addlast（）方法空指针异常

Question

public class Node {
    private String element;
    private Node next;

    public void setNext(Node newNext) {
       next = newNext;
    }

public class SLinkedList {

    protected Node head;
    protected Node tail;
    protected long size;

    public void addLast(Node newNode) {
        newNode.setNext(null);
        tail.setNext(newNode);
        tail = newNode;
        size = size + 1;
}

public class TestLinkedList {
public static void main(String args[]) {

    SLinkedList myList = new SLinkedList();

    //create 3 node
    Node m = new Node("Tom1", null);
    Node n = new Node("Tom2", null);
    Node p = new Node("Tom3", null);



    //add 1 node at the end
    myList.addLast(m);


    //print
    System.out.println(myList);
    System.out.println("Size=\t"+myList.getSize());
}

执行时“myList.addLast（m）;它在”tail.setNext（newNode）;“处显示nullpointerexception。” 我只是想知道发生了什么？

Answer 1

在您的代码中，

tail.setNext(newNode); // tail isn't initialized yet
tail = newNode;

相反，

tail = newNode;    
tail.setNext(newNode);