How are Java generics different from C++ templates? Why can’t I use int as a parameter?

Java generics are so different from C++ templates that I am not going to try to list the differences here. (See What are the differences between “generic” types in C++ and Java? for more details.)

In this particular case, the problem is that you cannot use primitives as generic type parameters (see JLS §4.5.1: “Type arguments may be either reference types or wildcards.”).

However, due to autoboxing, you can do things like:

List<Integer> ints = new ArrayList<Integer>();
ints.add(3); // 3 is autoboxed into Integer.valueOf(3)

So that removes some of the pain. It definitely hurts runtime efficiency, though.