Q about interfaces: implicit conversion

[Gian Franco]

Hello all,
In a quiz I got the question: Implicit converting an interface
to a class type is never allowed. (TRUE or FALSE?)
I think it's TRUE, because classes always implement interfaces
and therefore only a class can be seen as IS-A interface. But not
the other way round.
The answer though is false and says: an interface type can be converted
to a class type, only if the class implements the interface.
I think this is right only if in the question it hadn't explicity
mentioned the word 'implicit'.
Am I right?
Best regards,
Gian Franco Casula

[Madhav Lakkapragada]

I think it's TRUE, because classes always implement interfaces
and therefore only a class can be seen as IS-A interface. But not
the other way round.
This the design/implementation part.
The answer though is false and says: an interface type can be converted
to a class type, only if the class implements the interface.
I think this is right only if in the question it hadn't explicity
mentioned the word 'implicit'.
Am I right?

This statement talks about variable types, not about the interface
or class. You seem to be mixing concepts here.....
The mention of the word [i]implicit[/b] in the question is valid, IMO.
To me it means that you don't have to type-cast.
Other comments welcome....

[Bijesh Krishnadas]

IMHO the answer is TRUE. An interface type cannot be implicitly converted to a class-type.
For e.g. code below will not compile

public interface Interface1 
{
}
public class Implementor implements Interface1
{
  public Implementor()
  {
  }
  public static void main(String[] args)
  {
    Implementor implementor = null;
    Interface1 interfac = new Implementor();
    implementor = interfac;
  }
}

U get a compile error on line "implementor = interfac;" But it would compile if changed to "implementor = (Implementor)interfac;"
Am I right?
Bijesh

[Corey McGlone]

Originally posted by Bijesh Krishnadas:

Am I right?

That is correct. A reference to an interface is never implicitly cast as an implementing class. One possible reason for this might be the fact that multiple classes might implement the same interface. Therefore, it is impossible to know which class to cast the object as.
You can explicitly cast an object, but it is never done implicitly in this frame of reference.

[CH Lee]

Errrrr......one bartender said the answer is FALSE (to use the IMPLICIT in that reference) but another bartender said it is TRUE (as interface is never implicitly cast as an implementing class). I am a bit confused. Could someone please explains this further. Thank you.
Cheers.

[Bijesh Krishnadas]

There is no implicit conversion of
> Interface type into implementing class
nor of
> super-class type into sub-class.
Explicit conversion will compile but runs the risk of ClassCastException if improper casting is done
Bijesh

[Gian Franco]

Hi there,
coming back to my original question.
The authors of the quiz reformulated the question:
Question: Converting an interface type to a class type is never allowed.
Options:
True
False
Correct Answer:
B
Explanation.
An interface type can be converted to a class type, only if the class implements the interface.
A reference to an interface is never implicitly cast as an implementing class.

Best regards,
Gian Franco Casula