Wrapper reference variables can be null : Boxing Unboxing « Utility Classes « SCJP

1. SCJP
2. Utility Classes
3. Boxing Unboxing

public class MainClass{
      static Integer x;
      public static void main(String [] args) {
        doStuff(x);
      }
      static void doStuff(int z) {
        int z2 = 5;
        System.out.println(z2 + z);
      } 
    }

x doesnt refer to an Integer object, so theres no value to unbox.