How do you swap data between two nodes? (Java)

Question

I've been working on a project using Nodes and for this project we have to order the Nodes based on the priority (0 - 2 with 0 being the highest and 2 being the lowest) that the item within the Nodes hold (in decreasing order). Now I have got this working partially as when it actually does the swap it turns both items to be the same (I'm only trying to swap the items). For example: I would enter Node_1 which holds prio of 0, then Node_2 which holds prio of 1, it should then be ordered as:

Node_2(holds prio 1) --> Node_1(holds prio 0)

then it should run the method to order them as (in decreasing priority):

Node_1(holds prio 0) --> Node_2(holds prio 1)

but instead it just turns both nodes to be the same (same priority):

Node_1(holds prio 0) --> Node_1(holds prio 0)

this stays the same no matter how many nodes I add to it. They all turn to the same Node with the highest priority. Any help would be great.

Code:

private void sortJobs() {
    Node p, q, r;

    p = jobs;
    q = null;
    r = null;
    //only runs if there is more than 1 job
    while (p != null && p.next != null) {
        q = p;
        p = p.next;

        if (q.item.getPriority() > p.item.getPriority()) {
            r = q;
            q.item = p.item;
            p.item = r.item;


        }

    }

}

Please let me know if there is a better way to do this as I'm still learning.

Answer 1

Something more like this maybe:

private void sortJobs(Node p) {

    if (p != null && p.next != null) {

        Node q = p.next;

        Item pItem = p.getItem();
        Item qItem = q.getItem();

        // check for nulls???  Safety???
        if (qItem.getPriority() < pItem.getPriority()) {

            p.setItem(qItem);
            q.setItem(pItem);

         }

         // almost forgot the recursion
         sortJobs(q);

    }

}

You keep the nodes in the same order, but you swap the items in them. We don't need the copy into a third dummy value trick here because we already have references to the two items (we needed the references anyway to grab the priorities).

Answer 2

This has been said in Andreas's answer, but i'll elaborate further. Since this is Java, your temp (r in this case) has just a reference to q. That's why when you change write q = p; , r also changes entirely. Because they reference the same instance of the object. You need to create a temporary item to swap properly. The correct fix would be to have r be an item object, then setting p.item = r . Hope this helps.

Answer 3

Is it possible to use PriorityQueue instead of implementing sort function?

PriorityQueue<Node> pq = new PriorityQueue(new Comparator<Node>(){
    public int compare(Node a, Node b){
        return a.item.getPriority()-b.item.getPriority();
    }
});
p=jobs;
if(p==null) return p;
//add all jobs in PriorityQueue
while (p != null) {
    pq.add(p);
    p=p.next;
}
//Change next pointer
Node head=pq.poll();
Node prev=head;
while(pq.size()>0){
    Node n = pq.poll;
    prev.next=n;
    prev=n;
}
return head;