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This noncompliant code example violates the third condition in the contract. This requirement states 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.
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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);
}
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
}
}
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