generics - What does Map<?, ?> mean in Java? -

what map<?, ?> mean in java?
i've looked online can't seem find articles on it.

edit : found on mp3 duration java

? indicates placeholder in value not interested in (a wildcard):

hashmap<?, ?> foo = new hashmap<integer, string>(); 

and since ? wildcard, can skip , still same result:

hashmap foo = new hashmap<integer, string>(); 

but can used specify or subset generics used. in example, first generic must implement serializable interface.

// fail because httpservletrequest not implement serializable hashmap<? extends serializable, ?> foo = new hashmap<httpservletrequest, string>();  

but it's better use concrete classes instead of these wildcards. should use ? if know doing :)