Java - Enums - Logical circular reference [duplica

问题:

This question already has an answer here:

• Java enum- Cannot reference a field before it is defined 8 answers

Imagine the following made up example:

public enum Hand {
  ROCK(SCISSORS),
  PAPER(ROCK),
  SCISSORS(PAPER);

  private final Hand beats;

  Hand(Hand beats) {
    this.beats = beats;
  }
}

I will get an error Illegal forward reference for forward referencing SCISSORS.

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Is there a way to handle such forward references in Java?

Or how would you model such a situation, where you have a logical circular reference between several enums values?

回答1:

You cannot assign SCISSORS to ROCK before it is defined. You can, instead, assign the values in a static block.

I have seen a lot examples where people use String values in the constructors, but this is more concrete to assign the actual values after they have been declared. This is encapsulated and the beats instance variable cannot be changed (unless you use reflection).

public enum Hand {
    ROCK,
    PAPER,
    SCISSORS;

    private Hand beats;

    static {
        ROCK.beats = SCISSORS;
        PAPER.beats = ROCK;
        SCISSORS.beats = PAPER;
    }

    public Hand getBeats() {
        return beats;
    }

    public static void main(String[] args) {
        for (Hand hand : Hand.values()) {
            System.out.printf("%s beats %s%n", hand, hand.getBeats());
        }
    }
}

Output

ROCK beats SCISSORS
PAPER beats ROCK
SCISSORS beats PAPER