HashSet Vs TreeSet

HashSet:

• class offers constant time performance for the basic operations (add, remove, contains and size).
• it does not guarantee that the order of elements will remain constant over time
• iteration performance depends on the 'initial capacity' and the 'load factor' of the HashSet.
  • It's quite safe to accept default load factor but you may want to specify an initial capacity that's about twice the size to which you expect the set to grow.

TreeSet:

• guarantees log(n) time cost for the basic operations (add, remove and contains)
• guarantees that elements of set will be sorted [ascending, natural, or the one specified by you via it's constructor]
• doesn't offer any tuning parameters for iteration performance
• offers few handy methods to deal with the ordered set like first(), last(), headSet(), and tailSet() etc

Important points:

• Both guarantee duplicate-free collection of elements
• It is generally faster to add elements to the HasSet and then convert the collection to a TreeeSet for a duplicate-free sorted traversal.
• None of these implementation are synchronized. That is if multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.
• LinkedHashSet is in some sense intermediate between HashSet and TreeSet. Implemented as a hash table with a linked list running through it, however it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet