public class BinaryTree extends BinaryTreeBasis {
  public BinaryTree() {
  }  // end default constructor

  public BinaryTree(Object rootItem) {
    super(rootItem);
  }  // end constructor

  public BinaryTree(Object rootItem, 
                    BinaryTree leftTree, 
                    BinaryTree rightTree) {
    root = new TreeNode(rootItem, null, null);
    attachLeftSubtree(leftTree);
    attachRightSubtree(rightTree);
  }  // end constructor

  public void setRootItem(Object newItem) {
    if (root != null) {
      root.setItem(newItem);
    }
    else {
      root = new TreeNode(newItem, null, null);
    }  // end if
  }  // end setRootItem

  public void attachLeft(Object newItem) {
    if (!isEmpty() && root.getLeft() == null) {
      // assertion: nonempty tree; no left child
      root.setLeft(new TreeNode(newItem, null, null));
    }  // end if
  }  // end attachLeft

  public void attachRight(Object newItem) {
    if (!isEmpty() && root.getRight() == null) {
      // assertion: nonempty tree; no right child
      root.setRight(new TreeNode(newItem, null, null));
    }  // end if
  }  // end attachRight

  public void attachLeftSubtree(BinaryTree leftTree) 
                                throws TreeException {
    if (isEmpty()) {
      throw new TreeException("TreeException:  Empty tree");
    }
    else if (root.getLeft() != null) {
      // a left subtree already exists; it should have been 
      // deleted first
      throw new TreeException("TreeException: " + 
                           "Cannot overwrite left subtree");
    }
    else {
      // assertion: nonempty tree; no left child
      root.setLeft(leftTree.root);
      // don't want to leave multiple entry points into 
      // our tree
      leftTree.makeEmpty(); 
    }  // end if
  }  // end attachLeftSubtree

  public void attachRightSubtree(BinaryTree rightTree)  
                                 throws TreeException {
    if (isEmpty()) {
      throw new TreeException("TreeException:  Empty tree");
    }
    else if (root.getRight() != null) {
      // a right subtree already exists; it should have been 
      // deleted first
      throw new TreeException("TreeException: " + 
                          "Cannot overwrite right subtree");
    }
    else {
      // assertion: nonempty tree; no right child
      root.setRight(rightTree.root);
      // don't want to leave multiple entry points into 
      // our tree
      rightTree.makeEmpty(); 
    }  // end if
  }  // end attachRightSubtree
  
  protected BinaryTree(TreeNode rootNode) {
    root = rootNode;
  }  // end protected constructor

  public BinaryTree detachLeftSubtree()  
                         throws TreeException {
    if (isEmpty()) {
      throw new TreeException("TreeException:  Empty tree");
    }
    else {
      // create a new binary tree that has root's left 
      // node as its root
      BinaryTree leftTree;
      leftTree = new BinaryTree(root.getLeft());
      root.setLeft(null);
      return leftTree;
    }  // end if
  }  // end detachLeftSubtree

  public BinaryTree detachRightSubtree() 
                         throws TreeException {
    if (isEmpty()) {
      throw new TreeException("TreeException:  Empty tree");
    }
    else {
      BinaryTree rightTree;
      rightTree = new BinaryTree(root.getRight());
      root.setRight(null);
      return rightTree;
    }  // end if
  }  // end detachRightSubtree

} // end BinaryTree