How to get value type of a map in Java?

Possible Duplicate:
Get generic type of java.util.List

I have a Map and I want to get the type of T from an instance of that Map. How can I do that?

e.g. I want to do something like:

Map<String, Double> map = new HashMap<String, Double>();
...
String vtype = map.getValueType().getClass().getName(); //I want to get Double here

Of course there's no such 'getValueType()' function in the API.