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The general usage contract for {{compareTo()}} from Java SE 6 API \[[API 2006|AA. Bibliography#API 06], (numbering added\]) states that

1. The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff if y.compareTo(x) throws an exception.)
2. The implementor must also ensure that the relation is transitive: (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0.
3. Finally, the implementor must ensure that x.compareTo(y) == 0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.
4. It is strongly recommended, but not strictly required, that (x.compareTo(y) == 0) == x.equals(y). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is " Note: this class has a natural ordering that is inconsistent with equals."

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return either -1, 0, or 1 depending on whether the value of the expression is negative, zero or positive.

Never Implementations must never violate any of the first three conditions when implementing the compareTo() method. Implementations should conform to the fourth condition whenever possible.

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However, this game violates the notion of required transitivity property because Rock rock beats Scissorsscissors, Scissors scissors beats Paperpaper, but Rock sock does not beat Paperpaper.

Compliant Solution (Rock-Paper-Scissors)

This compliant solution implements the same game but does not use without using the Comparable interface.

class GameEntry {
  public enum Roshambo {ROCK, PAPER, SCISSORS}
  private Roshambo value;

  public GameEntry(Roshambo value) {
    this.value = value;
  }

  public int beats(Object that) {
    if (!(that instanceof Roshambo)) {
      throw new ClassCastException();
    }
    GameEntry t = (GameEntry) that;
    return (value == t.value) ? 0
      : (value == Roshambo.ROCK && t.value == Roshambo.PAPER) ? -1
      : (value == Roshambo.PAPER && t.value == Roshambo.SCISSORS) ? -1
      : (value == Roshambo.SCISSORS && t.value == Roshambo.ROCK) ? -1
      : 1;
  }
}

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Automated Detection

The Coverity Prevent Version 5.0 MUTABLE_COMPARISON checker Some static analysis tools can detect cases where the instances where compareTo() method is reading reads from a non-constant nonconstant field. If the non-constant nonconstant field is were modified, the value of compareTo() might change, which may would break program invariants.

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Bibliography

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