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  • Any reference variable whose type is the same as the class from which the object was instantiated.
  • Any reference variable whose type is a superclass of the class from which the object was instantiated.
  • Any reference variable whose type is an interface that is implemented by the class from which the object was instantiated.
  • Any reference variable whose type is an interface that is implemented by a superclass of the class from which the object was instantiated, and
  • A few other cases involving the class and interface hierarchy.

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

Type Object is completely generic

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.

Doesn't work in all cases

However, it is not possible to perform a successful cast to convert the type of a reference 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 an interface hierarchy. For example, a reference cannot be successfully cast to the type of a sibling or a cousin in the class hierarchy.

Downcasting

When we cast a reference along the class hierarchy in a direction 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 happens automatically, and the use of a cast operator is not required.

A sample program

The program named Poly02 , 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 extends the class named Object .

(Recall that it is not necessary to explicitly state that a class extends the class named Object . Any class that does not explicitly extend some other class will automatically extend Object by default. The class named A is shown to extend Object here simply for clarity of presentation.)

Listing 1 . Definition of the class named A.
class A extends Object{ //this class is empty}//end class A

The class named A is empty. It was included in this example for the sole purpose of adding a layer of inheritance to the class hierarchy.

The class named B

Listing 2 shows the definition of the class named B . This class extends the class named A .

Listing 2 . Definition of the classnamed B.
class B extends A{ public void m(){System.out.println("m in class B"); }//end method m()}//end class B

The method named m()

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..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
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.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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