SEI CERT Oracle Coding Standard for Java

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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:

The implementor must ensure sgn(x.compareTo(thumbs up) ) == -sgn(y.compareTo(error) ) for all x and y. (This implies that x.compareTo(thumbs up) must throw an exception iff y.compareTo(error) throws an exception.)

The implementor must also ensure that the relation is transitive: (x.compareTo(thumbs up) >0 && y.compareTo(z)>0) implies x.compareTo(z)>0.

Finally, the implementor must ensure that x.compareTo(thumbs up) ==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.

Do not violate any of five conditions while overriding the compareTo method.

Noncompliant Code Example

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.

public final class Card implements Comparable{
  private String suit;
  private int rank;

  public Card(String s, int r) {
    if (s == null)
      throw new NullPointerException();
    suit = s;
    rank = r;
  }

  public boolean equals(Object o) {
    if (o instanceof Card){
      Card c=(Card)o;
      return suit.equals(c.suit) || (rank == c.rank);
    }
    return false;
  }

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

  public static void main(String[] args) {
    Card a = new Card("Clubs", 2);
    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
  }
}

Compliant Solution

Make sure you fulfill the contract, and make sure your corresponding equals method matches with compareTo.

public final class Card implements Comparable{
  private String suit;
  private int rank;

  public Card(String s, int r) {
    if (s == null)
      throw new NullPointerException();
    suit = s;
    rank = r;
  }

  public boolean equals(Object o) {
    if (o instanceof Card){
      Card c=(Card)o;
      return suit.equals(c.suit) && (rank == c.rank);
    }
    return false;
  }

  //this method fulfills its contract
  public int compareTo(Object o){
    if (o instanceof Card){
      Card c=(Card)o;
      if(suit.equals(c.suit)) return c.rank - rank;
      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
  }
}

Risk Assessment

Violating the general contract when overriding the compareTo() method can lead to unexpected results.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

MET30-J

low

unlikely

medium

P2

L3

Automated Detection

TODO

References

Java API

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