java - 获取链接列表空指针，我不明白为什么

该程序应允许用户将整数插入“链表”中，并始终对其进行排序。我被困在为什么在将值返回其他方法后，此时会出现空指针异常的原因。感觉到我现在正在盘旋。我有虚拟打印语句以尝试找出问题所在。

类节点：

public class Node {
  Comparable data;
  Node next;

  Node(Node n, Comparable a) {
    this.data = a;
    this.next = n;
  }
}

类SortedLinkedList：

public class SortedLinkedList {
  Node head = null;
  private Comparable SortedLinkedList;


  public SortedLinkedList() {
    this.head = null;
    this.SortedLinkedList = SortedLinkedList ;
  }

  public Node insert(Node head, Comparable a){
        if (head == null || (head.data.compareTo(a)> 0))
        {
            System.out.println("In insert first if");
            head = new Node( head, a);
            //head = new Node(head, 22);
            System.out.println("Head = " + head.data + " before     return");
            return head;
        }
        Node pointer = head;
            while (pointer.next != null)
            {
                if (pointer.next.data.compareTo(a) > 0){
                    System.out.println("In insert, in while, in if");
                    break;
                }
                pointer = pointer.next;
            }
        return head;
  }

  public void print(Node head){
    System.out.println("In print outside of for" + head);
    for (Node pointer = head; pointer != null ; pointer = pointer.next)
    {
        System.out.println("In print");
        System.out.print(pointer.data);
    }
  }
}

类TestInteger

public class TestInteger implements Comparable{
    // This is the User Interface for manipulating the List

    static SortedLinkedList sll = new SortedLinkedList ();

    public static void nodeMenu() {
        Node head = sll.head;
        System.out.println(head);
        int option;

        while(true){
            System.out.println();
            System.out.println("**** Integer Node Menu ****");
            System.out.println("****************************");
            System.out.println("** 1. Insert              **");
            System.out.println("** 2. Delete              **");
            System.out.println("** 3. Clear               **");
            System.out.println("** 4. Smallest            **");
            System.out.println("** 5. Largest             **");
            System.out.println("** 6. Return to Main Menu **");
            System.out.println("****************************");

            Scanner sc = new Scanner(System.in);
        try{

            option = sc.nextInt();
            switch (option){
            case 1:{
                try{
                    System.out.println("Type an integer to insert: ");
                    int x = sc.nextInt();
                    Integer insertItem = new Integer(x);
                    sll.insert(head, insertItem);

                    System.out.println("After insert back in case1 head = " + head.data);
                    sll.print(head);
                }catch(InputMismatchException e){
                    System.out.println("Enter only integers");
                    sc.nextLine();
                }
                nodeMenu();
            }

在类SortedLinkedList中的实际插入方法中，它可以正确打印，但在类TestInteger中获得空指针。输出如下：

1
Type an integer to insert:
5
In insert first if
Head = 5 before return
Exception in thread "main" java.lang.NullPointerException
at CS_240_HW_2.TestInteger.nodeMenu(TestInteger.java:58)
at CS_240_HW_2.Main.mainMenu(Main.java:52)
at CS_240_HW_2.Main.main(Main.java:30)

最佳答案

您的代码有很多更改。例如，您只需要一个头，并且只需要一个Node类即可获取数据。 SortedLinkedList类可以是实用程序类，它仅具有一些用于以特定方式愚弄节点的方法。

因此，我建议更改为Node。这个类保存所有数据，除了头部本身。

public class Node {
  Comparable data;
  Node next;

  Node(Comparable a) {
    this.data = a;
  }
}

然后将这些更改更改为插入器类。此类只是一些有用的方法，用于对链表和/或其节点进行便利操作。

public class SortedLinkedList {

    public Node insert(Node head, Comparable a){

        Node curr = head;
        Node prev = null;
        while (curr != null && curr.data.compareTo(a) > 0) {
            prev = curr;
            curr = curr.next;
        }
        if (prev = null) {
            return new Node(a);
        }
        prev.next = new Node(a);
        prev.next.next = curr;
        return head;
    }

    // print doesn't need changing
}

对于测试类，无需太多更改：

public class TestInteger implements Comparable{
    // This is the User Interface for manipulating the List

    static SortedLinkedList sll = new SortedLinkedList ();
    Node head = null;

    public static void nodeMenu() {

        // no changes in this part ...

                    Integer insertItem = new Integer(x);
                    head = sll.insert(head, insertItem);
                }catch(InputMismatchException e){
                    System.out.println("Enter only integers");
                    sc.nextLine();
                }
            }

确保取出对nodeMenu()的递归调用

关于java - 获取链接列表空指针，我不明白为什么，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/16407691/