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Such a conversion will often result in the loss of data, and that loss is the responsibility of the programmerwho performs the cast.

Assignment compatibility for references

Assignment compatibility for references doesn't involve range issues, as is the case with primitives. Rather, the reference to an object instantiated from agiven class can be assigned to (is assignment-compatible with):

  1. Any reference variable whose type is the same as the class from which the object was instantiated.
  2. Any reference variable whose type is a superclass of the class from which the object was instantiated.
  3. Any reference variable whose type is an interface that is implemented by the class from which the object was instantiated.
  4. Any reference variable whose type is an interface that is implemented by a superclass of the class from which the object was instantiated.
  5. A couple of other cases involving interfaces that extend other interfaces.

In this module, we are interested only in cases 1 and 2 above. We will be interested in the other cases in future modules involving interfaces.

Such an assignment does not require the use of a cast operator.

Type Object is completely generic

As mentioned earlier, a reference to any object can be assigned to a reference variable of the type Object , because the Object class is a superclass of every other class.

Converting reference types with a cast

Assignments of references, other than those listed above, require the use of a cast operator to purposely change the type of the reference.

However, it is not possible to perform a successful cast to convert the type of a reference to another type in all cases.

Generally, a cast can only be performed among reference types that fall on the same ancestral line of the class hierarchy, or on an ancestral line of aninterface hierarchy. For example, a reference cannot be successfully cast to the type of a sibling or a cousin in the class hierarchy.


When we cast a reference along the class hierarchy in a direction away from the root class Object toward the leaves , we often refer to it as a downcast .

While it is also possible to cast in the direction from the leaves to the root , this conversion happens automatically, and the use of a cast operator is not required.


A sample program is provided that illustrates much of the detail involved in type conversion, method invocation, and casting with respect to referencetypes.

Discussion and sample code

The program named Polymorph02 , shown in Listing 11 near the end of the module illustrates the use of the cast operator with references.

When you examine that program, you will see that two classes named A and C each extend the class named Object . Hence, we might say that they are siblings in the class hierarchy.

Another class named B extends the class named A . Thus, we might say that A is a child of Object , and B is a child of A .

The class named A

The definition of the class named A is shown in Listing 1 . This class implicitly extends the class named Object by default.

Questions & Answers

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
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.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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