尝试将节点添加到链接列表

Question

We've started with linked list and nodes in class and I think I understood how they work. But at one part, where we have to add the node to the linked list, I'm having some issues to get it to work. The mistake occurs in the method "append". My IDE doesn't tell me much about the issue.

My class LinkedList:

public class LinkedList {
    public Node head = null;

    public void insert(Node n) {
        n.next = head;
        head = n;
    }

    public Node search(int nummer) {
        Node current = head;

        while (current != null) {
            if (current.element == nummer)
                return current;
            current = current.next;
        }
        return null;
    }

    public int count() {
        int c = 0;

        for (Node n = head; n != null; n = n.next) {
            c++;
        }
        return c;
    }

    public void append(Node n) {
        if (head == null){
            head = new Node(n, null);
        }
        else {
            Node p = head;
            while (p.a != null){
                p = (Node) p.a;
            }
        p.a = new Node(n, null);}
    }    
}

My Node-class:

public class Node {
    public int element = 0;
    public Node next = null;
    Object a;

    public Node(int e, Node n) {
    this.element = e;
    this.next = n;
    }

    public Node(int e) {
    this.element = e;
    }

}

Answer 1

Your are treating the field a as if it were the pointer to the next node in the list. It is not, it is actually the data contained in each list node. Instead, modify your append() method to use next :

public void append(Node n) {
    if (head == null) {
        head = new Node(n, null);
    }
    else {
        Node p = head;
        // walk down the list from the head until reaching the end
        while (p.next != null) {
            p = (Node) p.next;
        }
        // then append the new Node to the end of the list
        p.next = n;
    }
}

Note that ideally you should have getter and setter methods for your Node class. I might suggest something like this:

public class Node {
    private int element = 0;   // no idea what this is for
    private Node next = null;
    private Object a;

    public Node(int e, Node n) {
        this.element = e;
        this.next = n;
    }

    public Node(int e) {
        this.element = e;
    }

    public void setNext(Node next) {
        this.next = next;
    }

    public Node getNext() {
        return next;
    }

    public void setA(Object a) {
        this.a = a;
    }

    public Object getA() {
        return a;
    }
}

Using this updated definiton of Node your append() method would become:

public void append(Node n) {
    if (head == null) {
        head = new Node(n, null);
    }
    else {
        Node p = head;
        // walk down the list from the head until reaching the end
        while (p.getNext() != null) {
            p = p.getNext();
        }
        // then append the new Node to the end of the list
        p.setNext(n);
    }
}