Fun with constants in Java

The following code does not compile:

public class A {    public static void main(String[] args) {        int i = Integer.parseInt(args[0]);        switch (i) {            case X: System.out.println("ONE!"); break;            case Y: System.out.println("TWO!"); break;            default: System.out.println("ZERO!"); break;        }    }    public static final int X = B.getNumber("X");    public static final int Y = B.getNumber("Y");}class B {    public static int getNumber(String s) {        if (s.equals("X")) {            return 1;        }        else if (s.equals("Y")) {            return 2;        }        else {            return 0;        }    }}

The compile error is: constant expression required            case X: System.out.println("ONE!"); break;                 ^ constant expression required            case Y: System.out.println("TWO!"); break;                 ^2 errors

I think I'll leave explaining why as an exercise for the reader. (Or for a future post. ;-)