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Baldwin shows you how to use method overriding to cause the behavior of a method inherited into a subclass to be appropriate for an object instantiated from the subclass.

Revised: Thu Mar 31 10:37:32 CDT 2016

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This module is one of a series of modules designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.

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This module builds on the previous module . It is recommended that you study that module before embarking on this module.

The program discussed in this module extends a Radio class to produce a new class that simulates an upgraded car radio containing a tape player.

Method overriding is used to modify the behavior of a method of the Radio class named playStation , to cause that method to behave appropriately when a tape has been inserted into the tape player.

Discussion and sample code

Inheriting methods and variables

When you define a class that extends another class, an object instantiated from your new class will contain all of the methods and all of the variables defined in your new class. The object will also contain all of the methods and all of the variables defined in all of the superclasses of your new class.

The behavior of the methods

The behavior of the methods defined in a superclass and inherited into your new class may, or may not, be appropriate for an object instantiated from your new class. If those methods are appropriate, you can simply leave them alone.

Overriding to change behavior

If the behavior of one or more methods defined in a superclass and inherited into your new class is not appropriate for an object of your new class, you can change that behavior by overriding the method in your new class.

How do you override a method?

To override a method in your new class, simply reproduce the name, argument list, and return type of the original method in a new method definition in your new class. Then provide a body for the new method. Write code in that body to cause the behavior of the overridden method to be appropriate for an object of your new class.

Here is a more precise description of method overriding taken from the excellent book entitled The Complete Java 2 Certification Study Guide , by Roberts, Heller, and Ernest:

"A valid override has identical argument types and order, identical return type, and is not less accessible than the original method. The overriding method must not throw any checked exceptions that were not declared for the original method."

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