Java 双重链接列表-再次出现空指针异常

我得到了一个异常，它似乎是我在用一个空节点做事情。有人能解释一下我是怎么做的吗？构造函数应该是什么样子的？我看到它是空的，或者带有头节点和尾节点

    //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++;

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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
}

你的情况是这样的：

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

---

好像是头。setPrev（n）。应该改为设置为null吗？啊，它应该是n.setPrev（）XD