The Essence of OOP Using Java, Inheritance, Part 1

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The Essence of OOP Using Java, Inheritance, Part 1
By Richard G. Baldwin
January 3, 2002

Java Programming Notes #1604

Preface

Preview

Discussion and Sample Code

Summary

What's Next

Complete Program Listing

Preface
This lesson is one of a series of lessons designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.
The first lesson in the group was entitled The Essence of OOP Using Java, Objects, and Encapsulation. That lesson, and each of the lessons following that one, has provided explanations of certain aspects of the essence of Object-Oriented Programming using Java.   The previous lesson was entitled The Essence of OOP using Java, Classes.
Essence
My dictionary provides several definitions for the word essence. Among those definitions are the following:

The property necessary to the nature of a thing

The most significant property of a thing

Thus, this miniseries will describe and discuss the necessary and significant aspects of OOP using Java.
I will attempt to provide that information in a high-level format, devoid of any requirement to understand detailed Java syntax. In those cases where an understanding of Java syntax is required, I will attempt to provide the necessary syntax information in the form of sidebars or other obvious formats.
Therefore, if you have a general understanding of computer programming, you should be able to read and understand the lessons in this miniseries, even if you don't have a strong background in the Java programming language.
Viewing tip
You may find it useful to open another copy of this lesson in a separate browser window. That will make it easier for you to scroll back and forth among the different listings while you are reading about them.
Supplementary material
I recommend that you also study the other lessons in my extensive collection of online Java tutorials. You will find those lessons published at Gamelan.com. However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there. You will find a consolidated index at Baldwin's Java Programming Tutorials.

Preview
Extending a class
This lesson shows you how to extend an existing class to create a new class. The new class is the blueprint for a new type.
Inheritance and code reuse
The existing class is often called the superclass and the new class is often called the subclass. This is the mechanism for class inheritance in Java. Inheritance provides a formal mechanism for code reuse.
The subclass inherits all of the variables and all of the methods defined in the superclass.
Car radios with tape players
A class from a previous lesson (whose objects represent car radios) is extended to define a new class, whose objects represent expanded car radios that contain tape players.
Sending messages to the object
Objects of the new class know how to respond to messages for inserting, playing, and removing a tape, in addition to those messages appropriate for objects of the original Radio class.

Discussion and Sample Code
The three pillars of OOP
In an earlier lesson, I explained that most books on OOP will tell you that in order to understand OOP, you must understand the following three concepts:

Encapsulation

Inheritance

Polymorphism

I agree with that assessment.
Encapsulation
The first lesson in this series provided an explanation of encapsulation.
Inheritance
This lesson will provide an explanation of inheritance. I will use another simple program to explain the concept of inheritance.
Polymorphism
Polymorphism is the most complex of the three, and will be explained in future lessons.
A new data type
Whenever you define a class in Java, you cause a new data type to become available to the program. Therefore, whenever you need a new data type, you can define a new class to make that type available.
Extending a class
Defining a new class (to create a new type) can involve a lot of effort. Sometimes you have an option that can greatly reduce the effort required to create your new type. If a class (type) already exists that is close to what you need, you can often extend that class to produce a new class that is closer to what you need. In many cases, this will require much less effort than that required to start from scratch and define a new class to establish a new type.
The ability to extend one class into another new class is the essence of inheritance.
According to the current jargon, the new class is called the subclass and the class that was extended is called the superclass.
What is inherited?
The subclass inherits all of the variables and all of the methods defined in (or inherited into) the superclass, almost as if you had completely defined the new class from scratch, and had reproduced all of the code already defined in the existing superclasses.
Code reuse
Therefore, inheritance often makes it possible to define a new class with a minimum requirement to write new code by formally reusing the code that was previously written into the superclasses.
Sometimes you can get by with simply extending the existing class. Sometimes, however, it is necessary to make changes to the existing class to improve its ability to be extended in a meaningful way. (That is the case with the sample program discussed in this lesson, but the next lesson will show you how to avoid that problem.) It all depends on how the existing class was designed in the first place.
The Radio class
A previous program defined a class named Radio. Objects instantiated from the Radio class (see the previous lessons for a discussion of instantiating objects) were intended to simulate car radios. (Note that the car radios simulated by objects of the Radio class didn't have built-in tape players.)
The Combo class
In this lesson, I will use inheritance to extend the Radio class into a new class named Combo. Objects instantiated from the Combo class are intended to simulate car radios with a built-in tape player.
A complete listing of the new program is shown in Listing 9 near the end of the lesson.
Will discuss in fragments
As usual, I will discuss this program in fragments. I will begin my discussion with the definition of the new class named Combo. Then I will come back and discuss the class named Radio and the driver class named Radio02.
The combo class
The code in Listing 1 shows the beginning of the class named Combo. (Note that the boldface was added for emphasis only. Java source code does not use boldface, Italics, or other such typographical features.)

class Combo extends Radio{    public Combo(){//constructor     System.out.println(            "Combo object constructed"); }//end constructor Listing 1

Two new items
There are two new items in Listing 1 that you did not see in the code in the previous lesson.
Combo extends Radio
First, the class named Combo extends the class named Radio. This means that an object instantiated from the Combo class will contain all of the variables and all the methods defined in the Combo class, plus all the variables and methods defined in the Radio class, and its superclasses. (The variables and methods of the superclass are inherited into the subclass.)
An explicit constructor
Second, the class named Combo defines an explicit constructor.
Defining a constructor is optional
When defining a new class, it is not necessary to define a constructor. If you don't define a constructor, a default constructor will be provided automatically.
Why define a constructor?
The intended purpose of a constructor is to initialize the instance variables belonging to the new object. However, constructors can do other things as well. In this case, I used an explicit constructor to display a message when the object is instantiated from the class named Combo.
Brief discussion of constructors
I'm not going to discuss constructors in detail at this point. However, I will give you a few rules regarding constructors.

