BST中的空指针

Question

I am attempting a 'competition' that is run through a BST, i have implementation of players that are entered through a file provided ect ect.

Basic Premisiss of Implementation-

The plan is to use a dummy node for creation and running the tree.

roots left becomes the dummy , then the left of the dummy is the first player entered, the right then assigned as dummy. The process is repeated untill there are two teams left, then left and right are set to the remianing two teams.

Below is the code i have created to preform such a task!

public class BT {

    private Node root;

    private static class Node {
        String name;
        int score;
        Node left;
        Node right;

        Node(String newName) {
            name = newName;
            score = 0;
            left = null;
            right = null;
        }
    }

    public void BinaryTree(){
        root = null;
    }

    private Node insert(Node node, String player) {
        node = new Node(player);
        return node; // in any case, return the new pointer to the caller
    }

    public void setup(ArrayList<String> players, String setup) {
        Node dummy = root;
        if (setup == "left") {    
            dummy = dummy.left;
            for(int i = players.size()-1; i>=0; i--) {
                if(i>2) {
                    dummy.left = insert(dummy.left, players.get(i));
                } else if (i <= 2 ){
                    dummy = dummy.left;
                    dummy.left = insert(dummy.left , players.get(i));
                    dummy.right = insert(dummy.left, players.get(i));
                } //end else if
            } //end For
        } else if (setup == "right") {
            dummy= dummy.right;
            for(int i = players.size()-1; i>=0 ; i--) {
                if(i>2) {
                    dummy.left = insert(dummy.left, players.get(i));
                } else if (i<=2){
                    dummy = dummy.left;
                    dummy.left = insert(dummy.left , players.get(i));
                    dummy.right = insert(dummy.left, players.get(i));
                }//end else if
            }//end For
        } //end if
    }
}

However when i run this class i am getting Null Pointer on the first dummy = dummy.left;

I understand there is nothing there persay but a node left of root does exist due to creation of the root itself surely?

If anyone can highlight stupid errors i would greatly appreciate it.

(I understand the method in inputting players is a little odd but it should give the result i wish for, although not the normal method for inputting data to a tree.)

Answer 1

Seemed to fix the problem i was having, .equals was a smart change i cant believe i forgot about that so thank you.

It seems like a simple idea i have implemented to fix, and you sparked the thought CupawnTae so thank you also.

I was being silly and the break down was essentially.

1.) Root was created, so setting dummy.left was valid. Root existed therefore even though it had not value left node existed. 2.)Left node from the root was not a node itself, so would not own is own left node so threw a NPE.

code of what i changed is below -

    public void setup(ArrayList<String> players, String setup) {
    Node dummy;

    if (setup.equals("left")) {

        dummy = new Node("not played");
        root.left = dummy;
        dummy = root.left;
        System.out.println(dummy.name);

        for (int i = players.size() - 1; i >= 0; i--) {

            if (i > 1) {
                dummy.right = new Node("not played");
                dummy = dummy.right;

                dummy.left = insert(dummy.left, players.get(i));
                dummy = dummy.left;

                System.out.println(dummy.name);
            }

            else if (i <= 2) {
                dummy.right = insert(dummy.right, players.get(i));
                System.out.println(dummy.right.name);

            }
        }
    }

    else if (setup.equals("right")) {

        dummy = new Node("not played");
        root.right = dummy;
        dummy = root.right;
        System.out.println(dummy.name);

        for (int i = players.size() - 1; i >= 0; i--) {

            if (i > 1) {
                dummy.right = new Node("right not");
                dummy = dummy.right;

                dummy.left = insert(dummy.left, players.get(i));
                System.out.println(dummy.left.name);
            } else {
                dummy.right = insert(dummy.right, players.get(i));
                System.out.println(dummy.right.name);
            }
        }
    }

}

This seems to have cracked it, left debugging printouts in so they are ignoreable.

Thank you all :)