为什么会出现这样的报错

Question

package dd;

import java.util.Scanner;

class Node {

    public double data;

    public dd.Node leftNode;

    public dd.Node rightNode;

    public Node(double n) {
        data = n;
    }

    public void display() {
        System.out.print(this.data + "\t");
    }
}

class Tree {
    private dd.Node root;

    public void insert(double data) {
        dd.Node newNode = new dd.Node(data);
        if (root == null) {
            root = newNode;
        }
        dd.Node current = root;
        dd.Node parentNode = null;

        while (current != null) {
            parentNode = current;
            if (current.data > data) {
                current = current.leftNode;
                if (current == null) {
                    parentNode.leftNode = newNode;
                }
            } else {
                current = current.rightNode;
                if (current == null) {
                    parentNode.rightNode = newNode;
                }
            }
        }
    }

    public void preOrderTraverse() {
        preOrderTraverse(root);
        System.out.println();
    }

    private void preOrderTraverse(Node node) {
        if (node == null)
            return;

        node.display();
        preOrderTraverse(node.leftNode);
        preOrderTraverse(node.rightNode);
    }
}

public class Main {
    public static void showmenu(){
        System.out.println("-------------------------\n");
        System.out.println("a to add \n");
        System.out.println("b to show \n");
        System.out.println("c to quit\n");
        System.out.println("-------------------------\n");
    }

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

        double a;
        char ch;
        Scanner sc = new Scanner(System.in);
        showmenu();
        ch = sc.next().charAt(0);
        while (ch != 'c') {
            switch (ch) {
                case 'a':
                    System.out.println("Enter a numble :\n");
                    a = sc.nextDouble();
                    tree.insert(a);
                    break;
                case 'b':
                    System.out.println("Here is the result:\n");
                    tree.preOrderTraverse();
                    break;
                case 'c':
                    break;
                default:
                    break;

            }
            showmenu();
            ch = sc.next().charAt(0);
        }
    }
}

Answer 1

根节点为空直接返回

    if (root == null) {
            root = newNode;
            return;
        }