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Classes that override the Object.equals() method must also override the Object.hashCode() method. The java.lang.Object class requires that any two objects that compare equal using the equals() method must produce the same integer result when the hashCode() method is invoked on the objects [API 2006].

The equals() method is used to determine logical equivalence between object instances. Consequently, the hashCode() method must return the same value for all equivalent objects. Failure to follow this contract is a common source of defects.

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This compliant solution overrides the hashCode() method so that it generates the same value for any two instances that are considered to be equal by the equals() method. Bloch discusses the recipe to generate such a hash function in detail [Bloch 2008].

public final class CreditCard {
  private final int number;
  
  public CreditCard(int number) {
    this.number = 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 int hashCode() {
    int result = 17;
    result = 31 * result + number;
    return result;
  }

  public static void main(String[] args) {
    Map<CreditCard, String> m = new HashMap<CreditCard, String>();
    m.put(new CreditCard(100), "4111111111111111");
    System.out.println(m.get(new CreditCard(100)));
  }
}

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Bibliography

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