java bounded wildcards understanding issue -


this question has answer here:

i new java , read many articles on net ? super t or ? extends t still don't it. here example:

public class a<t> {      private t t;      public t gett() {         return t;     }      public void sett(t t) {         this.t = t;     }      public void doextends(a<? extends t> b) {         t t = b.gett();      //ok         this.sett(b.gett()); // ok         b.sett(this.gett()); // error     }     public void dosuper(a<? super t> b) {         t t = b.gett();       // error         this.sett(b.gett()); // error         b.sett(this.gett()); // ok     } }

why there errors? question different 1 referenced in comments

the thing follows:

in doextends(a<? extends t> b) specify b of type a encapsulating may unknown subclass of t. such fine t t = b.gett() since b.gett() produces extends t (so can cast t). same holds true this.sett(b.gett()) - equivalent this.sett(t). let's @ method's last line b.sett(...). said before a contains unknown subclass of t. want stuff in t (this.gett() produces t) , cannot sure t matches subclass may inside b - that's why doesn't work.

next on dosuper(a<? super t> b) - here b contains unknown superclass of t or maybe t itself. that's why last line - setting value - fine. this.gett() produces t can cast of superclasses. when value b.gett() known it's unknown superclass of t not guaranteed t. don't know class be.


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