java - Using generic bounded wildcards when testing Maps with junit -

i'm trying understand why junit assertion giving me compile time error:

map<string, set<string>> actual = methodtotest(); assertthat(result, hasentry("foo", new hashset<string>(arrays.aslist("bar")))); 

if write way works fine:

map<string, set<string>> actual = methodtotest(); set<string> expected = new hashset<string>(arrays.aslist("bar")); assertthat(result, hasentry("foo", expected)); 

the compiler error first example is:

the method assertthat(t, matcher<? super t>) in type assert not applicable arguments (map<string,set<string>>, matcher<map<? extends string,? extends hashset<string>>>) 

hashset<string> subtype of set<string> why isn't working?

hashset<string> subtype of set<string> true.

however, matcher<map<string,hashset<string>>> not subset of matcher<map<string,set<string>>>. remember list<string> not subtype of list<object>.

the assertthat method expects argument of type matcher<? super map<string, set<string>>> incompatibe matcher<map<string,hashset<string>>>.