Question

I have a recurring problem where I have a JList which I wish to update with new contents. I’m using a DefaultListModel which provides methods for adding new content to the list but when using these methods I find that some proportion of the calls result in a completely blank JList. Whether or not the update works seems to be random, and not related to the data which is sent.

Below is a simple program which demonstrates the problem. It simply generates a list of an increasing size to update a JList, but when run the list contents appear and disappear seemingly at random.

As far as I can tell I’m following the correct API to do this, but I guess there must be something fundamental I’m missing.

import java.awt.BorderLayout;
import javax.swing.*;

public class ListUpdateTest extends JPanel {

    private JList list;
    private DefaultListModel model;

    public ListUpdateTest () {
        model = new DefaultListModel();
        list = new JList(model);

        setLayout(new BorderLayout());

        add(new JScrollPane(list),BorderLayout.CENTER);
        new UpdateRunner();
    }

    public void updateList (String [] entries) {
        model.removeAllElements();
        for (int i=0;i<entries.length;i++) {
            model.addElement(entries[i]);
        }
    }

    private class UpdateRunner implements Runnable {

        public UpdateRunner () {
            Thread t = new Thread(this);
            t.start();
        }

        public void run() {

            while (true) {
                int entryCount = model.size()+1;

                System.out.println("Should be "+entryCount+" entries");

                String [] entries = new String [entryCount];

                for (int i=0;i<entries.length;i++) {
                    entries[i] = "Entry "+i;
                }

                updateList(entries);

                try {
                    Thread.sleep(1000);
                } 
                catch (InterruptedException e) {}
            }
        }   
    }

    public static void main (String [] args) {

        JDialog dialog = new JDialog();
        dialog.setContentPane(new ListUpdateTest());
        dialog.setSize(200,400);
        dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        dialog.setModal(true);
        dialog.setVisible(true);
        System.exit(0);
    }

}

Any pointers would be very welcome.

Answer 1

Look at this code:

import java.awt.BorderLayout;
import javax.swing.*;
import javax.swing.SwingWorker;
import java.util.Arrays;
import java.util.List;
public class ListUpdateTest extends JPanel {

    private JList list;
    private DefaultListModel model;

    public ListUpdateTest () {
        model = new DefaultListModel();
        list = new JList(model);

        setLayout(new BorderLayout());

        add(new JScrollPane(list),BorderLayout.CENTER);
        (new UpdateRunner()).execute();
    }

    public void updateList (List<String> entries) {
        model.removeAllElements();
        for (String entry : entries) {
            model.addElement(entry);
        }
    }
    private class UpdateRunner extends SwingWorker<List<String>, List<String>>{

        @Override
        public List<String> doInBackground() {
            while (true) {
                int entryCount = model.size()+1;

                System.out.println("Should be "+entryCount+" entries");

                String [] entries = new String [entryCount];

                for (int i=0;i<entries.length;i++) {
                    entries[i] = "Entry "+i;
                }

                publish(Arrays.asList(entries));

                try {
                    Thread.sleep(1000);
                }
                catch (InterruptedException e) {}
            }
            return null;
        }
        @Override
        protected void process(List<List<String>> entries) {
            for (List<String> entry : entries) {
                updateList(entry);
            }
        }
        @Override
        protected void done() {
            updateList(Arrays.asList("done"));
        }
    }

    public static void main (String [] args) {

        JDialog dialog = new JDialog();
        dialog.setContentPane(new ListUpdateTest());
        dialog.setSize(200,400);
        dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        dialog.setModal(true);
        dialog.setVisible(true);
        System.exit(0);
    }

}

Implemented by SwingWorker. Works smooth.