Passing Integer to int and vice versa

Question

Just wondered why is it possible to pass Integer as argument where method parameter is of int type and vice versa?

public class Salmon {

 public static Integer foo(Integer a, int b){
  return a - b;
 }
 public static void main(String[] args) {
  Integer a = 10;
  int b = 1;
  foo(b, a);
 }
}

Answer 1

This is auto-boxing and auto-unboxing. Basically the compiler puts in calls to Integer.valueOf() or x.intValue() appropriately.

The exact mechanism isn't actually specified, but the relevant sections of the spec are 5.1.7 and 5.1.8.