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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

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
Do somebody tell me a best nano engineering book for beginners?
s. Reply
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?
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
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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