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

Noncompliant Code Example

This program implements the classic game of Roshambo, using the compareTo() operator to determine the winner of a game.

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However, this game violates the notion of transitivity, since Rock beats Scissors, Scissors beats Paper, but Rock does not beat Paper.

Compliant Solution

This compliant solution implements the game, but does not use 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;
  }
}

Risk Assessment

Violating the general contract when implementing the compareTo() method can result in unexpected results, possibly leading to invalid comparisons and information disclosure.

Automated Detection

The Coverity Prevent Version 5.0 MUTABLE_COMPARISON checker can detect the instances where compareTo method is reading from a non constant field. If the non-constant field is modified, the value of compareTo might change, which may break program invariants.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Other Languages

This guideline appears in the C++ Secure Coding Standard as ARR40-CPP. Use a valid ordering rule.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] method [compareTo()|http://java.sun.com/javase/6/docs/api/java/lang/Comparable.html#compareTo(java.lang.Object)]
\[[JLS 2005|AA. Bibliography#JLS 05]\]

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