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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

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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