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Classes that overrides the {{Object.equals()}} method must also override the {{Object.hashCode()}} method.  The Java {{java.lang.Object}} class requires that any two objects that compare equal using the {{equals(Object)}} method must produce the same integer result when the {{hashCode()}} method is invoked on the objects \[[API 2006|AA. Bibliography#API 06]\]. 

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This noncompliant code example stores a <credit card number, string> pair into associates credit card numbers with strings using a HashMap and subsequently attempts to retrieve the string value associated with the a credit card number. The expected retrieved value is Java; the actual retrieved value is null. The cause of this erroneous behavior is that the CreditCard class overrides the equals() method but fails to override the hashCode() method. Consequently, the default hashCode() method returns a different value for each object, even though the objects are logically equivalent; these differing values lead to examination of different hash buckets, which prevents the get() method from finding the intended value.

public final class CreditCard {
  private final int number;

  public CreditCard(int number) {
    this.number = (short) number;
  }

  public boolean equals(Object o) {
    if (o == this) {
      return true;
    } 
    if (!(o instanceof CreditCard)) {
      return false;
    }
    CreditCard cc = (CreditCard)o;
    return cc.number == number; 
  }

  public static void main(String[] args) {
    Map<CreditCard, String> m = new HashMap<CreditCard, String>();
    m.put(new CreditCard(100), "Java");
    // Assuming Integer.MAX_VALUE is the largest number for card
    System.out.println(m.get(new CreditCard(100)));  
  }
}

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