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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

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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