java - LinkedList NullPointerException(需要帮助实现 LinkedList)

Question

我需要实现一个 LinkedList，到目前为止，我已经编写了在列表中按顺序插入值的方法。我有我的节点 front 作为我的类的实例数据，当创建我的第一个值并尝试将 front 的 next 值设置为指向我的新节点时，我得到了一个空指针异常。

public class Class1 {
private Node front = null;//Linked List pointer
private int comparisons = 0; //# of comparisons in Lists
//private LinkedList<Node> orderLink = new LinkedList<Node>();
public Class1() {

}

/*public Class1(int x){

}*/

public void insert (int num){
    Node current = null;
    Node previous = null;
    boolean placed = false;
    if (front == null){ //generate first node of list
        Node first = new Node(num, null);
        front.setNext(first);
        placed = true;
    }
    previous = front;
    current = front.getNext();
    Node step = new Node(num, null);
    if (placed == false){
    do{
        if (step.getData() < current.getData() && step.getData() > previous.getData() || step.getData() == current.getData() || step.getData() == previous.getData()){ //if the new data is between previous and current, place. If equals current, place.
            //Insert into List
            step.setNext(current);
            previous.setNext(step);
            placed = true;
        }
        if (previous == front && step.getData() < current.getData()){ //separate case for first node
            step.setNext(current);
            previous.setNext(step);
            placed = true;
        }
        if (current.getNext() == null && step.getData() > current.getData()){ //case for last node
            current.setNext(step);
            placed = true;
        }
        //move a space up the list
        previous = current;
        current = current.getNext();

    }while(previous.getNext() != null || placed == false);
    }

}

public int search(int num){
    int nodeIndex = 0;

    return 1; //Return index of num
}
public void delete (int num){
    //delete num from the list
}
public void traverse(){
    System.out.println(front.getNext());
    System.out.println(front.getNext().getNext());
    System.out.println(front.getNext().getNext().getNext());
    System.out.println(front.getNext().getNext().getNext().getNext());
    System.out.println(front.getNext().getNext().getNext().getNext().getNext());
}
public int getComparisons(){
    return comparisons;
}

public class Node {
    private Node next;
    private int data;
    public Node(){
        next = null;
    }
    public Node(int data, Node next){
        this.next = next;
        this.data = data;
    }
    public Node getNext(){
        return next;
    }
    public void setNext(Node nextNode){
        next = nextNode;
    }
    public int getData(){
        return data;
    }
    public void setData(int data){
        this.data = data;
    }
}

}

public class User {
public static void main(String[] args){
    Class1 test = new Class1();
    test.insert(1);
    test.insert(3);
    test.insert(5);
    test.insert(2);
    test.insert(4);

    test.traverse();
}

}

我刚刚开始学习 LinkedLists，我不确定我实现了什么错误。任何帮助将不胜感激。您实际上只需要查看我的插入方法和内部 Node 类。

Answer 1

您在这里遇到问题:

front.setNext(first);

您不能对 null 对象进行任何操作(并且在 front 为 null 的情况下)。大概这里应该是:

front = first;