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.
– The underlying data structure is Hashtable
– Heterogeneous objects are allowed
– Insertion order is not preserved and it is based on hashcode of the objects.
– null insertion is possible.
TreeSet:
– TreeSet class has a member reference variable of type NavigableMap. In fact, TreeSet make use of unique key property of Map’s to ensure no duplicate elements. There is a dummy value used for this instance member variable .
-The underlying data structure is balanced tree.
– Guarantees log(n) time cost for the basic operations (add, remove and contains)
– Heterogeneous objects are not allowed by defalut.
– Insertion order is not preserved and all the objects are inserted according to some sorting order.
– As the first element only null insertion is possible and in all other cases we will get NullPointerException (After null insertion other insertion not possible)
– 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 a few handy methods to deal with the ordered set like first(), last(), headSet(), and tailSet() etc
– we can construct a constructor your own rules for what the order should be using a comparable or comparator.
The TreeSet implementations useful when you need to extract elements from a collection in a sorted manner. It is generally faster to add elements to the HasSet then convert the collection to a TreeeSet for sorted traversal.
To optimize HashSet space usage , you can tune initial capacity and load factor. TreeSet has no tuning options, as the tree is always balanced, ensuring log(n0 performance for insertions, deletions and queries.