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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

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
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.
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