equals() (javadoc) must define an equality relation (it must be reflexivesymmetric, and transitive). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value). Furthermore, o.equals(null) must always return false.

hashCode() (javadoc) must also be consistent (if the object is not modified in terms of equals(), it must keep returning the same value).

The relation between the two methods is:

Whenever a.equals(b), then a.hashCode() must be same as b.hashCode().

In practice:

If you override one, then you should override the other.

Use the same set of fields that you use to compute equals() to compute hashCode().

Use the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Langlibrary. An example:

public class Person {     private String name;     private int age;     // ...     public int hashCode() {         return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers             // if deriving: appendSuper(super.hashCode()).             append(name).             append(age).             toHashCode();     }     public boolean equals(Object obj) {         if (obj == null)             return false;         if (obj == this)             return true;         if (obj.getClass() != getClass())             return false;         Person rhs = (Person) obj;         return new EqualsBuilder().             // if deriving: appendSuper(super.equals(obj)).             append(name, rhs.name).             append(age, rhs.age).             isEquals();     } } 

Also remember:

When using a hash-based Collection or Map such as HashSetLinkedHashSetHashMapHashtable, orWeakHashMap, make sure that the hashCode() of the key objects that you put into the collection never changes while the object is in the collection. The bulletproof way to ensure this is to make your keys immutable, which has also other benefits.