e>HashSet</code> must have consistent equality
and hashCode implementations. This means that the equals operation
must define a stable equivalence relation on the elements (reflexive,
symmetric, transitive, and consistent over time), and that the hashCode
must consistent with equality, so that the same for objects that are
considered equal.</p>
<p>The set allows <code>null</code> as an element.</p>
<p>Most simple operations on <code>HashSet</code> are done in (potentially amortized)
constant time: <a href="