Using Enum's ordinal value in switch-case statement

Question

For my project I am using enums, and I need to implement the switch-case statement, where ordinal numbers of values of the specific Enum are checked, like this way:

        switch ( variable )
        {
        case MyEnum.A.ordinal():
            return true;
        case MyEnum.B.ordinal():
            return true;
        default:
            return false;
        }

Note: return values are only an example.

Unfortunately, Eclipse (I'm using 1.6 JDK) gives my compilation error "case expressions must be constant expressions". What I should do? Is there any other method than static lookup table, described here: Convert from enum ordinal to enum type ?

Answer 1

This is how is done, provided you have a serialized ordinal somehow, somewhere. Usual way to persist an enum is by its name, not ordinal, though. Also you should not use ordinal in normal circumstances unless trying to implement something like EnumMap/Set. Of course, the enum can be just a port from C alike stuff and dealing with the inevitable int, needs a transform to the Enum object.

Just use Enum.values() to obtain an array ordered by ordinal(), since the array is cloned each time, keeping a ref towards is ok.

enum E{
 A, B, C...   
}

final static E[] vals = E.values();//copy the values(), calling values() clones the array
boolean f(int variable){
  switch(vals[variable]){
  case A:
...
  case B:
...
//break;
  default:
...
   }
}

---

Just noticed you need only true and false, that's a Set type of behavior. You can use java.util.EnumSet or a simple long, if feeling brave (and not having more than 64 enum constants). for example:

private static <E extends Enum> long ord(E e){
  return 1L<<e.ordinal();
}

static final long positiveSet = ord(E.A)+ord(E.B);
boolean f(int ordinal){
  return 0!=(positiveSet&(1L<<ordinal));
}

Answer 2

First of all, you should not rely on the ordinal that much. If possible make your variable a String (and transform to enum using Enum.valueOf(string) or at best make it enum.

If you really can't, then use enum.values()[ordinal].Then use the enum in the switch.