Java,Swing - JPanel doesn't appear in JFrame as expected

Question

I got a java class called PleaseWait and want to call it whenever a heavy task is in progress. When my program does a heavy task, in the first row in my actionListener I set a variable of this class setVisible(true) then set setVisible(true) at the end of the actionListener.

Somehow the JPanel in this class does not appear when I call it, it's just a window with title as set and white blank content. Here's my code:

public class PleaseWait extends JFrame{

public PleaseWait(){

    Toolkit toolkit = Toolkit.getDefaultToolkit();

    Dimension screenDimensions = toolkit.getScreenSize();
    setSize(300,100); //set size based on screen size
    setTitle("Please wait");
    Container container = getContentPane();
    setLocation(new Point(screenDimensions.width*1/4+200, screenDimensions.height*1/4+200)); //set location based on screen size

    JPanel panel = new JPanel();
    JLabel wait = new JLabel("Please wait");
    Dimension buttonsSize = new Dimension(300,100);
    panel.setPreferredSize(buttonsSize);
    wait.setPreferredSize(buttonsSize);
    panel.setLayout(new BorderLayout());
    panel.add(wait, BorderLayout.CENTER);
    container.add(panel);

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setResizable(false); //unresizable
}

Answer 1

The key is not in the code you've posted, but in this line:

and want to call it whenever a heavy task is in progress

You're running a "heavy" task, and while you're running it, Swing is not painting this GUI, because you're likely running that task on the Swing event thread, and doing so freezes the thread, and your GUI.

Solution: use a background thread such as is obtainable through a SwingWorker, to run the "heavy" task.

Other side issues:

• This appears to be a "dependent" or "sub" window off of the main application. If so, it should not be a JFrame since an application should only have one main application window, but rather it should be a JDialog.
• You're using setPreferredSize(...) and hard-coding your component sizes, something fraught with problems.

e.g.,

import java.awt.*;
import java.awt.Dialog.ModalityType;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;

import javax.swing.*;

public class TestPleaseWait {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            MainPanel mainPanel = new MainPanel();
            JFrame frame = new JFrame("Application");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(mainPanel);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

@SuppressWarnings("serial")
class MainPanel extends JPanel {
    private static final int PREF_W = 600;
    private static final int PREF_H = 450;

    public MainPanel() {
        add(new JButton(new AbstractAction("Without Background Thread") {
            {
                putValue(MNEMONIC_KEY, KeyEvent.VK_O);
            }

            @Override
            public void actionPerformed(ActionEvent e) {
                final PleaseWait wait = new PleaseWait();
                wait.setVisible(true);
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e1) {
                }
                wait.setVisible(false);
            }
        }));
        add(new JButton(new AbstractAction("With Background Thread") {
            private JDialog waitDialog = null;
            private MyWaitPanel myWaitPanel = new MyWaitPanel();

            {
                putValue(MNEMONIC_KEY, KeyEvent.VK_W);
            }

            @Override
            public void actionPerformed(ActionEvent e) {
                if (waitDialog == null) {
                    Component component = MainPanel.this;
                    Window win = SwingUtilities.getWindowAncestor(component);
                    waitDialog = new JDialog(win, "Please Wait", ModalityType.APPLICATION_MODAL);
                    waitDialog.add(myWaitPanel);
                    waitDialog.pack();
                    waitDialog.setLocationRelativeTo(win);
                }

                new Thread(() -> {
                    try {
                        Thread.sleep(4000);
                    } catch (InterruptedException e1) {
                    }
                    SwingUtilities.invokeLater(() -> {
                        waitDialog.setVisible(false);
                    });

                }).start();
                waitDialog.setVisible(true);
            }
        }));
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }
}

@SuppressWarnings("serial")
class MyWaitPanel extends JPanel {
    private JProgressBar progressBar = new JProgressBar();

    public MyWaitPanel() {
        progressBar.setIndeterminate(true);

        JLabel waitLabel = new JLabel("Please Wait", SwingConstants.CENTER);
        waitLabel.setFont(waitLabel.getFont().deriveFont(Font.BOLD, 40));

        int ebGap = 10;
        setBorder(BorderFactory.createEmptyBorder(ebGap, ebGap, ebGap, ebGap));
        setLayout(new BorderLayout(ebGap, ebGap));
        add(waitLabel, BorderLayout.PAGE_START);
        add(progressBar);
    }
}

@SuppressWarnings("serial")
class PleaseWait extends JFrame {

    public PleaseWait() {

        Toolkit toolkit = Toolkit.getDefaultToolkit();

        Dimension screenDimensions = toolkit.getScreenSize();
        setSize(300, 100); // set size based on screen size
        setTitle("Please wait");
        Container container = getContentPane();
        setLocation(new Point(screenDimensions.width * 1 / 4 + 200,
                screenDimensions.height * 1 / 4 + 200));

        JPanel panel = new JPanel();
        JLabel wait = new JLabel("Please wait");
        Dimension buttonsSize = new Dimension(300, 100);
        panel.setPreferredSize(buttonsSize);
        wait.setPreferredSize(buttonsSize);
        panel.setLayout(new BorderLayout());
        panel.add(wait, BorderLayout.CENTER);
        container.add(panel);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setResizable(false); // unresizable
    }
}