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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

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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