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This noncompliant code example violates the third condition (transitivity) in the contract. This requirement states that the objects that compareTo()
considers equal (by returning 0) must be ordered identically when compared to other objects. For example, it may be necessary to compare a card with any other card to check whether both belong to the same suit or have the same rank. When neither of these conditions is true, compareTo()
is expected to order the cards based on rank alone. This situation may arise in a game like Uno or Crazy Eights, where one can place a card on the pile only when it shares a suit or rank with the topmost card on the pile.
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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 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 } } |
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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
}
}
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