To add a single element: the add() method: public boolean add(Object element) : TreeSet « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. TreeSet

The element to add must implement the Comparable interface or the TreeSet constructor must have been passed a Comparator.

If both of these are not true then a ClassCastException will be thrown.

import java.util.*;

import java.util.Collections;
import java.util.Set;
import java.util.TreeSet;

public class MainClass {
  public static void main(String args[]) throws Exception {
    String elements[] = { "A", "C", "D", "G", "F" };
    Set set = new TreeSet(Collections.reverseOrder());
    for (int i = 0, n = elements.length; i < n; i++) {
      set.add(elements[i]);
    }
    System.out.println(set);
  }
}

[G, F, D, C, A]