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Choosing to implement the Comparable interface represents a commitment that the implementation of the compareTo() method adheres to the general contract for that method regarding how the method is to be called. Library classes such as TreeSet and TreeMap accept Comparable objects and use the associated compareTo() methods to sort the objects. However, a class that implements the compareTo() method in an unexpected way can cause undesirable results.

The general contract for compareTo() from Java SE 8 API [API 2014] states that

Note: I merely modified the equivalent rule for equals to make this rule.
It also seems like I'm sort of just copying from the Java standard,
but I can't think of any reason why if we have an equals rule
we should not have a compareTo rule, since it is so often used with equals.
This rule could be extended to deal with Comparator as well; since they go together.

The general usage contract for compareTo() has been put forth verbatim from the Java specification:

1. The implementor must ensure sgn(x.compareTo

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1. (y)) == -sgn(y.compareTo

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1. (x)) for all x and y. (This implies that x.compareTo

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1. (y) must throw an exception

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1. if y.compareTo

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1. (x) throws an exception.)
2. The implementor must also ensure that the relation is transitive: (x.compareTo

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1. (y) > 0 && y.compareTo(z)

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1. > 0) implies x.compareTo(z)

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1. > 0.
2. Finally, the implementor must ensure that x.compareTo

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1. (y) == 0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z))

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1. for all z.
2. 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.

Implementations must never violate any of the first three conditions when implementing the compareTo() method. Implementations should conform to the fourth condition whenever possibleDo not violate any of five conditions while overriding the compareTo method.

Noncompliant Code Example (Rock-Paper-Scissors)

This noncompliant code example violates the third condition in the contract.
Consider a Card that considers itself equal to any card of the same suit; otherwise it orders based on rank.program implements the classic game of rock-paper-scissors, using the compareTo() operator to determine the winner of a game:

public final class CardGameEntry implements Comparable {
   private String suit;public enum Roshambo {ROCK, PAPER, SCISSORS}
  private intRoshambo rankvalue;

  public CardGameEntry(String s, int rRoshambo value) {
    if (sthis.value == null)
      throw new NullPointerException() value;
    suit = s;
    rank = r;
  }

  public booleanint equalscompareTo(Object othat) {
    if (!(othat instanceof CardGameEntry)) {
      Card c=(Card)o;
      return suit.equals(c.suit) || (rank == c.rankthrow new ClassCastException();
    }
    returnGameEntry false;
  }

  //this method violates its contract
  public int compareTo(Object o){t = (GameEntry) that;
    ifreturn (o instanceof Card){
      Card c=(Card)o;
      if(suit.equals(c.suit)) return 0;value == t.value) ? 0
      return c.rank - rank;
    }
    throw new ClassCastException();
  }

  public static void main(String[] args) {
    Card a = new Card("Clubs", 2);: (value == Roshambo.ROCK && t.value == Roshambo.PAPER) ? -1
      : (value == Roshambo.PAPER && t.value == Roshambo.SCISSORS) ? -1
    Card b =: new Card("Clubs", 10);
    Card c = new Card("Hearts", 7);
    System.out.println(a.compareTo(b)); //returns 0
    System.out.println(a.compareTo(c)); //returns a negative number
    System.out.println(b.compareTo(c)); //returns a positive number(value == Roshambo.SCISSORS && t.value == Roshambo.ROCK) ? -1
      : 1;
  }
}

However, this game violates the required transitivity property because rock beats scissors, scissors beats paper, but rock does not beat paper.

Compliant Solution

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(Rock-Paper-Scissors)

This compliant solution implements the same game without using the Comparable interface:Make sure you fulfill the contract, and make sure your corresponding equals method matches with compareTo.

publicclass finalGameEntry class{
 Card implementspublic Comparable{
enum Roshambo private String suit;{ROCK, PAPER, SCISSORS}
  private intRoshambo rankvalue;

  public CardGameEntry(String s, int rRoshambo value) {
    if (s == null)
      throw new NullPointerException();
    suit = s;
    rank = r;
  this.value = value;
  }

  public booleanint equalsbeats(Object othat) {
    if (!(othat instanceof CardGameEntry)) {
      throw Cardnew c=ClassCastException(Card)o;
    }
  return suit.equals(c.suit) && (rank GameEntry t == c.rank)(GameEntry) that;
    }
    return false;
  }

  //this method fulfills its contract
  public int compareTo(Object o){
(value == t.value) ? 0
      if: (ovalue instanceof Card){
      Card c=(Card)o;
      if(suit.equals(c.suit)) return c.rank - rank;== Roshambo.ROCK && t.value == Roshambo.PAPER) ? -1
      return: suit.compareTo(c.suit);
    }
    throw new ClassCastException();
  }

  public static void main(String[] args) {
    Card a = new Card("Clubs", 2);
    Card b = new Card("Clubs", 2);
    Card c = new Card("Hearts", 7);
    System.out.println(a.compareTo(b)); //returns 0
    System.out.println(a.compareTo(c)); //returns a negative number
    System.out.println(b.compareTo(c)); //returns a negative number(value == Roshambo.PAPER && t.value == Roshambo.SCISSORS) ? -1
      : (value == Roshambo.SCISSORS && t.value == Roshambo.ROCK) ? -1
      : 1;
  }
}

Risk Assessment

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

Automated Detection

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TODO

References

Automated detections of violations of this rule is infeasible in the general case.

Tool	Version	Checker	Description
CodeSonar	Include Page CodeSonar_V CodeSonar_V	JAVA.CLASS.MCS	Missing Call to super (Java)
Coverity	7.5	FB.RU_INVOKE_RUN	Implemented

Related Guidelines

Bibliography

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