11 months late, but I think this question is ready for some Java Wildcard stuff.
This is a syntactical feature of Java. Suppose you have a method:
public <T> void Foo(Collection<T> thing)
And suppose you don't need to refer to the type T in the method body. You're declaring a name T and then only using it once, so why should you have to think of a name for it? Instead, you can write:
public void Foo(Collection<?> thing)
The question-mark asks the the compiler to pretend that you declared a normal named type parameter that only needs to appear once in that spot.
There's nothing you can do with wildcards that you can't also do with a named type parameter (which is how these things are always done in C++ and C#).
