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Interface type is not used

The program makes no use of the interface as a type, and exhibits no polymorphic behavior (no decision among methods having the same signature is required) .

The class named A inherits an abstract method named intfcMethod from the interface and must define it. (Otherwise, it would be necessary to declare the class named A abstract.)

The interface is not a particularly important player in this program.

Back to Question 3

Answer 2

A. Compiler Error

Explanation 2

Simple hierarchical polymorphic behavior

This program is designed to test your knowledge of simple hierarchical polymorphic behavior.

Implement the interface named X

This program defines a class named A that extends a class named Base , and implements an interface named X , as shown in the following code fragment.

class A extends Base implements X{ public void inherMethod(){System.out.print( " A-inherMethod ");}//end inherMethod()public void intfcMethod(){ System.out.print("A-intfcMethod ");}//end intfcMethod() }//end class Ainterface X{ public void intfcMethod();}//end X

Override and define some methods

The class named A overrides the method named inherMethod , which it inherits from the class named Base . It also provides a concrete definition of the method named intfcMethod , which is declared in the interface named X .

Store object's reference as superclass type

The program instantiates an object of the class named A and assigns that object's reference to a reference variable of type Base , as shown in the following code fragment.

Base myVar2 = new A();

Oops! Cannot call this method

So far, so good. However, the next fragment shows where the program turns sour. It attempts to call the method named intfcMethod on the object's reference, which was stored as type Base .


Polymorphic behavior doesn't apply here

Because the class named Base does not define the method named intfcMethod , hierarchical polymorphic behavior does not apply. Therefore a reference to the object being stored as type Base cannot be used to directly call the method named intfcMethod , and the program produces a compiler error.

What is the solution?

Hierarchical polymorphic behavior is possible only when the class defining the type of the reference (or some superclass of that class) contains a definition for the method that is called on the reference.

There are a couple of ways that downcasting could be used to solve the problem in this case.

Back to Question 2

Answer 1

A. Compiler Error

Explanation 1

I put this question in here just to see if you are still awake.

Can store reference as interface type

A reference to an object instantiated from a class can be assigned to any reference variable whose declared type is the name of an interface implementedby the class from which the object was instantiated, or implemented by any superclass of that class.

Define two classes and an interface

This program defines a class named A that extends a class named Base . The class named Base extends Object by default.

The program also defines an interface named X .

Instantiate an object

The following statement instantiates an object of the class named A , and attempts to assign that object's reference to a reference variable whosetype is the interface type named X .

X myVar2 = new A();

Interface X is defined but not implemented

None of the classes named A , Base , and Object implement the interface named X . Therefore, it is not allowable to assign a reference to an object of the classnamed A to a reference variable whose declared type is X . Therefore, the program produces the following compiler error under JDK 1.3:

Ap131.java:20: incompatible types found : Arequired: X X myVar2 = new A();

Back to Question 1


Questions & Answers

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.
what is biological synthesis of nanoparticles
Sanket Reply
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