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Interface type is not used
The program makes no use of the interface as a type, and exhibits no polymorphic behavior (no decision among methods having the same signature is required) .
The class named A inherits an abstract method named intfcMethod from the interface and must define it. (Otherwise, it would be necessary to declare the class named A abstract.)
The interface is not a particularly important player in this program.
A. Compiler Error
Simple hierarchical polymorphic behavior
This program is designed to test your knowledge of simple hierarchical polymorphic behavior.
Implement the interface named X
This program defines a class named A that extends a class named Base , and implements an interface named X , as shown in the following code fragment.
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 Ainterface X{
public void intfcMethod();}//end X Override and define some methods
The class named A overrides the method named inherMethod , which it inherits from the class named Base . It also provides a concrete definition of the method named intfcMethod , which is declared in the interface named X .
Store object's reference as superclass type
The program instantiates an object of the class named A and assigns that object's reference to a reference variable of type Base , as shown in the following code fragment.
Base myVar2 = new A(); Oops! Cannot call this method
So far, so good. However, the next fragment shows where the program turns sour. It attempts to call the method named intfcMethod on the object's reference, which was stored as type Base .
myVar2.intfcMethod(); Polymorphic behavior doesn't apply here
Because the class named Base does not define the method named intfcMethod , hierarchical polymorphic behavior does not apply. Therefore a reference to the object being stored as type Base cannot be used to directly call the method named intfcMethod , and the program produces a compiler error.
What is the solution?
Hierarchical polymorphic behavior is possible only when the class defining the type of the reference (or some superclass of that class) contains a definition for the method that is called on the reference.
There are a couple of ways that downcasting could be used to solve the problem in this case.
A. Compiler Error
I put this question in here just to see if you are still awake.
Can store reference as interface type
A reference to an object instantiated from a class can be assigned to any reference variable whose declared type is the name of an interface implementedby the class from which the object was instantiated, or implemented by any superclass of that class.
Define two classes and an interface
This program defines a class named A that extends a class named Base . The class named Base extends Object by default.
The program also defines an interface named X .
Instantiate an object
The following statement instantiates an object of the class named A , and attempts to assign that object's reference to a reference variable whosetype is the interface type named X .
X myVar2 = new A(); Interface X is defined but not implemented
None of the classes named A , Base , and Object implement the interface named X . Therefore, it is not allowable to assign a reference to an object of the classnamed A to a reference variable whose declared type is X . Therefore, the program produces the following compiler error under JDK 1.3:
Ap131.java:20: incompatible types
found : Arequired: X
X myVar2 = new A(); -end-
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