We could add a public printKeys method to BST:

public class BST<Key extends Comparable<Key>, Value>
{
  /**
   * prints the keys to standard output in sorted order
   */
  public void printKeys()
  {
    printKeys(root);
  }

  /**
   * prints the keys in the subtree t to standard output
   * in sorted order
   */
  private void printKeys(Node t)
  {
    /**
     * if t is empty, return
     * print keys in the left subtree in sorted order
     * print t.key
     * print keys in the right subtree in sorted order
     */

  }

}