<< Chapter < Page Chapter >> Page >
Listing 6 . A new object of type A.
var = new A(); ((A)var).m();

A new object of type A

The code in Listing 6 instantiates a new object of the class named A , and stores the object's reference in the original reference variable named var of type Object .

(As a side note, this overwrites the previous contents of the reference variable with a new reference and causes the object whose reference was previously stored there to become eligible for garbage collection.)

Downcast and call the method

Then the code in Listing 6 casts the reference down to type A , (the type of the object to which the reference refers) , and calls the method named m() on the downcast reference.

The output

As you would probably predict, this produces the following output on the computer screen:

m in class A

In this case, the version of the method defined in the class named A , (not the version defined in B ) was executed.

Not polymorphic behavior

In my view, this is not polymorphic behavior (at least it isn't a very useful form of polymorphic behavior) . This code simply converts the type of the reference from type Object to the type of the class from which the object was instantiated, and calls one of its methods. Nothing special takes place regarding a selection among different versions of the method.

Some authors may disagree

While some authors might argue that this is technically runtime polymorphic behavior, in my view at least, it does not illustrate the real benefits of runtime polymorphic behavior. The benefits of runtime polymorphic behavior generally accrue when the actual type of the object is a subclass of the type of the reference variable containing the reference to the object.

Once again, what is runtime polymorphism?

As I have discussed in this module, runtime polymorphic behavior based on class inheritance occurs when

  • The type of the reference is a superclass of the class from which the object was instantiated.
  • The version of the method that is executed is the version that is either defined in, or inherited into, the class from which the object was instantiated.

More than you ever wanted to hear

And that is probably more than you ever wanted to hear about runtime polymorphism based on class inheritance.

A future module will discuss runtime polymorphism based on the Java interface. From a practical viewpoint, you will find the rules to be similar but somewhat different in the case of the Java interface.

A very important concept

As an example of the importance of runtime polymorphism in Java, the entire event-driven graphical user interface structure of Java is based on runtime polymorphism involving the Java interface.


Polymorphism manifests itself in Java in the form of multiple methods having the same name.

From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

  • Method overloading
  • Method overriding through class inheritance
  • Method overriding through the Java interface

This module discusses method overriding through class inheritance.

With runtime polymorphism based on method overriding, the decision as to which version of a method will be executed is based on the actual type of object whose reference is stored in the reference variable, and not on the type of the reference variable on which the method is called.

The decision as to which version of the method to call cannot be made at compile time. That decision must be deferred and made at runtime. This is sometimes referred to as late binding.

This is illustrated in the sample program discussed in this module.

What's next?

In the next module, I will continue my discussion of the implementation of polymorphism using method overriding through class inheritance, and I will concentrate on a special case in that category.

Specifically, I will discuss the use of the Object class as a completely generic type for storing references to objects of subclass types, and explain how that results in a very useful form of runtime polymorphism.


This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Java OOP: Runtime Polymorphism through Class Inheritance
  • File: Java1612.htm
  • Published: 02/27/02

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 misappropriated copies of my modules from cnx.org, converted them to Kindle books, andplaced them for sale on Amazon.com showing me as the author. I receive no compensation for those sales and don't know who doesreceive compensation. If you purchase such a book, please be aware that it is a bootleg copy of a module that is freelyavailable on cnx.org.

Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

Complete program listing

A complete listing of the program is shown in Listing 7 below.

Listing 7 . Listing 7 : Complete program listing.
/*File Poly03.java Copyright 2002, R.G.BaldwinThis program illustrates downcasting and polymorphic behaviorProgram output is:m in class B m in class Bm in class A **************************************/class A extends Object{ public void m(){System.out.println("m in class A"); }//end method m()}//end class A //===================================//class B extends A{ public void m(){System.out.println("m in class B"); }//end method m()}//end class B //===================================//public class Poly03{ public static void main(String[]args){ Object var = new B();//Following will compile and run ((B)var).m();//Following will also compile // and run due to polymorphic// behavior. ((A)var).m();//Following will not compile //var.m();//Instantiate obj of class A var = new A();//Call the method on it ((A)var).m();}//end main }//end class Poly03


Questions & Answers

how can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
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
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
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
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now

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
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Object-oriented programming (oop) with java' conversation and receive update notifications?