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

Question

What does Map<?, ?> mean in Java?
I've looked online but can't seem to find any articles on it.

edit : I found this on MP3 Duration Java

Answer 1

Map<?,?> means that at compile time, you do not know what the class type of the key and value object of the Map is going to be.

Its a wildcard type. http://download.oracle.com/javase/tutorial/extra/generics/wildcards.html

Answer 2

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

HashMap<?, ?> foo = new HashMap<Integer, String>();

And since ? is a wildcard, you can skip it and still get the same result:

HashMap foo = new HashMap<Integer, String>();

But they can be used to specify or subset the generics to be used. In this example, the first generic must implement the Serializable interface.

// would fail because HttpServletRequest does not implement Serializable
HashMap<? extends Serializable, ?> foo = new HashMap<HttpServletRequest, String>(); 

But it's always better to use concrete classes instead of these wildcards. You should only use ? if you know what your are doing :)