TreeSet §

TreeSet is basically an implementation of a self-balancing binary search tree like a Red-Black Tree. The elements in TreeSet are sorted.

to init a TreeSet:

TreeSet treeSet = new TreeSet();
 
// specify sorting order
TreeSet treeSet = new TreeSet(Comparator comp);
 
// build a TreeSet with elements from a collection
TreeSet treeSet = new TreeSet(Collection col);

methods of TreeSet：

• add(Object o): shouldn’t add null to a TreeSet, otherwise it will throw NullPointException.
• addAll(Collection co)
• contains(Object o)
• ceiling(E e): This method returns the least element in this set greater than or equal to the given element, or null if there is no such element.
• floor(E e): This method returns the greatest element in this set less than or equal to the given element, or null if there is no such element.
• higher(E e): This method returns the least element in this set strictly greater than the given element, or null if there is no such element.
• first(): his method will return the first element in TreeSet if TreeSet is not null else it will throw NoSuchElementException.
• last(): This method will return the last element in TreeSet if TreeSet is not null else it will throw NoSuchElementException.
• clear()
• etc.

Time complexity §

The Time complexity for TreeSet’s methods add, remove and contains is O(logn).

NOT Thread-safe §

 The implementation of a TreeSet is not synchronized. To make TreeSet be Thread-safe, it can be inited likes:

TreeSet ts = new TreeSet(); 
Set syncSet = Collections.synchronziedSet(ts);