Question

Please help me with this:

If Lion IS-A Animal and given Cage<T>:

Cage<? extends Animal> c = new Cage<Lion>(); // ok,

but

Set<Cage<? extends Animal>> cc = new HashSet<Cage<Lion>>(); // not ok

What I don’t see here?

Answer 1

When assigning to a variable (Set<T>) with a non-wildcard generic type T, the object being assigned must have exactly T as its generic type (including all generic type parameters of T, wildcard and non-wildcard). In your case T is Cage<Lion>, which is not the same type as Cage<? extends Animal>.

What you can do, because Cage<Lion> is assignable to Cage<? extends Animal>, is use the wildcard type:

Set<? extends Cage<? extends Animal>> a = new Set<Cage<Lion>>();