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Inheritance provides a formal mechanism for code reuse.
This module modifies slightly, and then extends the Radio class from a previous module to define a new class named Combo . Objects of the Combo class simulate car radios that contain tape players. Objects of the Combo class know how to respond to messages for inserting, playing, and removing a tape, in addition to those messages appropriate for an object of the Radio class.
The changes that were required in the definition of the Radio class provide for the fact that it is not possible to play a radio station and to play a tape at the same time. This change was necessary because the original designer of the Radio class (this author) didn't design that class with the idea of extending it to include a tape player. This points out the importance of thinking ahead when defining a new class.
In the next module, I will show 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.
I will also show you how to use method overriding to eliminate the above requirement to modify the Radio class before extending it.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have misappropriated copies of my modules from cnx.org, converted them to Kindle books, andplaced them for sale on Amazon.com showing me as the author. I receive no compensation for those sales and don't know who doesreceive compensation. If you purchase such a book, please be aware that it is a bootleg copy of a module that is freelyavailable on cnx.org.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
A complete listing of the program is shown in Listing 9 below.
The primary difference between this program and the program in the earlier module (whose objects simulate car radios) is the inclusion in this program of a new class named Combo . The class named Combo extends the original Radio class to create a new type of radio that also contains a tape player.
| Listing 9 . The program named Radio02. |
|---|
/*File Radio02.java
Copyright 2002, R.G.BaldwinSimulates the manufacture and use of a
combination car radio and tape player.This program produces the following
output on the computer screen:Combo object constructedButton 3 programmed
Play RadioPlaying the station at 93.5 Mhz
Insert TapeTape is in
Radio is offPlay Radio
Remove the tape firstRemove Tape
Tape is outRadio is on
Play RadioPlaying the station at 93.5 Mhz
Play TapeInsert the tape first
Insert TapeTape is in
Radio is offPlay Tape
Tape is playingRemove Tape
Tape is outRadio is on
Play RadioPlaying the station at 93.5 Mhz
**************************************/public class Radio02{
//This class simulates the// manufacturer and the human user
public static void main(String[] args){Combo myObjRef = new Combo();
myObjRef.setStationNumber(3,93.5);myObjRef.playStation(3);
myObjRef.insertTape();myObjRef.playStation(3);
myObjRef.removeTape();myObjRef.playStation(3);
myObjRef.playTape();myObjRef.insertTape();
myObjRef.playTape();myObjRef.removeTape();
myObjRef.playStation(3);}//end main
}//end class Radio02//===================================//
class Radio{//This class simulates the plans from
// which the radio object is created.protected double[] stationNumber =new double[5];protected boolean tapeIn = false;
//---------------------------------//public void setStationNumber(int index,double freq){
stationNumber[index]= freq;
System.out.println("Button "+ index + " programmed");
}//end method setStationNumber//---------------------------------//public void playStation(int index){
System.out.println("Play Radio");if(!tapeIn){
System.out.println(" Playing the station at "
+ stationNumber[index]+ " Mhz");
}else{System.out.println(
" Remove the tape first");}//end if/else
}//end method playStation}//end class Radio//===================================//
class Combo extends Radio{public Combo(){//constructorSystem.out.println(
"Combo object constructed");}//end constructor
//---------------------------------//public void insertTape(){System.out.println("Insert Tape");
tapeIn = true;System.out.println(" Tape is in");
System.out.println(" Radio is off");
}//end insertTape method//---------------------------------//public void removeTape(){
System.out.println("Remove Tape");tapeIn = false;
System.out.println(" Tape is out");
System.out.println(" Radio is on");
}//end removeTape method//---------------------------------//public void playTape(){
System.out.println("Play Tape");if(!tapeIn){
System.out.println(" Insert the tape first");
}else{System.out.println(
" Tape is playing");}//end if/else
}//end playTape}//end class combo |
-end-
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