Java链表-NullPointerException

Question

所以我的代码中有一些错误，但我不确定他们告诉我要更改什么。这是我的第一个链表代码。如果有人可以帮助我，我将不胜感激。

这是我的链接列表

  public class MyLinkedList<E> 
 {
private Node<E> head = null;

public void add(E element)
{
    if(size() == 0)
    {
        head = new Node<E>(element);
        return;
    }

    Node<E> cursor = head;

    while (cursor.next != null)
    {
        cursor = cursor.next;
    }

    cursor.next = new Node<E>(element);
}


public void add(int index, E element)
{
    Node<E> cursor = head;
    E temp, before;

    for(int x = 0; x < index; x++)
    {
        cursor = cursor.next;
    }

    before = cursor.content; 
    cursor.content = element;

    while(cursor.next != null)
    {
        cursor = cursor.next;
        temp = cursor.content;
        cursor.content = before;
        before = temp;
    }

    add(before);
}

public boolean remove(E element)
{
    Node<E> cursor = head;

    if (head.content == element)
    {
        head = cursor.next;
        return true;
    }

    while(cursor.next != null)
    {
        if (cursor.next.content == element)
        {
            cursor.next = cursor.next.next;
        }
        else
        {
            cursor = cursor.next;
        }

    }

    if (cursor.next == null)
    {
        return false;
    }

    return true;
}

public E remove(int index)
{
    E result = null;

    if (index < 0 || index >= size())
    {
        return null;
    }

    Node<E> cursor = head;

    for (int x = 0; x < index; x++)
    {
        cursor = cursor.next;
    }

    result = cursor.content;

    cursor = head;
    for (int x = 0; x < index - 1; x++)
    {
        cursor = cursor.next;
    }

    if(index != 0)
    {
        cursor.next = cursor.next.next;
    }
    else
    {
        head = cursor.next;
    }
    return result;
}

public E set(int index, E element)
{
    Node<E> cursor = head;
    E temp;
    for (int x = 0; x < index; x++)
    {
        cursor = cursor.next;
    }

    temp = cursor.content;
    cursor.content = element;
    return temp;
}

public boolean contains(E element)
{
    Node<E> cursor = head;

    while(cursor != null)
    {
        if(cursor.content == element)
        {
            return true;
        }
        cursor = cursor.next;
    }

    return false;
}

public E get(int index)
{
    Node<E> cursor = head;
    if (index < 0 || index >= size())
    {
        return null;
    }

    for (int x = 0; x < index; x++)
    {
        cursor = cursor.next;
    }

    return cursor.content;
}

public int indexOf(E element)
{
    Node<E> cursor = head;
    int index = 0;

    while (cursor != null)
    {
        if(cursor.content == element)
        {
            return index;
        }

        index++;
        cursor = cursor.next;
    }

    return -1;
}

public boolean isEmpty()
{
    if (size() == 0)
    {
        return true;
    }
    return false;
}

public int size()
{
    Node<E> cursor = head;
    int count = 0;

    while (cursor != null)
    {
        count++;
        cursor = cursor.next;
    }
    return count;
}

public void dumpList()
{
    Node<E> cursor = head;

    while (cursor != null)
    {
        System.out.println(cursor.content);
        cursor = cursor.next;
    }
}
 }

这是我的节点代码

  public class Node<E>
  { 
  public E content;
public Node<E> next;

public Node(E content)
{
    this.content = content;
}

public Node(E content, Node<E> next)
{
    this(content);
    this.next = next;
}

public String toString()
{
    return content.toString();
}
   }

这是我们正在测试的代码

  public class Demo4
  {
public static void main(String[] args)
{
    MyLinkedList<String> t = new MyLinkedList<String>();

    t.add("Santa Maria");
    t.add("Los Angeles");
    t.add("Ventura");
    t.add("Thousand Oaks");
    t.add(0, "Orcutt");
    t.add(5, "Pismo");
    t.add(3, "San Luis Obispo");
    t.set(1, "London");
    t.set(0, "San Diego");
    t.set(6, "Tokyo");
    t.add("Westlake");

    t.remove("Santa Maria");
    System.out.println("was Tokyo found? " + t.remove("Tokyo"));
    t.remove("Westlake");
    System.out.println("was Dubai found? " + t.remove("Dubai"));
    t.remove("Pismo");

    System.out.println("Remove index 5. It contained: " + t.remove(5));
    System.out.println("Remove index 0. It contained: " + t.remove(0));
    System.out.println("Remove index 2. It contained: " + t.remove(2));
    System.out.println("Here's what's left over");
    for (int x = 0; x < t.size(); x++)
    {
        System.out.println(t.get(x));
    }

    System.out.println("--------");
    System.out.println("Cool!  I didn't crash!");
}
   }


 my error in eclipse is the following
Exception in thread "main" java.lang.NullPointerException
at MyLinkedList.add(MyLinkedList.java:35)
at MyLinkedListDemo.main(MyLinkedListDemo.java:12)

Answer 1

看，Eclipse 正在告诉您人类可以告诉您的一切:)

这是你的错误:

Exception in thread "main" java.lang.NullPointerException
at MyLinkedList.add(MyLinkedList.java:35)
at MyLinkedListDemo.main(MyLinkedListDemo.java:12)

当调用 add() 方法时，Eclipse 在文件 MyLinkedList.java 的第 35 行中显示“存在空指针异常”。这实际上是在第 12 行调用了 MyLinkedListDemo.java 的 main() 。

现在在该行上放置一个调试点，您将看到什么是 null 以及为什么它是 null。 当您尝试在 null 上调用某些内容时，您会得到一个 NPE