// extrait de la classe TreeSet
public class TreeSet extends AbstractSet
        implements SortedSet, Cloneable, java.io.Serializable {
  //...
  public TreeSet() {
    this(new TreeMap());
  }
  //...
}

TreeSet: Can contain only unique values and it is sorted in ascending order
TreeMap: Can contain only unique keys and keys are sorted in ascending order.

- TreeSet sorts the order and don't accept null