在Java中通过链表实现优先级队列

Question

我正在使用链接列表实现Java中的Priority Queue，设置优先级顺序，我试图将所有链接列表对象转移到数组进行排序，然后再次返回到Linked列表（优先排序后）。我尝试了这段代码，但我得到了＃34;空指针异常＆＃34;每次我将Linked List Object传递给Array。我该怎么办？

public class Node {

    Node next;
    Object data;
    int pref;

    Node() {
        this(0, null);
    }

    Node(Object m, Node s) {
        setData(m);
        next = s;
    }

    Node(Object m, Node s, int pref) {
        setData(m);
        next = s;
        this.pref = pref;
    }

    void setData(Object m) {

        data = m;
    }

    public Object getData() {
        return data;
    }

    public void setPref(int pref){
        this.pref = pref;
    }

    public int getPref(){
        return pref;
    }
}




 public class PriorityQueue(){
    public Node head;
    PriorityQueue(){
    head = null;
    }
    public boolean enQueue(Object v, int p) {

            Node newNode = new Node();
            newNode.setData(v);
            newNode.setPref(p);

            if (head == null) {

                newNode.next = head;
                head = newNode;
                return true;

            } else {

                Node current = head;

                try {

                    while (current.next != null) {
                        current = current.next;
                    }
                    current.next = newNode;
                    settingPriorityLevel();
                    return true;

                } catch (java.lang.OutOfMemoryError e) {
                    System.out.println(e.getMessage());
                    return false;
                }
            }

        }

        public void settingPriorityLevel(){

            Node arr[] = null;      //converting LinkedList to array for sorting purpose
            Node current = head;
            Node temp;

            for(int i = 0; i <= size(); i++){
                arr[i] = current;

                current = current.next;
            }

            for(int i=0; i < arr.length; i++){              //sorting
                for(int j = i+1; j < arr.length; j++){
                    if(arr[i].pref > arr[j].pref){
                    temp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = temp;
                    }
                }
            }

            for(int i = 0; i < arr.length; i++){        //shifting back to LinkedList
                current = arr[i];
                current = current.next;
            }


        }
    }