java - Doubly-Wildcard Generic Type error -

i trying define operator ++ custom map type this:

@override public mutablemap<k, v> $plus$plus(map<? extends k, ? extends v> map) {     hashmap<k, v> copy = this.copy();     map.$plus$plus$eq(map);     return copy; } 

the ++= operator defined this:

public void $plus$plus$eq(map<? extends k, ? extends v> map); 

however, compiler complains on map.$plus$plus$eq(map); line, following crazy error:

the method $plus$plus$eq(map<? extends capture#10-of ? extends k, ? extends capture#11-of ? extends v>) in type map<capture#10-of ? extends k,capture#11-of ? extends v> not applicable arguments (map<capture#12-of ? extends k,capture#13-of ? extends v>) 

as can see in screenshot, none of solutions provided eclipse work, yet alone make sense:

i have been working java generics quite while now, having developed own generic type system custom programming (whose library trying code), have never had error before.

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edit: interestingly, casting map argument raw type (map) seems fix problem.

map.$plus$plus$eq((map) map); 

however, changing cast (map<?, ?>) (which eclipse's second solution does) causes similar error.

it works if use raw type map, lose type safety. don't use raw types. exist compatibility java 1.4 , older, didn't have generics.

it doesn't work wildcards because compiler doesn't know exact types wildcards ? stand for. reason same question why can't call add() on list<? extends t>.

note wildcard not mean can use whatever types of objects type of key extends k , type of value extends v. instead, means have map keys of specific, unknown type k, , values of specific, unknown type v. can't call $plus$plus$eq on such map because compiler doesn't know exact types, can't check them.

in principle, wildcards of map call method on, might stand types different wildcards of map pass argument - though can see in case must same because use same object map in case.

you can fix using type parameters instead of wildcards:

@override public <kk extends k, vv extends v> mutablemap<k, v> $plus$plus(map<kk, vv> map) {     hashmap<k, v> copy = this.copy();     map.$plus$plus$eq(map);     return copy; }