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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.


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.


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

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


Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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