why map interface do not extend Collection interface though it is a part of Collection framework

[ajay mittal]

Hi all,

I want to ask that though Map interface is a part of Collection framework but still it do not extends Collection interface.

Why is it so.

Regards,
Ajay Mittal

[William Brogden]

Exactly how would you have a Map implement the methods of Collection?

For example, what would add( Object x ) mean since a Map needs both a key and a value?

Bill

[Saral Saxena]

Hi Folks,
Yeah but is this is the only technical reason that why map is not included..??

[Bear Bibeault]

What other reason would be needed?

[Saifuddin Merchant]

The Collection Interface has nothing to do with the Collection Framework (well it does, but that's other point). They just happen to share a name.

Collection framework = Collection Interface + Map Interface + possible future interfaces.

For e.g. consider 'Table' - in Google Guava - if something like that or a Tuple ever get added to Collection Framework it most probably would have a new Interface.

Probably someone should have tried using a different name for the 'Collection Interface' or probably the Map is a later addition to the Collection framework and the folks an 'Sun' decided not to rename the framework to Collection-Map framework!

Well let me leave you a questions - why is Java called Java when it has nothing to do with Java?

[ajay mittal]

Hi all,

Thanks a lot all for your opinions and i do rely on it. But is this the only reason for not extending the Collection interface. In the next versions of jdk ,they could even make a interface Map that extends Collection interface and have it's own methods also as put(key,value).
I think there are some more reasons we need to peep through.

Ajay Mittal

[fred rosenberger]

I don't think you really understand what it means to extend an interface.

BY DEFINITION, if a class extends an interface, it MUST implement each and every method in said interface. The Collections interface has a method, as William pointed out, that looks like this:

add(Object X)

Even if there were a "interface Map extends Collections", anything that implemented that interface would still have to have an "add (Object X)", regardless of what other new methods were declared. And a Map simply cannot meet this requirement.

[Stephan van Hulst]

And another thing, the point of the Collection interface is that it's generic. A Map is *not* a collection of just anything. A Map is a collection of key-value pairs. That's a very specific restriction, and so it makes no sense to let it extend the generic collection interface.