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

Extending a class

This module 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 module (whose objects represent car radios) is extended to define a new class, whose objects represent expanded car radios that contain tape players. (Yes, at one point in history, car radios did contain tape players instead of CDs.)

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 that are appropriate for objects of the original Radio class.

The three pillars of OOP

In an earlier module, I explained that most books on OOP will tell you that in order to understand OOP, you must understand the following three concepts:

I agree with that assessment.

Encapsulation

The first module in this series provided an explanation of encapsulation.

Inheritance

This module (and some modules to follow) 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 modules.

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 is 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 also 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 module, but the next module will show you how to avoid that issue.) 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 modules 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 module, 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 module.

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 .

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

Two new items

There are two new items in Listing 1 that you did not see in the code in the previous modules.

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.

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:

(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

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:

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 module 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\

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

Confirm that the tape is ready

Calling the method named playTape can be thought of as sending a message to the Combo object asking it to play the tape. The code in the playTape method checks to confirm that the value stored in the tapeIn variable is true before executing the request 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 .

class Radio{
  protected double[] stationNumber = 
                         new double[5];
  protected boolean tapeIn = false

Tape status

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

  protected boolean tapeIn = false;

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 module, 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 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")

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 .

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 Image 1 .

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

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 Image 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.

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.

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.

/*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

-end-