The theory (for the language lawyers and the mathematically inclined):
equals()
(javadoc) must define an equivalence relation (it must be reflexive, symmetric, 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 Lang library. An example:
public class Person {
private String name;
private int age;
// ...
@Override
public int hashCode() {
return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers
// if deriving: appendSuper(super.hashCode()).
append(name).
append(age).
toHashCode();
}
@Override
public boolean equals(Object obj) {
if (!(obj instanceof Person))
return false;
if (obj == this)
return true;
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 HashSet, LinkedHashSet, HashMap, Hashtable, or WeakHashMap, 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.
Best Answer
Yes, it is important if your item will be used as a key in a dictionary, or
HashSet<T>
, etc - since this is used (in the absence of a customIEqualityComparer<T>
) to group items into buckets. If the hash-code for two items does not match, they may never be considered equal (Equals will simply never be called).The GetHashCode() method should reflect the
Equals
logic; the rules are:Equals(...) == true
) then they must return the same value forGetHashCode()
GetHashCode()
is equal, it is not necessary for them to be the same; this is a collision, andEquals
will be called to see if it is a real equality or not.In this case, it looks like "
return FooId;
" is a suitableGetHashCode()
implementation. If you are testing multiple properties, it is common to combine them using code like below, to reduce diagonal collisions (i.e. so thatnew Foo(3,5)
has a different hash-code tonew Foo(5,3)
):Oh - for convenience, you might also consider providing
==
and!=
operators when overridingEquals
andGetHashCode
.A demonstration of what happens when you get this wrong is here.