java - 三叉树实现中出现空指针异常

Question

我正在尝试在 java 中实现三叉树。但我的代码中出现空指针异常

Exception in thread "main" java.lang.NullPointerException
at TreeStructure.printNode(TreeStructure.java:67)
at TreeStructure.printNode(TreeStructure.java:64)
at TreeStructure.runTree(TreeStructure.java:95)
at TreeStructure.main(TreeStructure.java:109)

我的主要树代码是

public class TreeStructure 
{


  public Node root;


    /**
     * @param args
     */
    public void insert(Node node, String value)
    {
        if(value.compareTo(node.value)<0)
        {
            if (node.left!=null)
            {
                insert(node.left,value);
            }
            else
            {
                System.out.println("Inserted"+value+"to left of"+ node.value);
                node.left=new Node(value);
                node.left.parent=node;
            }

        }
        else if(value.compareTo(node.value)>0)
        {
            if(node.right!=null)
            {
                insert(node.left,value);
            }
            else
            {
                System.out.println("Inserted"+value+"to right of "+node.value);
                node.right=new Node(value);
                node.right.parent=node;
            }
        }
        else if(value==node.value)
        {
            if(node.middle==null)
            {
                insert(node.middle,value);
            }
            else
            {
                System.out.println("Inserted"+value+"to middle of"+node.value);
                node.middle=new Node(value);
                node.middle.parent=node;
            }



        }

    }

    public void printNode(Node node)
    {
        if(node!=null)
        {
            printNode(node.left);
            System.out.println("Tree traversal"+node.value);
        }
        if(node.parent!=null)
        {
            System.out.println("parent is"+node.parent.value);
        }
        else
        {
            System.out.println("");
            printNode(node.middle);
            printNode(node.right);
        }
        ///need to add some code here
    }
    public void TestInsert(Node start)
    {
        System.out.println("Testcase Insert");
        insert (root,"Astana");
        insert(root,"Asgrad");
        insert(root,"Lychs");
        insert (root,"Brac");
        insert(root,"London");
        insert(root,"Arad");
        insert(root,"Kathmundu");
        printNode(start);
    }
    public void runTree()
    {
         root=new Node("Asgrad");
         System.out.println("Tree build with"+ root.value);
         printNode(root);
         TestInsert(root);


    }




    public static void main(String[] args) 
    {

        // TODO Auto-generated method stub
        TreeStructure  tree=new TreeStructure();
        tree.runTree();

    }

}

我的节点类是

    public class Node 
{
    Node parent;
    Node left;
    Node right;
    Node middle;
    String value;
    public  Node(String value)
    {
        this.parent=null;
        this.left=null;
        this.right=null;
        this.middle=null;
        this.value=value;


    }



}

我正在尝试解决这个问题，但我无法找出错误。如果有人帮我找到问题，那就太好了......提前致谢

Answer 1

You have a few problems, but just focusing on your current exception . . . you need to change this:

if(node.parent!=null)

to this:

if(node != null && node.parent != null)

and you need to learn how to read a stack-trace. :-)

Answer 2

Look here:

    if(node.parent !=null) // node might be null, exception might be thrown
    {
       [..]
    }
    else // node might be null
    {
        System.out.println("");
        // here node might be null and the exception can be thrown
        printNode(node.middle);
        printNode(node.right);
    }