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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(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 associates credit card numbers with strings using a HashMap and subsequently attempts to retrieve the string value associated with a credit card number. The expected retrieved value is 4111111111111111; 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), "4111111111111111");
    System.out.println(m.get(new CreditCard(100)));  
  }
}

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.
Note that by specifying the credit card number in main(), these code examples violate rule MSC03-J. for the sake of brevity.

Compliant Solution

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 \[[Bloch 2008|AA. Bibliography#Bloch 08]\].

import java.util.Map;
import java.util.HashMap;

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 int hashCode() {
    int result = 7;
    result = 37 * 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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Automated detection of classes that override only one of equals() and hashcode() is straightforward. Sound static determination that the implementations of equals() and hashcode() are mutually consistent is not feasible in the general case, although heuristic techniques may be useduseful.

Related Guidelines

Bibliography

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