NullPointerException 构建二叉搜索树

Question

我正在用 Java 制作二叉搜索树，但在尝试向树中添加节点时遇到了 NullPointerException。附上我目前拥有的代码截图。

import java.util.Scanner;
import java.io.FileWriter;
import java.io.IOException;

/**
 * Write a description of class BinaryTree here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
public class BinaryTree<T extends Comparable<T>> { 

    public TreeNode<T> root = null;
    
    public void add(T value) {
        if(root == null) {
            root = new TreeNode<T>(value, 0);
        } else {
            add(root, value, 0);
        }
    }
    
    private void add(TreeNode<T> node, T entry, int rank) {
        if(entry.compareTo(node.value) < 0) {
            if(node.left == null) {
                node.left = new TreeNode<T>(entry, rank + 1);
            } else {
                add(node.left, entry, rank + 1);
            } 
        } 
        if(node.right == null) {
            node.right = new TreeNode<T>(entry, rank + 1);
        } else {
            add(node.right, entry, rank + 1);
            }
    }
    
    public void printInOrder() {
        printInOrder(root);
        System.out.println();
    }
    
    private void printInOrder(TreeNode<T> node) {
        if(node.left != null) {
            printInOrder(node.left);
        }
        System.out.println(node.value);
        if(node.right != null) {
            printInOrder(node.right);
        }
    }
    
    public void printPreOrder() {
        printPreOrder(root);
        System.out.println();
    }
    
    private void printPreOrder(TreeNode<T> node) {
        if(node != null) {
            System.out.println(node.value);
            printPreOrder(node.left);
        }
        if(node.right != null) {
            printPreOrder(node.right);
        }
    }
    
    public void printPostOrder() {
        printPostOrder(root);
        System.out.println();
    }
    
    private void printPostOrder(TreeNode<T> node) {
        if(node.left != null) {
            printPostOrder(node.left);
        }
        if(node.right != null) {
            printPostOrder(node.right);
        }
        System.out.println(node.value);
    }
    
    public void printTreeGraph() {
         Queue<TreeNode<T>> queue = new Queue<>();
         queue.push(root);
         try {
             FileWriter treeFile = new FileWriter("rankOrderTreeGraph.txt");
             treeFile.write("digraph Tree {");         
                 while(! queue.isEmpty()) {
                 TreeNode<T> node = queue.pop();
                 if(node.parent != null) {
                     treeFile.write("\t" + node.parent.value + "->" + node.value + ";");
                     System.out.println(node.value + ";");
                 }
                 if(node.left != null) {
                     queue.push(node.left);
                 }
                 if(node.right != null) {
                     queue.push(node.right);
                 }   
            }
            treeFile.write("}");
            treeFile.close();
            } catch (IOException e) {
                System.out.println("An error occurred");
            }
    }
    
    public static void main(String[] args) {
        BinaryTree<String> tree = new BinaryTree<>();
        System.out.println("Enter name, enter 'quit' when finished");
        Scanner scanner = new Scanner(System.in);
        String name = scanner.nextLine();
        while(! name.equals("quit")) {
            tree.add(name);
            name = scanner.nextLine();
        }
        System.out.printf("Choose how to print tree: %n" +
            "1: Inorder Traversal %n" + "2: Preorder Traversal %n" +
            "3: Postorder Traversal %n" + "4: As a GraphViz Dot File %n" +
            "q: Quit %n");
        String choice = scanner.nextLine();
        while(! choice.equals("q")) {
            if(choice.equals("1")) {
                tree.printInOrder();
                System.out.println();
            }
            if(choice.equals("2")) {
                tree.printPreOrder();
                System.out.println();
            }
            if(choice.equals("3")) {
                tree.printPostOrder();
                System.out.println();
            }
            if(choice.equals("4")) {
                tree.printTreeGraph();
            } else {
                System.out.println("Error");
            }
            System.out.println(">");
            choice = scanner.nextLine();
        }
    }
}

/**
 * Write a description of class TreeNode here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
public class TreeNode<T extends Comparable<T>> {
    public T value;
    public TreeNode<T> left;
    public TreeNode<T> right;
    public int rank;
    public TreeNode<T> parent;
    
    public TreeNode(T value, int rank) {
        value = this.value;
        left = null;
        right = null;
        this.rank = rank;
    }
}
    

当我尝试将节点添加到列表时，我得到一个 NullPointerException，它指向私有方法 add 中的 if(entry.compareTo(node.value) < 0)。