object's reference as an interface type
The interesting part of the program is shown in the following code fragment.
void doIt(){
Base myVar1 = new Base();myVar1.inherMethod();
X myVar2 = new A();myVar2.intfcMethod();
System.out.println("");}//end doIt() The above fragment instantiates a new object of the class named A , and saves a reference to that object in a reference variable of the declared type X .
How many ways can you save an object's reference?
Recall that a reference to an object can be held by a reference variable whose type matches any of the following:
Save object's reference as implemented interface type
In this program, the type of the reference variable matches the interface named X , which is implemented by the class named A .
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