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Except for type boolean , values of primitive types can be assigned to any variable whose type represents a range that is as wide or wider than the range of the value's type. (Values of type boolean can only be assigned to variables of type boolean .)

With respect to reference types, the reference to an object instantiated from a given class can be assigned to any of the following without the use of a cast:

  • Any reference variable whose type is the same as the class from which the object was instantiated.
  • Any reference variable whose type is a superclass of the class from which the object was instantiated.
  • Any reference variable whose type is an interface that is implemented by the class from which the object was instantiated.
  • Any reference variable whose type is an interface that is implemented by a superclass of the class from which the object was instantiated.
  • A few other cases involving the class and interface hierarchy.

Assignments of references, other than those listed above, require the use of a cast to change the type of the reference.

It is not always possible to perform a successful cast to convert the type of a reference. Whether or not a cast can be successfully performed depends on the relationship of the classes involved in the class hierarchy.

A reference to any object can be assigned to a reference variable of the type Object , because the Object class is a superclass of every other class.

When we cast a reference along the class hierarchy in a direction from the root class Object toward the leaves, we often refer to it as a downcast .

Whether or not a method can be called on a reference to an object depends on the current type of the reference and the location in the class hierarchy where the method is defined. In order to use a reference of a class type to call a method, the method must be defined at or above that class in the class hierarchy.

A sample program is provided that illustrates much of the detail involved in type conversion, method invocation, and casting with respect to reference types.

What's next?

I will begin the discussion of runtime polymorphism through method overriding and inheritance in the next module.

I will demonstrate that for runtime polymorphism, the selection of a method for execution is based on the actual type of object whose reference is stored in a reference variable, and not on the type of the reference variable on which the method is called.


This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Java OOP: Polymorphism, Type Conversion, Casting, etc.
  • File: Java1610.htm
  • Published: 02/26/02

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 listings

A complete listing of the program is shown in Listing 11 below.

Listing 11 . Complete program listing.
/*File Poly02.java Copyright 2002, R.G.BaldwinThis program illustrates downcasting Program output is:m in class B**************************************/ class A extends Object{//this class is empty }//end class A//===================================// class B extends A{public void m(){ System.out.println("m in class B");}//end method m() }//end class B//===================================// class C extends Object{//this class is empty }//end class C//===================================// public class Poly02{public static void main(String[] args){Object var = new B(); //Following will not compile//var.m(); //Following will not compile//((A)var).m(); //Following will compile and run((B)var).m();//Following will compile and run B v1 = (B)var;//Following will not execute //C v2 = (C)var;//Following will not compile //C v3 = (B)var;}//end main }//end class Poly02


Questions & Answers

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.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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