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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.

Run the program

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.

Summary

You have learned about abstract methods, abstract classes, and overridden methods in this module. Very importantly, you have learned about overriding the toString method.

What's next?

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.

Miscellaneous

This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Java OOP: Abstract Methods, Abstract Classes, and Overridden Methods
  • File: Java3008.htm
  • Published: 08/02/12
Disclaimers:

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.

Complete program listings

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-

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
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John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
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emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
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Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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