The incomparable types error is telling you that it doesn't make sense to compare two things that cannot possibly be equal.

For example, there's no point in Integer.valueOf(0) == "", because they're not the same types; nor is one a supertype of the other. It will always be false.

The compiler will prevent the a == b if both are class types (as opposed to interfaces), and both a = b and b = a would be disallowed.

So, you are being told that a Class<? extends T[]> cannot be equal to a Class<Object[]>, because you can't assign a reference of one type to a variable of the other type.

By casting one of the references to Object, the compiler no longer knows (/thinks) that the types are definitely not related - because Object is a supertype of everything, so "nor is one a supertype of the other" is no longer true - so the compiler allows the check.

One thing that is redundant in that method is U. There's no need for it, just use Object[] in the parameter type instead.