Set
- cannot contain duplicates

TreeSet
- does not preserve the insertion order of elements but elements are sorted by keys
- does not allow insertion of Heterogeneous objects (throw ClassCastException at Runtime)
- excellent choice for storing large amounts of sorted information which are supposed to be accessed quickly because of its faster access and retrieval time
- operations like printing n elements in sorted order takes O(n) time
- insertion of null into a TreeSet throws NullPointerException

HashSet
- permits the null element
- does not guarantee the constant order of elements over time (objects are inserted based on their hash code)
- very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important
- a hash table stores information by using a mechanism called hashing (the informational content of a key is used to determine a unique value, called its hash code)