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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

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
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
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
Daniel
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
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
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
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
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.
Tarell
what is the actual application of fullerenes nowadays?
Damian
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.
Tarell
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.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
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.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
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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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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