Question

Sometimes I just don’t get generics. I often use the most generic version of the collections in the code. For instance if I need a set of just anything I would write something like:

Set<?> set1 = new HashSet<Object>();

It is allowed by the compiler and why shouldn’t it – Set<?> is a as general as Set<Object> (or even more generic..). However if I use “generics of generics” making it “more generic” just doesn’t work:

Set<Class<?>> singletonSet = new HashSet<Class<Object>>(); // type mismatch

What is going on? Why is Set<Object> assignable to Set<?> and Set<Class<Object>> isn’t assignable to Set<Class<?>>?

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I always find a way around these kinds of problems but in this case I really want to know why this isn’t allowed and not a work-around.

Answer 1

Generics are not covariant in Java. This question may help you:

java generics covariance