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Base myVar = new A();
((A)myVar).test(); The downcast is redundant
As you learned in the discussion of Question 8 , the downcast isn't required, and it has no impact on the behavior of the program in Question 8 .
This program behaves exactly the same with the second statement in the above fragment replaced by the following statement, which does not contain a downcast.
myVar.test(); Again, you need to know when downcasting is required, when it isn't required, and to make use of that knowledge to downcast appropriately.
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, as shown in the following fragment.
Base myVar = new A();
((A)myVar).test(); This would be a simple case of polymorphic behavior were it not for the downcast shown in the above fragment.
The downcast is redundant
Actually, the downcast was placed there to see if you could determine that it is redundant. It isn't required, and it has no impact on the behavior of thisprogram. This program would behave exactly the same if the second statement in the above fragment were replaced with the following statement, which does notcontain a downcast.
myVar.test(); You need to know when downcasting is required, when it isn't required, and to make use of that knowledge to downcast appropriately.
B. Runtime Error
Storing a reference as a superclass type
You can store an object's reference in any reference variable whose declared type is a superclass of the actual class from which the object was instantiated.
May need to downcast later
Later on, when you attempt to make use of that reference, you may need to downcast it. Whether or not you will need to downcast will depend on what youattempt to do.
In order to call a method ...
For example, if you attempt to call a method on the reference, but that method is not defined in or inherited into the class of the reference variable,then you will need to downcast the reference in order to call the method on that reference.
Class Base defines method named test
This program defines a class named Base that defines a method named test .
Class A extends Base and overrides test
The program also defines a class named A that extends Base and overrides the method named test as shown in the following fragment.
class Base{
public void test(){System.out.print("Base ");};
}//end class Baseclass A extends Base{
public void test(){System.out.print("A ");
}//end test()}//end class A A new object of the class Base
The program instantiates a new object of the class Base and stores a reference to that object in a reference variable of type Base , as shown in the following fragment.
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