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Listing 6 . An ineffective downcast.

Still doesn't solve the problem

However, this still doesn't solve the problem, and the result is another compiler error. Again, it was necessary to convert the statement into a comment in order to get the program to compile.

What is the problem here?

The problem is that the downcast simply didn't go far enough down the inheritance hierarchy.

The class named A neither defines nor inherits the method named m() . The method named m() is defined in class B , which is a subclass of class A .

Therefore, a reference of type A is no more useful than a reference of type Object insofar as calling the method named m() is concerned.

The real solution

The solution to the problem is shown in Listing 7 .

Listing 7 . A downcast to type B.

The code in Listing 7 casts (temporarily converts) the reference value contained in the Object variable named var down to type B .

The method named m() is defined in the class named B . Therefore, a reference of type B can be used to call the method.

The code in Listing 7 compiles and executes successfully. This causes the method named m() to execute, producing the following output on the computer screen.

m in class B

A few odds and ends

Before leaving this topic, let's look at a couple more issues. The code in Listing 8 declares and populates a new variable of type B .

Listing 8 . Declare a variable of type B.
B v1 = (B)var;

The code in Listing 8 also uses a cast to:

  • Convert the contents of the Object variable to type B
  • Assign the converted reference to the new reference variable of type B.

A legal operation

This is a legal operation. In this class hierarchy, the reference to the object of the class B can be assigned to a reference variable of the types B , A , or Object .

Cannot be assigned to type C

However, the reference to the object of the class B cannot be assigned to a reference variable of any other type, including the type C . An attempt to do so is shown in Listing 9 .

Listing 9 . Cannot be assigned to type C.
//C v2 = (C)var;

The code in Listing 9 attempts to cast the reference to type C and assign it to a reference variable of type C .

A runtime error

Although the program will compile, it won't execute. An attempt to execute the statement in Listing 9 results in a ClassCastException at runtime. As a result, it was necessary to convert the statement into a comment in order to execute the program.

Another failed attempt

Similarly, an attempt to cast the reference to type B and assign it to a reference variable of type C , as shown in Listing 10 , won't compile.

Listing 10 . Another failed attempt.
//C v3 = (B)var;

The problem here is that the class C is not a superclass of the class named B . Therefore, a reference of type B is not assignment compatible with a reference variable of type C .

Again, it was necessary to convert the statement into a comment in order to compile the program.


This module discusses type conversion for both primitive and reference types.

A value of a particular type may be assignment compatible with variables of other types.

If the type of a value is not assignment compatible with a variable of a given type, it may be possible to perform a cast on the value to change its type and assign it to the variable as the new type. For primitive types, this will often result in the loss of information.

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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