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In other words, the behavior of the method named test , when called on a reference to an object of type A , is different from the behavior of the method named test when called on a reference to an object of type B .
The method definitions
The definitions of the two classes named A and B , along with the two versions of the overridden method named test are shown in the following fragment.
class A extends Base{
public void test(){System.out.print("A ");
}//end test()}//end class A
class B extends Base{public void test(){
System.out.print("B ");}//end test()
}//end class B Store a subclass object's reference as a superclass type
The program declares a reference variable of type Base , instantiates a new object of the class named A , and assigns that object's reference to the reference variable of type Base . Then it calls the method named test on that reference as shown in the following fragment.
Base myVar = new A();
myVar.test(); Polymorphic behavior applies
Simple polymorphic behavior causes the overridden version of the method named test , defined in the class named A , (as opposed to the versions defined in class Base or class B ) to be executed. This causes the letter A followed by a space character to be displayed on the standard output device.
Store another subclass object's reference as superclass type
Then the program instantiates a new object from the class named B , and assigns that object's reference to the same reference variable, overwriting thereference previously stored there. (This causes the object whose reference was previously stored in the reference variable to become eligible for garbagecollection in this case.)
Then the program calls the method named test on the reference as shown in the following fragment.
myVar = new B();
myVar.test(); Polymorphic behavior applies again
This time, simple polymorphic behavior causes the overridden version of the method named test , defined in the class named B , (as opposed to the versions defined in class Base or class A ) to be executed. Thiscauses the letter B followed by a space character to be displayed on the standard output device.
Once again, what is runtime polymorphic behavior?
With runtime polymorphic behavior, the method selected for execution is based, not on the type of the reference variable holding the reference to theobject, but rather on the actual class from which the object was instantiated.
If the method was properly overridden, the behavior exhibited by the execution of the method is appropriate for an object of the class from which theobject was instantiated.
D. A
Compiles and executes successfully
This program compiles and executes successfully causing the version of the method named test , which is overridden in the class named A to be executed. That overridden method is shown in the following fragment.
class A extends Base{
public void test(){System.out.print("A ");}//end test()
}//end class A So, what is the issue here?
The purpose of this program is to determine if you understand polymorphic behavior and the role of downcasting. Consider the following fragment taken fromthe program in Question 8 .
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