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Call the toString method
The first thing that this version of the println method does is to call the toString method on the incoming reference. Then it displays the string value returned by the toString method on the screen.
In this case, the overridden toString method returns the string "Baldwin" , which is what you see displayed in Figure 1 .
Runtime polymorphism
This is a clear example of an OOP concept known as runtime polymorphism .
Runtime polymorphism is much too complicated to explain in this module. However, I explain it in detail here and here . I strongly recommend that you study it there until you thoroughly understand it.
A critical concept
It is critical that you understand runtime polymorphism if you expect to go further in Java OOP.
It is almost impossible to write a useful Java application without making heavy use of runtime polymorphism. That is the foundation of the eventdriven Java graphical user interface system as well as the collections framework.
Call the overridden getData method
The next statement in Listing 1 calls the overridden getData method and displays the return value.
As you saw earlier, this method returns a copy of the random value that was received and saved by the constructor for the Prob04MyClass class in Listing 2 .
Display the original random value
Finally, the last statement in the main method in Listing 1 displays the contents of the instance variable named randomNumber . This variable contains the random value that was passed to the constructor for the Prob04MyClass earlier in Listing 1 .
The two values must match
Therefore, the final two statements in the main method in Listing 1 display the same random value. This is shown in the command line screen output in Figure 1 .
The program terminates
After displaying this value, the main method terminates causing the program to terminate.
I encourage you to copy the code from Listing 5 , compile it and execute it. Experiment with the code, making changes, and observing the results of yourchanges. Make certain that you can explain why your changes behave as they do.
You have learned about abstract methods, abstract classes, and overridden methods in this module. Very importantly, you have learned about overriding the toString method.
You will learn more about indirection, array objects, and casting in the next module.
While not a requirement of the course, you can select the following links to view optional online video lectures on the material in thismodule.
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 copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
A complete listing of the program discussed in this module is shown in Listing 5 below.
| Listing 5 . Complete program listing. |
|---|
/*File Prob04 Copyright 2001, R.G.Baldwin
Rev 12/16/08*********************************************************/
import java.util.*;abstract class Prob04{
public static void main(String[]args){
Random generator = new Random(new Date().getTime());int randomNumber = (byte)generator.nextInt();
Prob04 objRef = new Prob04MyClass(randomNumber);System.out.println(objRef);
System.out.println(objRef.getData());System.out.println(randomNumber);
}//end main//Declare the signature of an abstract class.
public abstract int getData();}//end class Prob04
//======================================================//class Prob04MyClass extends Prob04{
private int data;public Prob04MyClass(int inData){//constructor
System.out.println("Prob04");System.out.println("Dick");
data = inData;}//end constructor
public int getData(){//overridden abstract methodreturn data;
}//end getData()public String toString(){//overridden method
return "Baldwin";}//end overloaded toString()
}//end class Prob04MyClass |
-end-
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