The second two constructors are copy constructors : TreeSet « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. TreeSet

public TreeSet(Collection col)
public TreeSet(SortedSet set)

import java.util.Arrays;
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(Arrays.asList(elements));

    System.out.println(set);
  }
}

[A, C, D, F, G]