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An abstract method doesn't have a body.

Abstract classes and methods

A class that contains an abstract method must itself be declared abstract . However, an abstract class is not required to contain abstract methods.

Failed to declare the class abstract

In this program, the class named Base contains an abstract method named test , but the class is not declared abstract as required.

class Base{ abstract public void test();}//end class Base

Therefore, the program produces the following compiler error under JDK 1.3:

Ap122.java:24: Base should be declared abstract;it does not define test in Base class Base{

Back to Question 3

Answer 2

C. A

Explanation 2

If you missed this ...

If you missed this question, you didn't pay attention to the explanation for Question 1 .

Define a method in a subclass

This program defines a subclass named A that extends a superclass named Base . A method named test is defined in the subclass named A but is not defined in any superclass of the class named A .

Store a reference as a superclass type

The program declares a reference variable of the superclass type, and stores a reference to an object of the subclass in that reference variable as shown inthe following code fragment.

Base myVar = new A();

Downcast and call the method

Then the program calls the method named test on the reference stored as the superclass type, as shown in the following fragment.

((A)myVar).test();

Unlike the program in Question 1 , the reference is downcast to the true type of the object before calling the method named test . As a result, this program does not produce a compiler error.

Why is the cast required?

As explained in Question 1 , it is allowable to store a reference to a subclass object in a variable of a superclass type. Also,as explained in Question 1 , it is not allowable to directly call, on that superclass reference, a method of the subclass objectthat is not defined in or inherited into the superclass.

However, such a call is allowable if the programmer purposely downcasts the reference to the true type of the object before calling the method.

Back to Question 2

Answer 1

A. Compiler Error

Explanation 1

Define a method in a subclass

This program defines a subclass named A that extends a superclass named Base . A method named test , is defined in the subclass named A , which is not defined in any superclass of the class named A .

Store a reference as superclass type

The program declares a reference variable of the superclass type, and stores a reference to an object of the subclass in that reference variable as shown inthe following code fragment.

Base myVar = new A();

Note that no cast is required to store a reference to a subclass object in a reference variable of a superclass type. The required type conversion happensautomatically in this case.

Call a method on the reference

Then the program attempts to call the method named test on the reference stored as the superclass type, as shown in the following fragment.This produces a compiler error.

myVar.test();

The reason for the error

It is allowable to store a reference to a subclass object in a variable of a superclass type. However, it is not allowable to directly call, (on that superclass reference) , a method of the subclass object that is not defined in or inherited into the superclass.

The following error message is produced by JDK 1.3.

Ap120.java:18: cannot resolve symbol symbol : method test ()location: class Base myVar.test();

The solution is ...

This error can be avoided by casting the reference to type A before calling the method as shown below:

((A)myVar).test();

Back to Question 1

-end-

Questions & Answers

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
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
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
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
Daniel
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
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
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
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
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.
Tarell
what is the actual application of fullerenes nowadays?
Damian
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.
Tarell
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.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
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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