Why do I need to use the wrapper on my method

Question

I've been working on interview questions and as I was finished with the problem of finding the max depth of a tree I wanted to print the tree out as well. Going over some older code I worked on, I found the printPostOrder method. I found if I remove the wrapper code I can't use the method. I don't know enough about the subject and understand that I need to do more research, but could someone explain to me why it's necessary and why it's used. Or possibly some other examples so I can solidify my understanding of what is going on internally?

// Java program to find height of tree

// A binary tree node
  class Node {
int val;

Node left, right;

Node(int item) {
    val = item;
    left = right = null;
 }
}

class BinaryTree {
Node root;

/*
 * 
 * Through recursion we get both left and right side
 * depths. Then compare left and right side, pick the bigger one and add one to
 * it to account for the current node.
 */
int maxDepth(Node node) {
    if (node == null)
        return 0;
    else {
        /* compute the depth of each subtree */
        int leftDepth = maxDepth(node.left);
        int rightDepth = maxDepth(node.right);

        int bigger = Math.max(leftDepth, rightDepth);

        return bigger + 1;
    }

}

void printPostorder(Node node) {
    if (node == null)
        return;

    // first recur on left subtree
    printPostorder(node.left);

    // then recur on right subtree
    printPostorder(node.right);

    // now deal with the node
    System.out.print(node.val + " ");
}

// why does this wrapper make the above method function and not without it
void printPostorder() {
    printPostorder(root);
}

public static void main(String[] args) {
    BinaryTree tree = new BinaryTree();

    tree.root = new Node(1);
    tree.root.left = new Node(2);
    tree.root.right = new Node(3);
    tree.root.left.left = new Node(4);
    tree.root.left.right = new Node(5);
    tree.root.left.left.left = new Node(66);
    tree.root.left.left.right = new Node(77);
    tree.root.left.left.left.left = new Node(666);

    System.out.println("Height of tree is : " + tree.maxDepth(tree.root));
    tree.printPostorder();

}
}

Answer 1

Strictly speaking, you don't need a separate void printPostorder() method that takes no arguments; you could, instead, force the caller to call void printPostorder(Node node) , and pass the root of the tree as an argument:

    tree.printPostorder(tree.root);

But that makes for messy code. Callers expect to be able to just write tree.printPostorder() to print the tree, rather than having to pass in a redundant argument just because of how printPostorder is implemented internally.

(Incidentally, in a more-realistic program, I'd expect void printPostorder() to be public, and Node root and void printPostorder(Node node) to both be private, so as to not expose this sort of implementation detail.)