Java Set

Java TreeSet class extends AbstractSet and implements NavigableSet interface. It is very similar to HashSet class, except it stores the element in sorted order.

Java LinkedHashSet class extends HashSet and implements Set interface. It is very very similar to HashSet class, except if offers the predictable iteration order.

Java HashSet class implements the Set interface, backed by a hash table(actually a HashMap instance). If does not offer any guarantees as to the iteration order, and allows null element.