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Check the tape status
The code in Listing 7 uses tapeIn to check the tape status before attempting to tune the radio station and play the radio. If a tape is inserted, this method simply displays an error message instructing the user to remove the tape first.
So, what's the big deal with inheritance?
The fact that it was necessary for me to make changes to the class named Radio greatly reduced the benefit of inheritance in this case. However, even in this case, the use of inheritance eliminated the need for me to define a new class that reproduces all of the code in the class named Radio .
(In the next module, I will explain the process of overriding methods. I will show you how to use method overriding to accomplish these same purposes by extending the Radio class, without any requirement to modify the code in the Radio class. That will be a much better illustration of the benefits of inheritance.)
The driver class
The new driver class named Radio02 is shown in Listing 8 .
| Listing 8 . The class named Radio02. |
|---|
public class Radio02{
//This class simulates the// manufacturer and the human user
public static void main(String[] args){Combo myObjRef = new Combo() |
New object of the Combo class
The most significant change in this class (relative to the driver class named Radio01 in a previous module) is the statement that instantiates a new object of the Combo class (instead of the Radio class) .
All of the other new code in Listing 8 is used to send messages to the new object in order to exercise its behavior.
Program output
The Combo object responds to those messages by producing the screen output shown in Figure 1 .
| Figure 1 . Program output. |
|---|
Combo object constructed
Button 3 programmedPlay Radio
Playing the station at 93.5 MhzInsert Tape
Tape is inRadio is off
Play RadioRemove the tape first
Remove TapeTape is out
Radio is onPlay Radio
Playing the station at 93.5 MhzPlay Tape
Insert the tape firstInsert Tape
Tape is inRadio is off
Play TapeTape is playing
Remove TapeTape is out
Radio is onPlay Radio
Playing the station at 93.5 Mhz |
An exercise for the student
As the old saying goes, I will leave it as an exercise for the student to correlate the messages in Listing 8 with the output shown in Figure 1 .
Extending an existing class often provides an easy way to create a new type. This is primarily true when an existing class creates a type whose features are close to, but not identical to the features needed in the new type.
When an existing class is extended to define a new class, the existing class is often called the superclass and the new class is often called the subclass .
The subclass inherits all of the variables and all of the methods defined in the superclass and its superclasses.
Although not explicitly demonstrated in this module, whenever you declare a reference variable whose type is defined by a class, that variable can be usedto store references to objects instantiated from that class or instantiated from any subclass of that class. Furthermore, that reference can be used to callmethods defined in or inherited into that class with no requirement for a cast. If the method is defined in that class and overridden in the subclass, and ifthe object is actually of the subclass type, the overridden version will be executed. This is polymorphism.
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