java.lang.NullPointerException 尝试向循环链表添加元素时

Question

我正在尝试向循环链表添加一个元素，但每当我尝试访问该节点时它都会抛出 NullPointerException。这是我的代码：编辑：我已经阅读了 post 解释 NullPointerExceptions 但没有任何效果。

public class CircularLinkedList<T> {
static private class Node<T> {
    public T data;
    public Node<T> next;

    Node(T data, Node<T> next) {
        this.data = data;
        this.next = next;
    }
}

private Node<T> first;

public void addFirst(T x){
    if (first.data.equals(null)){
        first.data = x;
        first.next = first;
    }else{
        Node<T> second = first;
        first.data = x;
        first.next = second;
    }
}

编辑：我已经用 private Node<T> first = new Node(null, null); 初始化了节点，一直抛出错误。同样与first.data == null相比，现在它终止了进程。

编辑：完整代码如下：

public class CircularLinkedList<T> {
    static private class Node<T> {
        public T data;
        public Node<T> next;

        Node(T data, Node<T> next) {
            this.data = data;
            this.next = next;
        }
    }

    private Node<T> first = new Node(null, null);

    public void addFirst(T x){
        if (first.data == null){
            first.data = x;
            first.next = first;
        }else{
            Node<T> second = first;
            first = new Node(x, second);
        }
    }

    public void print() {
        while (first.data == null) {
            System.out.println(first.data);
            first = first.next;
        }
    }

    public static void main(String[] args) {
        CircularLinkedList<String> lst = new CircularLinkedList<String>();
        lst.addFirst("Z");
        lst.addFirst("A");
        lst.addFirst("B");
        lst.print();
    }
}

Answer 1

您在 first.data.equals(null) 语句中得到 NPE，因为最初 first 变量本身是 null。你必须在开始时像这样初始化它

private Node<T> first = new Node(null, null);

你还必须与 null 进行不同的比较

if (first.data == null) {

java.lang.NullPointerException 尝试向循环链表添加元素时

java.lang.NullPointerException when trying to add elements to a circular linked list

java

circular-list

data-structures