The context is what was said above: Java generics can't abstract over primitive types, they only work for objects. That is awful, and it is important, because this one limitation is the only thing that makes the implementation of generics in Java possible and simple.

If generics had had to actually support ArrayList<int>, they would have required significant changes to how ArrayList is stored in memory, and to how a lot functions which work with generic types are actually compiled. Instead, Java has the easy path in the implementation: since everything only works with reference types, there's no need for multiple copies of a function to work with different types.

Of course, this means that sum<Integer>(new Integer[1000000]) is going to be orders of magnitude slower than sumInts(new int[1000000]).