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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

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..
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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