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Objects of a class can be ordered relative to one another. One way to do this is for the class to implement the Comparable interface. Library classes like such as TreeSet and TreeMap will } accept {{Comparable objects and use their compareTo() methods to sort them. However, a class that implements the compareTo() method in an unexpected way could can cause undesirable results. For instance, a TreeSet reporting it does not contain an object that it really does contain, could lead to exploitable behavior.

The general usage contract for {{compareTo()}} has been put forth verbatim from the Java SE 6 API \[[API 06|AA. Java References#API 06]\] states that:

The implementpr 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 y.compareTo(x) throws an exception.)

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

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

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.

Do not violate any of these four the first three conditions when implementing the compareTo() method. Implementing the fourth condition is strongly recommended but is not necessary.

Noncompliant Code Example

This noncompliant code example violates the third condition (transitivity) in the contract. This requirement states that the objects that compareTo() considers equal (returns 0) must be ordered the same with respect to other objects. Consider a Card that considers itself equal to any card of For example, a card is to be compared against any other card to check whether both belong to the same suit or same rank; otherwise it orders have the same rank. If none of these conditions is true, the compareTo() is expected to order the objects based on rank alone. This might arise in a game like Uno or Crazy Eights, where you can only place a card on the pile that shares a suit or rank with the top card on the pile.

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); // bad
    }
    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;
      if((c.rank >= rank + Integer.MIN_VALUE) && 
      (c.rank <= rank + Integer.MAX_VALUE))
          // check for integer underflow/overflow
        return c.rank - rank; // order based on 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
  }
}

Here, the comparison between (a,c) yields that c is larger. ButHowever, the comparison (b,c) yields b as larger. This means b must be larger than a. However, comparing (a,b) results in the value 0 (same).

Compliant Solution

Make sure you fulfill This compliant solution ensures that the compareTo() contract is satisfied, and don't forget to make sure your the corresponding equals() method matches is consistent 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); // good
    }
    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) && (c.rank >= rank + Integer.MIN_VALUE) && (c.rank <= rank + Integer.MAX_VALUE)) 
        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", 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 negative number
  }
}

As per required by the ordering, c is larger than both a and b and the comparison (a,b) produces an " equal " result. This maintains the compareTo contract.

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