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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 TreeSet and TreeMap will 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 their the associated compareTo() methods to sort themthe objects. 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.unmigrated-wiki-markup

The general usage contract for {{compareTo()}} has been put forth verbatim from the Java SE 6 API \[[API 06|AA. Java References#API 06]\]:8 API [API 2014] states that

1. The implementor

The implementpr

1. 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

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

>0

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

>0

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

>0

1. > 0.
2. Finally, the implementor must ensure that x.compareTo(y) == 0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z))

,

1. for all z.
2. It is strongly recommended, but not strictly required, that (x.compareTo(y) == 0) ==

(

1. x.equals(y)

)

1. . Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is

"

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

Noncompliant Code Example (Rock-Paper-Scissors)

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 the same suit or same rank; otherwise it orders based on rank. 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.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;
  private int rank;

  public Card(String s, int r) {
    if (s == null)enum Roshambo {ROCK, PAPER, SCISSORS}
  private    throw new NullPointerException();
    suit = s;
    rank = r;
  }

  public boolean equals(Object oRoshambo value;

  public GameEntry(Roshambo value) {
    if (o instanceof Card){
      Card c = (Card)o;
      return suit.equals(c.suit) || (rank == c.rank); // bad
    }
    return false;
  }

//This method violates its contract
  this.value = value;
  }

  public int compareTo(Object othat) {
    if (o!(that instanceof CardGameEntry)) {
      Cardthrow cnew = ClassCastException(Card)o;
      if(suit.equals(c.suit)) }
    GameEntry t =  return 0(GameEntry) that;
     return if(c.rankvalue >== rank + Integer.MIN_VALUEt.value) // check for integer underflow? 0
      : (value return== cRoshambo.rankROCK - rank;
    }&& t.value == Roshambo.PAPER) ? -1
    throw new ClassCastException();
  }

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

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

Compliant Solution

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

Compliant Solution (Rock-Paper-Scissors)

This compliant solution implements the same game without using the Comparable interface:Make sure you fulfill the compareTo() contract, and don't forget to 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()this.value = value;
    suit = s;
    rank = r;
  }

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

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

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

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

Exceptions

...

(value == Roshambo.SCISSORS && t.value == Roshambo.ROCK) ? -1
      : 1;
  }
}

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation Automated detections of violations of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as ARR40-CPP. Use a Valid Ordering Rule.

References

is infeasible in the general case.

Related Guidelines

Bibliography

...

Image Added Image Added Image Added Wiki Markup\[[API 06|AA. Java References#API 06]\] [method compareTo()|http://java.sun.com/javase/6/docs/api/java/lang/Comparable.html#compareTo(java.lang.Object)] \[[JLS 05|AA. Java References#JLS 05]\]