implementation of Interface

[Selva Prasad Rajendran]

In one the test which i took had a question on interface. the answerrs that were in close relation to that question were an abstract class that implemented the interface and another one was an interface that extended the interface.

i marked the answer as the abstact class that implemented the interface but i was given wrong.

my doubt is whether an abstract class implementing an interface has to implement all its method or else should it redefine those methods if it is not implementing that method.

please clear me

[Joyce Lee]

Hi Selve,

Was it a mock question? The question could be ambiguous. You may want to post the mock question here.

An abstract class, just like any concrete class, can implement any number of interfaces. The difference is an abstract class can choose to implement part of an interface or simply leave it out for the concrete subclasses to implement.

An interface extends another interface.

Joyce
[ September 07, 2004: Message edited by: Joyce Lee ]

[Oneal Shaha]

Originally posted by Selva Prasad Rajendran:
my doubt is whether an abstract class implementing an interface has to implement all its method or else should it redefine those methods if it is not implementing that method.

If abstract class is implementing any interface it is free to implement its zero or more methods. But there is no compulsion in implementing any interface method.

But the concrete class which extends this abstract class has to implement all the abstract methods of the class plus the interface methods which are not implemeted be the abstract class

Hope this makes your doubt clear.

[Selva Prasad Rajendran]

i got the doubt cleared and i under stood but the question in one of the test engine was this


Given the following interface definition, which definition are valid


interface I
{ void setvalue(int val);
int getvalue( );
}


a) class A extends I {
int value;
void setValue(int val){
value=val;
}
int getvalue( ){
return value;
}
}

b) interface B extends I{
void increment( );
}



c)abstact class C implements I{

int getvalue( ){ return 0;}
abstract void increment( );
}

d)
interface D implements {

void increment( );
}


e) class E implements I {
int value;
public void setvalue(int val){ value= val;}
}



In this above question i marked b and c but in the answers its given that only b is the correct answer please explain me why c cant be the answer. but i feel that c is also correct because the absract calss can implement the interface and pass the impementation to the next concrete subclass that extends the abstact class.
please any one clear me

[Jimmy Praet]

An interface's methods are implicitly public abstract.

Class C declares getValue() with default package access, which makes it an illegal implementation of interface I.
[ September 08, 2004: Message edited by: Jimmy Praet ]