java - Issue with declaration of Map<String,Class<? extends Serializable>> -

java provides me <? extends class> way of filtering java classes can use build in case new hashmap, example:

i can that:

map<string,? extends serializable> map1 = new hashmap<string,string>(); 

it correct, because string implements serializable, compiler let me that.

but when try it:

map<string,genericclass<? extends serializable>> map2 = new hashmap<string, genericclass<string>>(); 

being genericclass it:

public class genericclass<t> { . . . } 

the compiler throw error saying:

type mismatch: cannot convert hashmap<string,genericclass<string>> map<string,genericclass<? extends serializable>> 

i know, happen?

maybe compiler cannot detect extends class being part of generic type.

you need use following:

map<string, ? extends genericclass<? extends serializable>> map2 =         new hashmap<string, genericclass<string>>(); 

nested wildcards different top-level wildcards - latter perform wildcard capture. result, hashmap<string, genericclass<string>> considered inconvertible map<string, genericclass<? extends serializable>>, because genericclass<? extends serializable> concrete type argument (and because generics aren't covariant).

see post further information on nested wildcards: multiple wildcards on generic methods makes java compiler (and me!) confused