<p>What issues / pitfalls must be considered when overriding <code>equals</code> and <code>hashCode</code>?</p>

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
.