java - 添加到链接列表的末尾

Question

我目前正在研究 Java 中的链表。我有一个示例程序，对于某些输入可以按预期工作，而对于其他输入则完全不起作用。

//Constructor in class List of People
ListOfPeople() {
    Person listHead = new Person("LIST HEAD");
    personList = listHead;
    lastPerson = listHead;
    numberPeople = 0;
}
//Adds person to the beginning of the list
public void addFirst(Person newP) {
    newP.next = personList.next;
    personList.next = newP;
    numberPeople++;
}
//Adds person to the end of the list
public void addLast(Person lastP) {
    lastPerson.next = lastP;
    lastPerson = lastP;
    numberPeople++;
}

对于 Person 类，我有以下代码:

//From the Person class
String name;
Person next;

Person(String n) {
    name = n;
    next = null;
}

假设我将两个不同的人添加到列表的开头:

Person mulan = new Person("Mulan");
myFriends.addFirst(mulan);
Person mushu = new Person("Mushu");
myFriends.addFirst(mushu);

然后，代码就可以正常运行了。我得到的输出是:“Mushu，Mulan”。但是，如果我在列表的开头添加一个人，在列表的末尾添加另一个人，我会收到 NullPointerException。如果我尝试在两个 Person 对象上调用 addLast(String name) 方法，似乎没有问题。

非常感谢任何提示!

Answer 1

尝试如下:

 // I left out getters and setters for brevity.
 class PersonNode {
      Person current;
      PersonNode next;
      PersonNode previous;
 }

 class PersonList {
       PersonNode head; 
       PersonNode tail;

      public PersonList(){ 
          head.previous = null;
          tail.next = null;
      }
      void addFront(Person p){ 
          if (head.person == null) {
                head.person = p; 
          }
          else {
               PersonNode temp = head; head= new PersonNode(p);               
               temp.previous = head; head.next = temp;
               }
          }
     void addBack(Person p) {
          if (tail.person == null) {
                tail.person = p; 
          }
          else {
              PersonNode temp = tail;
              tail= new PersonNode(p);               
              temp.next = tail;
              tail.previous = temp;
      }

     int count() {
          int c = 0;
          for(PersonNode n = head; head.next != null; n = head.next){
              if (n.Person !=null){
                  ++c;
              }
          }
          return c;
     }
}