java - 双向链表-再次出现空指针异常

Question

我收到此异常，这似乎是我正在使用空节点执行操作。有人可以解释我是怎么做的吗？构造函数应该是什么样子？我看到它是空的或带有 header 和标尾虚拟节点..

    //default constructor
public AddressList() {

}

    //get size
public int size() {
    return counter;
}

public void addEntryNode(){
    //create the new node
    EntryNode n = new EntryNode();
    //set the data
    System.out.println("Enter first name: ");
    n.myFirstName = keyboard.next();
    n.setFirstName(n.myFirstName);

    System.out.println("Enter Last Name: ");
    n.myLastName = keyboard.next();
    n.setLastName(n.myLastName);

    System.out.println("Enter Email Address: ");
    n.myEmail = keyboard.next();
    n.setEmail(n.myEmail);

    System.out.println("Enter Phone Number: ");
    n.myPhoneNum = keyboard.next();
    n.setPhoneNum(n.myPhoneNum);

    n.setIndex(index);

    //add nodes to head of list
    head.setPrev(n);
    n.setNext(head);
    head = n;

    //up the count and index 
    counter++;

Answer 1

嗯，这似乎是最有可能的解决办法。将 head 初始化为 null，并添加列表为空时的条件。

public AddressList() {
    head = null
}

你的情况如下:

if (head == null) {  // empty list
    head = n
}
else {
    head.setPrev(n);
    n.setNext(head);
    head = n;
}