Question

I run requestor thread

Task t = new Task(s1, s2, c, p);

Requestor req = new Requestor(queue, t);
req.start();

It add task to queue and run service thread

queue.add(task); // my class

Service ser = new Service(queue);
ser.start();

add method

public void add(Task t) {
    synchronized (mutex) {
        queue.add(t); // PriorityQueue<Task>

        list.clear(); // DefaultListModel

        for (Task ta : queue)
            list.addElement(ta);
    }
}

Service’s run method

    task = queue.pop(); // my class
...
    queue.add(task);

pop method

public Task pop() {
    synchronized (mutex) {
        ArrayList<Task> al = new ArrayList<Task>();

        Task tmp = queue.remove();
        al.add(tmp);

        while (tmp.isFinish() && !queue.isEmpty()) {
            tmp = queue.remove();
            al.add(tmp);
        }

        al.remove(al.size()-1);

        for (Task t : al) {
            queue.add(t);
        }

        list.clear();

        for (Task ta : queue)
            list.addElement(ta);

        return tmp;
    }
}

queue var is

    Comparator<Task> cmp = new TaskComparator();

    queue = new PriorityQueue<Task>(20, cmp);

comparator

 public class TaskComparator implements Comparator<Task> {

@Override
public int compare(Task o1, Task o2) {
    return o1.getPriority() - o2.getPriority();
}

}

generally this priority queue sort randomly, some time i can see

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 1 >= 0
at java.util.Vector.elementAt(Vector.java:427)
at javax.swing.DefaultListModel.getElementAt(DefaultListModel.java:70)
...

—-edit
GUI

private Queue queue;
...
    _add.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
...
            Task t = new Task(s1, s2, c, p);

            Requestor req = new Requestor(queue, t);
            req.start();
        }
    });
...
    JScrollPane scrollPane = new JScrollPane();
    contentPane.add(scrollPane, BorderLayout.CENTER);

    DefaultListModel model = new DefaultListModel();

    JList _list = new JList(model);
    scrollPane.setViewportView(_list);

    queue = new Queue(model);

— edit

I implements comparable to task class. If i return 1 it always add new o elements on bottom, when I return -1 it always add new element on top, but when

@Override
public int compareTo(Task o) {
    if (pr == o.getPriority())
        return 0;
    else if (pr > o.getPriority())
        return 1;
    else
        return -1;
}

It sort as it want

Answer 1

PriorityQueue is a heap, and it’s rebuilding when you use remove() method.

public void add(Task t) {
    synchronized (mutex) {
        queue.add(t);

        list.clear();

        ArrayList<Task> al = new ArrayList<Task>();

        while (queue.size() > 0)
            al.add(queue.remove());

        for (Task ta : al) {
            list.addElement(ta);
            queue.add(ta);
        }
    }
}