java - Why can't the Integer.class object be added to a Collection<Class<Number>> object? -


this question has answer here:

i thought have quite understanding of generics can't figure out why compiler error here:

    collection<class<number>> ncs = new hashset<class<number>>();                    ncs.add(number.class);      //ok!     ncs.add(integer.class);     //compiler error here

would nice if explain this. :)

edit: understand why it's not possible add integer.class object collection> pointed out in this question. don't understand why example same. of course doesn't work:

collection<? extends number> ns = new hashset<number>(); ns.add(new integer(1)); //compiler error here

but does:

collection<number> ns = new hashset<number>(); ns.add(new integer(1)); //ok!

as far understand initial code uses same concepts.

given classes x, y, , z, x subclass of y, x<z> subclass of y<z> (e.g. you've done collection , hashset), z<x> not subclass of z<y>, class<integer> instance not class<number> , can't put former in collection of latter. latter need wildcards, romeara has shown.

see this question.


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