Question from mock exam for SCJP 5

Question

Hi everyone,
I have encountered this question:
class Game{ }
class Cricket extends Game{ }
class Instrument{ }
class Guitar extends Instrument{ }
interface Player<E>{ void play(E e); }
interface GamePlayer<E extends Game> extends Player<E>{ }
interface MusicPlayer<E extends Instrument> extends Player{ }Identify valid declarations.
And one of the possible answers is (which is not correct):
class MidiPlayer implements MysicPlayer {
   public void play(Guitar) {}
}And the explanation given why it is a wrong answer is:
"Observe that this is a non-typed usage of MusicPlayer. Since MusicPlayer has not been typed to
anything, that means, it should be able to work with any object. Thus, MidiPlayer must have a
method play(Object )."
But this explanation in my opinion is wrong as play(Object) would not satisfy MusicPlayer
interface. It would have to be play(Instrument) ... Am I right here ?
Cheers,
Adrian

Answer 1

But this explanation in my opinion is wrong as play(Object) would not satisfy MusicPlayer
interface. It would have to be play(Instrument) ... Am I right here ?
Cheers,
AdrianYes, and no.
play(Object) WOULD satisfy the MusicPlayer interface, because Object is the super class of Instrument. The requirement for a class to implement an interface is that its parameter types be able to accept the types specified in the interface, and its return type be a type or subtype of the return type from the interface.
So technially, it should be instrument, BUT
It is perfectly valid to accept any super type of Instrument, which includes java.lang.Object. The point is that if I do this:
MusicPlayer musicPlayer = new MidiPlayer();
musicPlayer.play(new Guitar());This MUST be legal. Since MidiPlayer has a play operation that is void and accepts an Object (which a Guitar happens to be a subtype of), this is legal. The restriction is that the play method cannot take type Guitar, because not all Instruments are Guitars, and the play method cannot take String, for example, because an Instrument is not a String. But it can define a parameter of type Object, because all Instruments are Objects.
I'm not sure if that made much sense, but that's the answer you're looking for. Here's the relevent section from the JLS:
http://java.sun.com/docs/books/jls/third_edition/html/classes.html#38649
http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.8.3
http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.8.4
- Adam