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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 .

The method named m is neither defined in nor inherited into 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 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 .

Listing 6 . Try a downcast to type A.

//Following will not compile //((A)var).m();

Still doesn't solve the problem

However, this still doesn't solve the problem, and the result is another compiler error.

The method named m is neither defined in nor inherited into the class named A .

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

What is the problem here?

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

The class named A does not contain a definition of the method named m . Neither does it inherit the method named m . The method named m is defined in class B, which is a subclass of 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 . Try a downcast to type B.

//Following will compile and run ((B)var).m();

The code in Listing 7 casts (temporarily converts) the reference value contained in the Object variable named var 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 few more issues.

The code in Listing 8 declares and populates a new variable of type B.

Listing 8 . Assign var to v1.

//Following will compile and run B v1 = (B)var;

The code in Listing 8 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 .

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 .

While this operation is legal, it is usually not a good idea to have two different reference variables that contain references to the same object. Inthis case, the variables named var and v1 both contain a reference to the same 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 type C . An attempt to do so is shown in Listing 9 .

Questions & Answers

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
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
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.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
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Source:  OpenStax, Xna game studio. OpenStax CNX. Feb 28, 2014 Download for free at https://legacy.cnx.org/content/col11634/1.6
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