Java Enum linking to another Enum

Question

I would like to have enumerator in Java having other enum as attribute.

public enum Direction {
 Up(Down),
 Down(Up),
 Left(Right),
 Right(Left);
 private Direction opposite;
 Direction(Direction opposite){
 this.opposite = opposite;
 }
}

So I have different Direction, and for each I want to know the opposite. It is working fine for Down and Right, but I can't initialize Up because Down is not known yet (same fort Left).

How can I edit enum variables after initialisation ?

Answer 1

Put your initialization in a static block:

public enum Direction {
 Up, Down, Left, Right;
 private Direction opposite;
 static {
 Up.setDirection(Down);
 Down.setDirection(Up);
 Left.setDirection(Right);
 Right.setDirection(Left);
 }
 private void setDirection(Direction opposite) {
 this.opposite = opposite;
 }
 public String toString() {
 return this.name() + " (" + opposite.name() + ")";
 }
}

• Nicktar: setDirection has a misleading name. setOppositeDirection or setOpposite would be better.

Answer 2

One of the possible solution - you can encapsulate this login within the method

public Direction getOpposite() {
 switch (this) {
 case Up:
 return Down;
 case Down:
 return Up;
 case Left:
 return Right;
 case Right:
 return Left;
 }
 return null;
}

It will be the same interface for classes that will use this enum

Answer 3

Quick and dirty solution, put the initialization in a static block:

public enum Direction {
 Up,
 Down,
 Left,
 Right;
private Direction opposite;
public Direction opposite() {
 return this.opposite;
}
static {
 Up.opposite = Down;
 Down.opposite = Up;
 Left.opposite = Right;
 Right.opposite = Left;
 }
}

Answer 4

One trick is to leave the arguments as nulls:

enum Direction {
 Up(null),
 Down(Up),
 Left(null),
 Right(Left);

and set the "opposite of the opposite" to this in the constructor:

Direction(Direction opposite){
 this.opposite = opposite;
 if (opposite != null) {
 opposite.opposite = this;
 }
}

But this is merely a trick. I don't think it's good-looking code.