Printing the data and placement of tree elements

Question

I have to print elements of a tree based on the levels of the tree. I have to use recursion to complete this objective, however; I am only returning the root data. I apparently disregard the children.

/**
  * A binary tree in which each node has two children.
  */
public class BinaryTree {

    private Node root;

    /**
     * Constructs an empty tree.
     */
    public BinaryTree() {
        root = null;
    }

    /**
     * Constructs a tree with one node and no children.
     *
     * @param rootData the data for the root
     */
    public BinaryTree(Object rootData) {
        //
        root = new Node();
        root.data = rootData;
        root.left = null;
        root.right = null;
    }

    /**
     * Constructs a binary tree.
     *
     * @param rootData the data for the root
     * @param left the left subtree
     * @param right the right subtree
     */
    public BinaryTree(Object rootData, BinaryTree left, BinaryTree right) {
        //
        root = new Node();
        root.data = rootData;
        root.left = left.root;
        root.right = right.root;
    }

    class Node {

        public Object data;
        public Node left;
        public Node right;

        public String printTree(int level) {
            String strVal = "";
            strVal += root.data;
            if (root.left != null) {
                strVal += this.left.printTree(level + 1);
            }
            return strVal;
        }
    }

    /**
     * Returns the height of the subtree whose root is the given node.
     *
     * @param n a node or null
     * @return the height of the subtree, or 0 if n is null
     */
    private static int height(Node n) {
        //
        if (n == null) {
            return 0;
        } else {
            return 1 + Math.max(height(n.left), height(n.right));
        }
    }

    /**
     * Returns the height of this tree.
     *
     * @return the height
     */
    public Object data() {
        //
        return root.data;
    }

    /**
     * Gets the left subtree of this tree
     *
     * @return the left child of the root
     */
    public BinaryTree left() {
        //
        BinaryTree result = new BinaryTree();
        result.root = root.left;
        return result;
    }

    /**
     * Gets the right subtree of this tree
     *
     * @return the right child of the root
     */
    public BinaryTree right() {
        //
        BinaryTree result = new BinaryTree();
        result.root = root.right;
        return result;
    }

    /**
     * Sets a new right child
     *
     * @param child the new right child
     */
    public void setRight(BinaryTree child) {
        //
        BinaryTree result = new BinaryTree();
        result.root = root.right;
        result = child;
    }

    /**
     * Sets a new left child
     *
     * @param child the new left child
     */
    public void setLeft(BinaryTree child) {
        //
        BinaryTree result = new BinaryTree();
        result.root = root.left;
        result = child;
    }

    public void setData(Object data) {
        //
        root.data = data;
    }

    public boolean isLeaf() {
        //

        if (root.left == null && root.right == null) {
            return true;
        } else {
            return false;
        }

    }

    public String printTree() {
        //
        return root.printTree(0);
    }
   }

I don't understand why I'm only returning the root.

Answer 1

Your values for left and right are never being set, they will always be null

Also a better way would be like this

private String strVal = "";  // StringBuffer would be better

String result = printTree (root);

public String printTree(Node n) {

    if (node.left != null) {
        strVal += node.toString();
        printTree  (node.left);
    }
    if (node.right != null) {
        strVal += node.toString();
        printTree  (node.right);
    }
    return strVal;
}

The above method uses the traditional way of passing the node to the recursive method.