There is really no reason to use the Integer class for something as simple:
int x = 7, y = 4;
int d = y+x;
The difference is where the information is stored (primitives are on the stack, while Class instances are put on the heap). Primitives are the ones that you can add subtract and so on. Classes (with the exception of String) do not have those options.
(sorry for beating the topic to death, it took me a long time to reply, so i didnt see the other two answers getting posted)