public abstract class BinaryTreeBasis {
  protected TreeNode root;

  public BinaryTreeBasis() {
    root = null;
  }  // end default constructor

  public BinaryTreeBasis(Object rootItem) {
    root = new TreeNode(rootItem, null, null);
  }  // end constructor

  public boolean isEmpty() {
// Returns true if the tree is empty, else returns false.
    return root == null;
  }  // end isEmpty

  public void makeEmpty() {
// Removes all nodes from the tree.
    root = null;
  }  // end makeEmpty

  public Object getRootItem() throws TreeException {
// Returns the item in the tree’s root.
    if (root == null) {
      throw new TreeException("TreeException: Empty tree");
    }
    else {
      return root.getItem();
    }  // end if
  }  // end getRootItem

}  // end BinaryTreeBasis