Question

我目前正在研究Java中的链表。我有一个示例程序，对于某些输入按预期工作，而对其他输入完全没有。

//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;
}

假设我在列表的开头添加了两个不同的人：

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

然后，代码可以正常运行。我得到了输出：“Mushu，Mulan”。 HOWEVER ，如果我在列表的开头添加一个人，最后添加另一个人，我会得到一个NullPointerException。如果我尝试在两个Person对象上调用addLast（String name）方法，则似乎没有问题。

任何提示都非常感谢！

Answer 1

尝试类似：

 // 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;
     }
}

添加到链接列表的末尾

1 个答案: