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What does this allow you to do?
When a reference to an object is held by a reference variable whose type matches an interface implemented by the class from which the object wasinstantiated, that reference can be used to call any method declared in or inherited into that interface.
(That reference cannot be used to call methods not declared in or not inherited into that interface.)
In this simple case ...
The method named intfcMethod is declared in the interface named X and implemented in the class named A .
Therefore, the method named intfcMethod can be called on an object instantiated from the class named A when the reference to the object is held in a reference variable of the interface type.
(The method could also be called if the reference is being held in a reference variable of declared type A .)
The call to the method named intfcMethod causes the text A-intfcMethod to appear on the screen.
C. Base A-intfcMethod
What is runtime polymorphic behavior?
One way to describe runtime polymorphic behavior is:
The runtime system selects among two or more methods having the same signature, not on the basis of the type of the reference variable in which an object'sreference is stored, but rather on the basis of the class from which the object was originally instantiated.
Illustrates simple class and interface inheritance
The program defines a class named Base , and a class named A , which extends Base , and implements the interface named X , as shown in the following fragment.
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 Define an interface method
The interface named X declares a method named intfcMethod . A concrete definition of that method is defined in the class named A .
A new object of type Base
The code in the following fragment instantiates a new object of the class Base and calls its inherMethod . This causes the word Base to appear on the output screen. There is nothing special about this. This is a simple example of the use of an object's reference to call one of itsinstance methods.
void doIt(){
Base myVar1 = new Base();myVar1.inherMethod(); A new object of type A
The following fragment instantiates a new object of the class A and calls its intfcMethod . This causes the text A-intfcMethod to appear on the output screen. There is also nothing special about this. This is also a simple example of the use of anobject's reference to call one of its instance methods.
A myVar2 = new A();
myVar2.intfcMethod();System.out.println("");
}//end doIt() Not polymorphic behavior
The fact that the class named A implements the interface named X does not indicate polymorphic behavior in this case. Rather, this program is an example of simple class and interface inheritance.
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