According to the java.lang.Object specification:
"If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result."
Failure to follow this contract is a source of common bugs.
Non-Compliant Code Example
Even when the equals method conveys logical equivalence between classes, the hashCode method returns distinct numbers as opposed to returning the same values, as expected by the contract. This non-compliant example stores a credit card number into a HashMap and retrieves it. The expected retrieved value is Java, however, null is returned instead. The reason for this erroneous behavior is that the hashCode method is not overridden which means that a different bucket would be looked into than was used to store the original 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 m = new HashMap();
m.put(new CreditCard(100), "Java");
System.out.println(m.get(new CreditCard(100)));
}
}
Compliant Solution
This compliant solution shows how hashCode can be overridden so that the same value is generated for an instance. The recipe to generate such a hash function is described in Effective Java Language Programming, Item 8.
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 m = new HashMap();
m.put(new CreditCard(100), "Java");
System.out.println(m.get(new CreditCard(100)));
}
}
References
Effective Programming in Java. Item 8
java.lang.Object Documentation