The java.util.TreeSet class in the Java standard class library is an implementation of the Set interface using trees. This class uses a balanced binary search tree to keep elements in sorted order. The TreeSet class implements the Set interface, so Figure 28.1 describes TreeSet's methods as well.

In general, any Comparable objects may be placed into a TreeSet. This guarantees that the set will be in ascending order, as determined by the object’s compareTo method. Items in a TreeSet should also be mutually comparable, meaning that for any pair of elements e1 and e2 in the set, e1.compareTo(e2) will not throw a ClassCastException. The items in a TreeSet should all be of the same type.

Because of the balanced binary tree implementation, the TreeSet class provides an O(log n) run time for the operations add, remove, and contains.

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

Set <String> myTree = new TreeSet <String>();
myTree.add("Nancy");
myTree.add("David");
myTree.add("David");
System.out.println(myTree.size());

for(String temp : myTree){
  System.out.println(temp);
}

The output for this code fragment is

The size of the set is 2
David
Nancy