Assigning values to static members

Question

I have the following problem from nested classes category and I do not understand why the printed values for the 3 variables is the same 3131 and not 3131, 3132, 3133?

class X
{
static int x = 3131;
 
static class Y
{
    static int y = x++;
     
    static class Z
    {
        static int z = y++;
    }
    }
  }

 public class MainClass
  {
 public static void main(String[] args)
 {
    System.out.println(X.x);
     
    System.out.println(X.Y.y);
     
    System.out.println(X.Y.Z.z);
     }
  }