Java Map compiler error with generic

Question

// I know that this method will generated duplicate 
// trim keys for the same value but I am just
// trying to understand why we have a compile error:
// The method         


        

Answer 1

? is not a wildcard for any type but the unknown type. So the map only accepts ?-type objects and not String. One conclusion from this (rather strange) fact: We can't add values to collections that are parametized with ?.