difference between Map<?,?> and Map<Object,Object>

Question

when I declare a Map or Map <Object,Object> I can put anything in this Map

Map map = new HashMap();
map.put("");

but if I declare it as Map <?,?> I can put nothing in it

Map<?,?> map = new HashMap();
map.put("");

it will goes wrong why ?

Answer 1

Map<?,?> map = new HashMap<Integer, Integer>(); // compiles just fine!

? represents some fixed but unknown type. You can't put "" in a Map<Integer, Integer>, and a Map<?, ?> is allowed to be any type of Map, including a Map<Integer, Integer>.