Sets

Models a mathematical set; hence, no duplicates are allowed
boolean contains(Object o) returns true if the object is present; false otherwise
boolean add(Object o) returns true if the add is successful, which happens if the object was not already present
boolean remove(Object o) returns true if the object was there to be removed; returns false if the object was not there
HashSet is constant time in the average case for all three operations; it can be linear in the (rare) worst case
- equals() defaults to doing a reference comparison
- hashCode() defaults to using the reference
- If equals() is true for two objects, then hashCode() needs to return the same value for them
- Hence, when using HashSet, make sure that if equals() has been overridden for the object, that hashCode() is also overridden in a consistent manner
LinkedHashSet is slightly less efficient for the above operations, but it is more efficient when iterated
TreeSet is O(log n) for all three operations
- Only objects that implement the Comparable interface can be used
- Implements the SortedSet interface, which means that first and last elements can be requested, and consecutive subsets can be requested