TreeSet

TreeSet is similar to HashSet except that it sorts the elements in the ascending order while HashSet doesn’t maintain any order. TreeSet allows null element but like HashSet it doesn’t allow. Like most of the other collection classes this class is also not synchronized, however it can be synchronized explicitly like this: 
SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));


Example:



import java.util.TreeSet;
public class TreeSetExample {
     public static void main(String args[]) {
         // TreeSet of String Type
         TreeSet<String> tset = new TreeSet<String>();

         // Adding elements to TreeSet<String>
         tset.add("ABC");
         tset.add("String");
         tset.add("Test");
         tset.add("Pen");
         tset.add("Ink");
         tset.add("Jack");

         //Displaying TreeSet
         System.out.println(tset);

         // TreeSet of Integer Type
         TreeSet<Integer> tset2 = new TreeSet<Integer>();

         // Adding elements to TreeSet<Integer>
         tset2.add(88);
         tset2.add(7);
         tset2.add(101);
         tset2.add(0);
         tset2.add(3);
         tset2.add(222);
         System.out.println(tset2);
    }
 }

Output: You can see both the TreeSet have been sorted in ascending order implicitly.

[ABC, Ink, Jack, Pen, String, Test]
[0, 3, 7, 88, 101, 222]