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The class named B defines a method named m() . The behavior of the method is simply to display a message each time it is called.

The class named C

Listing 3 contains the definition of the class named C , which also extends Object .

Listing 3 . Definition of the class named C.
class C extends Object{ //this class is empty}//end class C

The class named C is also empty. It was included in this example as a sibling class for the class named A . Stated differently, it was included as a class that is not in the ancestral line of the class named B .

The driver class

Listing 4 shows the beginning of the driver class named Poly02 .

Listing 4 . Beginning of the class named Poly02.
public class Poly02{ public static void main(String[]args){ Object var = new B();

An object of the class named B

The code in Listing 4 instantiates an object of the class B and assigns the object's reference to a reference variable of type Object .

(It is important to note that the reference to the object of type B was not assigned to a reference variable of type B . Instead, it was assigned to a reference variable of type Object .)

This assignment is allowable because Object is a superclass of B . In other words, the reference to the object of the class B is assignment compatible with a reference variable of the type Object .

Automatic type conversion

In this case, the reference of type B is automatically converted to type Object and assigned to the reference variable of type Object . (Note that the use of a cast operator was not required in this assignment.)

Only part of the story

However, assignment compatibility is only part of the story. The simple fact that a reference is assignment compatible with a reference variable of a given type says nothing about what can be done with the reference after it is assigned to the reference variable.

An illegal operation

For example, in this case, the reference variable that was automatically converted to type Object cannot be used directly to call the method named m() on the object of type B . This is indicated in Listing 5 .

Listing 5 . An illegal operation.
//var.m();

An attempt to call the method named m() on the reference variable of type Object in Listing 5 resulted in a compiler error. It was necessary to convert the statement into a comment in order to get the program to compile successfully.

An important rule

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.

This case violates the rule

In this case, the method named m() is defined in the class named B , which is two levels down from the class named Object .

When the reference to the object of the class B was assigned to the reference variable of type Object , the type of the reference was automatically converted to type Object .

Therefore, because the reference is of type Object , it cannot be used directly to call the method named m() .

The solution is a downcast

In this case, the solution to the problem is a downcast. The code in Listing 6 shows an attempt to solve the problem by casting the reference down the hierarchy to type A .

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..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
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.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
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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
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