7.6 Ap0060: self-assessment, more on arrays  (Page 8/9)

Back to Question 9

Answer 8

B. Runtime Error

Explanation 8

Another ClassCastException

While it is allowable to assign an array reference to an array reference variable declared for a class that is further up the inheritance hierarchy (as illustrated earlier) , it is not allowable to cast an array reference down the inheritance hierarchy to a subclass of the original declared class for thearray.

This program declares a reference for, creates, and populates a two-element array for a class named Superclass . Then it downcasts that reference to a subclass of the class named Superclass . The compiler is unable to determine that this is a problem. However, the runtime system throws the following exception,which terminates the program at runtime.

Back to Question 8

Answer 7

C. 1 2

Explanation 7

Assignment to superclass array reference variable

This program illustrates that, if you have a reference to an array object containing references to other objects, you can assign the array object'sreference to an array reference variable whose type is a superclass of the declared class of the array object. (As we will see later, this doesn't work for array objects containing primitive values.)

What can you do then?

Having made the assignment to the superclass reference variable, whether or not you can do anything useful with the elements in the array (without downcasting) depends on many factors.

No downcast required in this case

In this case, the ability to display the contents of the objects referred to in the array was inherited from the class named Superclass . Therefore, it is possible to access and display a String representation of the objects without downcasting the array object reference from Superclass to the actual type of the objects.

Probably need to downcast in most cases

However, that will often not be the case. In most cases, when using a reference of a superclass type, you will probably need to downcast in order tomake effective use of the elements in the array object.

Back to Question 7

Answer 6

C. 1 2

Explanation 6

Straightforward array application

This is a straightforward application of Java array technology for the storage and retrieval of references to objects.

The program declares a reference to, creates, and populates a two-element array of a class named Subclass . The class named Subclass extends the class named Superclass , which in turn, extends the class named Object by default.

The super keyword

The class named Subclass doesn't do anything particularly useful other than to illustrate extending a class.

However, it also provides a preview of the use of the super keyword for the purpose of causing a constructor in a subclass to call a parameterizedconstructor in its superclass.

Setting the stage for follow-on questions

The main purpose for showing you this program is to set the stage for several programs that will be using this class structure in follow-on questions.