Correct. When those lines are executed on the event thread things are done like:

1) textField.setText("Loading started...");
2)  String page = loadUrl();
3) textField.setText("...Loading done");
4) update screen

And that gives the observed result.

You need to call loadUrl in a separate thread, but still update textField on the event thread as Swing components are usualy not threadsafe.

Example:

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class GuiPinger extends JFrame {
    private JTextField tf;
    private JTextArea ta;
    private JButton btn;
    public GuiPinger() {
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        setTitle("Pinger");
        setLayout(new BorderLayout());
        tf = new JTextField();
        JPanel top = new JPanel();
        top.setLayout(new BorderLayout());
        top.add(new JLabel("Host: "), BorderLayout.WEST);
        top.add(tf, BorderLayout.CENTER);
        getContentPane().add(top, BorderLayout.NORTH);
        ta = new JTextArea();
        getContentPane().add(new JScrollPane(ta), BorderLayout.CENTER);
        btn = new JButton("Ping");
        btn.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                ta.setText("");
                Thread t = new Thread(new Runnable() {
                    public void run() {
                        try {
                            Process p = Runtime.getRuntime().exec(new String[] { "ping", tf.getText() });
                            BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
                            String line;
                            while((line = br.readLine()) != null) {
                                final String line2 = line + "\n";
                                EventQueue.invokeLater(new Runnable() {
                                    public void run() {
                                        ta.append(line2);
                                    }
                                });
                            }
                            p.waitFor();
                        } catch (IOException e) {
                            JOptionPane.showMessageDialog(null, e.getMessage());
                        } catch (InterruptedException e) {
                            JOptionPane.showMessageDialog(null, e.getMessage());
                        }
                    }
                });
                t.start();
            }
        });
        getContentPane().add(btn, BorderLayout.SOUTH);
        setSize(800, 600);
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame f = new GuiPinger();
                f.setVisible(true);
            }
        });
    }
}

The long running thing (here excuting an external ping command) are done in a thread.

When the thread need to update the GUI it uses  EventQueue.invokeLater, which will cause it to be done in the event thread.

Another example:

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Paging extends JFrame implements ActionListener {
    private JTextField txt;
    private JButton btn;
    public Paging() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().setLayout(new BorderLayout());
        txt = new JTextField();
        txt.setText("");
        getContentPane().add(txt, BorderLayout.CENTER);
        btn = new JButton("Vis nyt billede");
        btn.addActionListener(this);
        getContentPane().add(btn, BorderLayout.SOUTH);
        this.setSize(300, 300);
    }
    public void load(String s) {
        txt.setText(s);
    }
    public void actionPerformed(ActionEvent ev) {
        if (ev.getSource() == btn) {
            T t = new T(this);
            t.start();
        }
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                Paging f = new Paging();
                f.setVisible(true);
            }
        });
    }
}

class T extends Thread {
    private static int n = 0;
    private Paging pag;
    public T(Paging dn) {
        this.pag = dn;
    }
    public void run() {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                pag.load("Nu viser vi billede af nogle pausefisk");
            }
        });
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
        }
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                n++;
                pag.load("Nu viser vi et rigtigt billede nummer " + n);
            }
        });
    }
}

Hello Arne!

I followed you example and did it like:

    public void actionPerformed(final ActionEvent e)
    {
        textfield.setText"Loading started...");

                T t = new T("...loading done!");
                t.start();
        }

....

    class T extends Thread
    {
        String message;
        public T(String m)
        {
            message = m;
        }
        public void run()
        {
            loadUrl();
                       
            SwingUtilities.invokeLater(new Runnable()
            {
                public void run()
                {
                    textfield.setText(message);            }
            });
        }
    }

Seems to work fine!!
If you see anything bad or something I did not get right just let me know!

How ever "mange tak"!
My best regards, really good examples you provided!!
Btw give a svar so I can reward you!

I would probably let the thread write both messages to be consistent.

But that is style not function.

og svar