Constructors (like methods) can be overloaded (I will explain what overloading means in a subsequent lesson).

The names of constructors must match the names of the classes in which they are defined.

A constructor signature never indicates a return type (such as void).

The code in a constructor never contains a return statement.

Instance methods
The new class named Combo defines three instance methods, each of which has to do with the handling of tape in the tape player:

insertTape

removeTape

playTape

(If you feel ambitious, you could upgrade this class even further to add features such as rewind, fast forward, pause, etc.).
The insertTape method
The entire method named insertTape is shown in Listing 2. This is the method that is used to simulate the insertion of a tape by the user.

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 Listing 2

The most significant thing about the code in Listing 2 is the assignment of the true value to the boolean variable named tapeIn. Other than setting the value of the tapeIn variable to true, the code in Listing 2 simply prints some messages to indicate what is going on.
What is tapeIn used for?
As you will see shortly, the value of the variable named tapeIn is used to determine if it is possible to play the tape or to play the radio.
According to that logic:

If tapeIn is true, it is possible to play the tape but it is not possible to play the radio.

If tapeIn is false, it is possible to play the radio, but it is not possible to play the tape.

tapeIn is not declared in the Combo class
It is also worthy of note that in this version of the program, the variable named tapeIn is not declared in the Combo class (this will change in the next lesson where the program uses method overriding). Rather, this variable is inherited from the Radio class that is extended by the Combo class.
The removeTape method
The removeTape method of the Combo class is shown in Listing 3. Its behavior is pretty much the reverse of the insertTape method, so I won't discuss it further.

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 Listing 3

The playTape method
Listing 4 shows the method named playTape defined in the new Combo class.

public void playTape(){     System.out.println("Play Tape");     if(!tapeIn){//tapeIn is false       System.out.println(             " Insert the tape first");     }else{//tapeIn is true       System.out.println(                   " Tape is playing");     }//end if/else }//end playTape Listing 4

Confirm that the tape is ready
The invocation of the method named playTape can be thought of as sending a message to the Combo object to play the tape. The code in the playTape method checks to confirm that the value stored in the tapeIn variable is true before implementing the incoming message to play the tape.
If tapeIn is false, an error message is displayed advising the user to insert the tape first.
If tapeIn is true, the method prints a message indicating that the tape is playing.
Modified Radio class
Listing 5 shows the definition of the modified version of the class named Radio. The significant changes that were made to this class are shown in boldface (a couple of other changes were made simply to print some information indicating what is going on while the program is executing).

class Radio{ 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){//tapeIn is false       System.out.println(           " Playing the station at "                + stationNumber[index]                + " Mhz");     }else{//tapeIn is true       System.out.println(             " Remove the tape first");     }//end if/else }//end method playStation    }//end class Radio Listing 5

Tape status
The first significant change that was made to the class named Radio is shown in Listing 6 below.

protected boolean tapeIn = false; Listing 6

The statement in Listing 6 declares and initializes a new instance variable named tapeIn. As explained earlier, this instance variable is used to indicate whether or not a tape is inserted. (The Combo class inherits this variable.)
Earlier in this lesson, I explained how the playTape method of the Combo class uses this value to determine whether or not to attempt to play a tape.
Change to the playStation method
The significant change that was made to the method named playStation of the Radio class is shown by the boldface code in Listing 7 below.

    if(!tapeIn){//tapeIn is false       System.out.println(           " Playing the station at "                + stationNumber[index]                + " Mhz");     }else{//tapeIn is true       System.out.println(             " Remove the tape first");     }//end if/else Listing 7

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 lesson, 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.

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 Listing 8

New object of the Combo class
The most significant change in this class (relative to the driver class named Radio01 in a previous lesson) is the single statement in boldface. Note that unlike the previous program, this program 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
That object responds to those messages by producing the following output on the computer screen:
Combo object constructed
Button 3 programmed
Play Radio
Playing the station at 93.5 Mhz
Insert Tape
Tape is in
Radio is off
Play Radio
Remove the tape first
Remove Tape
Tape is out
Radio is on
Play Radio
Playing the station at 93.5 Mhz
Play Tape
Insert the tape first
Insert Tape
Tape is in
Radio is off
Play Tape
Tape is playing
Remove Tape
Tape is out
Radio is on
Play Radio
Playing the station at 93.5 Mhz
Leave as 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 above.

Summary
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.
Inheritance provides a formal mechanism for code reuse.
This lesson modifies slightly, and then extends the Radio class from a previous lesson 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.

What's Next?
In the next lesson, 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.
Complete Program Listing
A complete listing of the program is shown in Listing 9 below.
The primary difference between this program and the program in the earlier lesson (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.

/*File Radio02.java Copyright 2002, R.G.Baldwin Simulates the manufacture and use of a combination car radio and tape player. This program produces the following output on the computer screen:    Combo object constructed Button 3 programmed Play Radio Playing the station at 93.5 Mhz Insert Tape Tape is in Radio is off Play Radio Remove the tape first Remove Tape Tape is out Radio is on Play Radio Playing the station at 93.5 Mhz Play Tape Insert the tape first Insert Tape Tape is in Radio is off Play Tape Tape is playing Remove Tape Tape is out Radio is on Play Radio Playing 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(){//constructor     System.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 Listing 9

About the author
Richard Baldwin is a college professor (at Austin Community College in Austin, TX) and private consultant whose primary focus is a combination of Java and XML. In addition to the many platform-independent benefits of Java applications, he believes that a combination of Java and XML will become the primary driving force in the delivery of structured information on the Web.
Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two. He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.
baldwin.richard@iname.com

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