Java enum- Cannot reference a field before it is defined

Question

I have an enum like the one below, but eclipse says that there are errors in the first definition of each opposite pair.

public enum Baz{
  yin(yang),    //Cannot reference a field before it is defined
  yang(yin),
  good(evil),   //Cannot reference a field before it is defined
  evil(good);

  public final Baz opposite;

  Baz(Baz opposite){
    this.opposite = opposite;
  }
}

What I want to accomplish is being able to use Baz.something.opposite to get the opposite object of Baz.something. Is there a possible workaround for this? Maybe an empty placeholder for yang and bad before yin and good are defined in this example?

Answer 1

You could try something like:

public enum Baz{
  yin("yang"),    
  yang("yin"),
  good("evil"),   
  evil("good");

  private String opposite;

  Baz(String opposite){
    this.opposite = opposite;
  }

  public Baz getOpposite(){
     return Baz.valueOf(opposite);
  }
}

and then reference it as

Baz.something.getOpposite()

That should accomplish what you are looking to do by looking up the enum value by it's string representation. I don't think you can get it to work with the recursive reference to Baz.

Answer 2

With the switch statement:

public enum Baz{
  yin,
  yang,
  good,
  evil;

  public Baz getOpposite() {
    switch (this) {
        case yin: return yang;
        case yang: return yin;
        case good: return evil;
        case evil: return good;
    }
    throw new AssertionError();
}

Or deferred initialization:

public enum Baz{
  yin,
  yang,
  good,
  evil;

  public Baz opposite;

  static {
    yin.opposite = yang;
    yang.opposite = yin;
    good.opposite = evil;
    evil.opposite = good;
  }
}

You might wish to make the mutable field private and provide a getter.

Answer 3

How about an EnumMap?

public enum Baz {
  yin,
  yang,
  good,
  evil;
  private static final Map<Baz, Baz> opposites = new EnumMap<Baz, Baz>(Baz.class);

  static {
    opposites.put(yin, yang);
    opposites.put(yang, yin);
    opposites.put(good, evil);
    opposites.put(evil, good);
  }

  public Baz getOpposite() {
    return opposites.get(this);
  }
}