Thread.start vs SwingUtilities - GUI update

Question

Hi
how come the GUI is not updated ( moving label ) using
        SwingUtilities.invokeAndWait(new AThread(label));when I use
        Thread t = new Thread(new AThread(label));
        t.start();the GUI is updated
Thanks
public class Test {
    public static void main(String[] args) throws Exception {
        JFrame frame = new JFrame("test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());
        frame.setSize(400, 400);
        frame.setVisible(true);
        JLabel label = new JLabel("000000");
        frame.getContentPane().add(label);
        //SwingUtilities.invokeLater(new AThread(label));
        SwingUtilities.invokeAndWait(new AThread(label));
        //Thread t = new Thread(new AThread(label));
        //t.start();
class AThread implements Runnable {
    private JLabel alabel;
    public AThread(JLabel alabel) {
        this.alabel = alabel;
    public void run() {
        try {
            while (alabel.getY() < 200) {
                Thread.sleep(50);
                Point p = alabel.getLocation();
                double x = p.getX();
                double y = p.getY();
                alabel.setLocation((int) x + 1, (int) y + 1);
                System.out.println( alabel.getY());
        } catch (InterruptedException e) {
        System.out.println("done");
}Edited by: sc3sc3 on Feb 8, 2008 10:06 AM

Answer 1

The invokeAndWait call waits for the thread to finish. So until that thread finishes, the event dispatch thread (EDT) cannot update and so you never see any changes to the label until the thread is complete.
In the second case, the EDT can incorrectly sporadically update the label because they are running concurrently. I say incorrectly here because you are modifying the label outside of the EDT which is an error and can have various unforeseen consequences. If you want to modify the label, you should queue another runnable to the EDT so the update can occur there. Something like
import java.awt.*;
import javax.swing.*;
public class TestLabelEdt {
    public static void main(String[] args) throws Exception {
        JFrame frame = new JFrame("test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());
        frame.setSize(400, 400);
        frame.setVisible(true);
        JLabel label = new JLabel("000000");
        frame.getContentPane().add(label);
        //SwingUtilities.invokeLater(new AThread(label));
        //SwingUtilities.invokeAndWait(new AThread(label));
        Thread t = new Thread(new AThread(label));
        t.start();
class AThread implements Runnable {
    private JLabel alabel;
    public AThread(JLabel alabel) {
        this.alabel = alabel;
    public void run() {
        try {
            while (alabel.getY() < 200) {
                Thread.sleep(50);
                Point p = alabel.getLocation();
                final double x = p.getX();
                final double y = p.getY();
                Runnable doRun = new Runnable() {
                    public void run() {
                        alabel.setLocation((int) x + 1, (int) y + 1);
                        System.out.println( alabel.getY());                              
                SwingUtilities.invokeLater( doRun );               
        } catch (InterruptedException e) {
        System.out.println("done");
}