Description Piotr Kobzda 2006-05-15 13:11:34 EDT

There is a bug in binary compatibility of enum 'switch tables' generated by the Eclipse compiler.
This bug probably applies to all Eclipse compiler versions supporting Java enums.


The following sample code is used to demonstrate the problem:

enum Action { ONE, TWO };

class Test {
   public static void main(String[] args) {
      for(Action a : Action.values()) {
         switch(a) {
         case ONE:
            System.out.println("one");
            break;
         case TWO:
            System.out.println("two");
            break;
         default:
            System.out.println("unknown action "+ a);
         }
      }
   }
}

The output of running above Test class is:

one
two


When an Action enum will evolve, for example to the following one:

enum Action { ONE, TWO, THREE };

The result of running Test class (without recompiling) will be:

one
two
one

Instead of expected:

one
two
unknown action THREE


Decompiled version of a Test class explains it in details:

// Decompiled by DJ v3.8.8.85 Copyright 2005 Atanas Neshkov  Date: 2006-05-15 18:39:57
// Home Page : http://members.fortunecity.com/neshkov/dj.html  - Check often for new version!
// Decompiler options: packimports(3) deadcode nonlb space 
// Source File Name:   Test.java

import java.io.PrintStream;

// Referenced classes of package :
//         Action

public class Test {

   public Test() {
   }

   public static void main(String args[]) {
      Action aaction[] = Action.values();
      int i = 0;
      for (int j = aaction.length; i < j; i++) {
         Action a = aaction[i];
         switch ($SWITCH_TABLE$Action()[a.ordinal()]) {
         case 0: // '\0'
            System.out.println("one");
            break;

         case 1: // '\001'
            System.out.println("two");
            break;

         default:
            System.out.println((new StringBuilder("unknown action ")).append(a).toString());
            break;
         }
      }

   }

   static int[] $SWITCH_TABLE$Action() {
      $SWITCH_TABLE$Action;
      if ($SWITCH_TABLE$Action == null) goto _L2; else goto _L1
_L1:
      return;
_L2:
      JVM INSTR pop ;
      int ai[] = new int[Action.values().length];
      try {
         ai[Action.ONE.ordinal()] = 0;
      }
      catch (NoSuchFieldError _ex) { }
      try {
         ai[Action.TWO.ordinal()] = 1;
      }
      catch (NoSuchFieldError _ex) { }
      return $SWITCH_TABLE$Action = ai;
   }

   private static int $SWITCH_TABLE$Action[];
}


One of the simplest ways to fix that is to add Arrays.fill(ai, -1), after 'ai' table creation in the above code.

The same will be achieved without adding additional processing using 1 (instead of 0) as a starting ordinal index for known enum constants references in the above like code generation (this is the way chosen by the Sun Java compiler).


Regards,
piotr