Why Map doesn't extend collection interface while List and Set can extend Collection interface

[Harikrishna Gorrepati]

Hi, Is there any reason Why Map doesn't extend collection interface while List and Set can extend Collection interface ?

[Paul Clapham]

Yes. A Map simply isn't a collection so there is no reason why it should implement Collection.

Also, you could ask yourself how the Map<K, V> interface could possibly implement the Collection<E> interface. When you see it stated in generic terms it's easier to see that it doesn't make sense.

[Harikrishna Gorrepati]

Paul Clapham wrote:Yes. A Map simply isn't a collection so there is no reason why it should implement Collection.

Also, you could ask yourself how the Map<K, V> interface could possibly implement the Collection<E> interface. When you see it stated in generic terms it's easier to see that it doesn't make sense.

hi Paul, There should be a reason for each and everything. Without reason, nothing will exist/happen. could you please provide more information on "Also, you could ask yourself how the Map<K, V> interface could possibly implement the Collection<E> interface. When you see it stated in generic terms it's easier to see that it doesn't make sense." I am not clear

[Paul Clapham]

If you were going to make Map extend Collection what would you do?

public interface Map<K, V> extends Collection<K>

... so that a Map is a collection of its keys... or

public interface Map<K, V> extends Collection<V>

... so that a Map is a collection of its values... or something else?

[Trivikram Kamat]

Hi Harikrishna,

It's true that Map has set of keys and collection of values in it.
So, it will be meaningless for Map to inherit Collection, which has only collection of values.

Collection<E> has methods like:

boolean add(E Element)

boolean addAll(Collection<? extends E> C)

won't mean much in Map, which has set of keys in addition.
That is why, Map<K, V> has second parameter in analogous functions, as follows:

Object put(K key, V value)

void putAll(Map <? extends K, ? extends V> map)