How to change/edit a JPanel object at startup/init

Question

I want to change a JPanel-object in an applet at startup/init. I can't figure out how to do this. I've made a simple example of my problem in which I clear the JPanel. It does not work when it is called by the init() method but it works when I press the button. What can I do to change the JPanel at startup/init?

import javax.swing.JApplet;
import javax.swing.JPanel;
import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class TestStartUpApplet extends JApplet {

    JPanel panel;   

    @Override
    public void init() {
        System.out.println("Init");
        erasePanel();
    }

    private void erasePanel() {
        Graphics g = panel.getGraphics();
        g.clearRect(0, 0, 117, 48);
    }

    public TestStartUpApplet() {
        getContentPane().setLayout(null);

        panel = new JPanel();
        panel.setBackground(Color.RED);
        panel.setBounds(35, 36, 117, 48);
        getContentPane().add(panel);

        JButton btnTest = new JButton("Test");
        btnTest.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                erasePanel();
            }
        });
        btnTest.setBounds(35, 108, 117, 25);
        getContentPane().add(btnTest);

    }
}

You should click the check mark next to the answer you found most useful in order to accept it. Thank you. — Code-Apprentice, Aug 03 '12 at 23:19

score 3 · Answer 1 · answered Aug 02 '12 at 21:58

Works just for me:

public class AppletPaintTest extends JApplet {

    @Override
    public void init() {

        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {

                setLayout(new BorderLayout());

                ImagePane pane = new ImagePane();
                pane.setBackground(Color.RED);
                pane.setOpaque(false); // Little trick

                add(pane);

            }
        });

    }

    @Override
    public void start() {
        super.start();
    }

    public class ImagePane extends JPanel {

        @Override
        protected void paintComponent(Graphics g) {

            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D)g.create();

            Insets insets = getInsets();
            int x = insets.left;
            int y = insets.top;
            int width = getWidth() - 1 - (insets.left + insets.right);
            int height = getHeight() - 2 - (insets.top + insets.bottom);

            g2d.setColor(getBackground());
            g2d.fillRect(x + 10, y + 10, width - 20, height - 20);

            g2d.dispose();

        }

    }

}

Thanks, helped me making my own panel class. – Paul Koning Aug 02 '12 at 22:33 — Paul Koning, Aug 02 '12 at 22:33

score 2 · Accepted Answer · answered Aug 02 '12 at 20:48

2

question

no idea why do you want to clear empty JPanel without any custom painting
what's wrong with JPanel with red Background
you clear JApplet before became visible on the screen
doesn't works correctly, because doesn't works anything

suggestions

don't use AbsoluteLayout, use LayoutManager instead
Graphics g = panel.getGraphics(); is usefull for printing to the printer, or save Graphics to the image, don't use this method for another reason
read how JApplet works
maybe to use JFrame in the case that you don't want to publish your GUI to the web browser
maybe to read tutorial about 2D Graphics

answered Aug 02 '12 at 20:48

mKorbel

109,525
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1&2) As I wrote, this is just an example. I am writing an application where I have a method that draws an image on a JPanel object. This has to be shown when the applet starts. Problem is the same. It works when I press a button but I cannot get it to work 'on startup'. 3) I guess that is the problem, so where can I put my method erasePanel() just after the moment the applet is visible? 4) That's why I posted my problem here ;-) Thanks for the suggestions. – Paul Koning Aug 02 '12 at 20:56
@Paul Koning start with 2D Graphics tutorial, [there are a few examples](http://stackoverflow.com/questions/tagged/java+swing+paintcomponent), painting for JPanale in the JApplet or JFrame or JDialog is the same, to try to avoiding to use method paint(), use only paintComponent() for Swing JComponents – mKorbel Aug 02 '12 at 21:07
@PaulKoning your making assumptions about how the paint engine works which are fundamentally wrong. Try calling something like invalidate &/or repaint in the applets run method, rather then trying to circumvent the paint process (which is what your erase method is trying to do) – MadProgrammer Aug 02 '12 at 21:17
Thanks, it's clear to me that I have to change the code, I made my own panel class and now it works. – Paul Koning Aug 02 '12 at 22:31
@PaulKoning: That's the right choice; also consider this [hybrid approach](http://stackoverflow.com/a/11372932/230513) for flexibility and this discussion of [`contentPane()`](http://stackoverflow.com/a/11769153/230513). – trashgod Aug 02 '12 at 23:21

score 2 · Answer 3 · answered Aug 02 '12 at 22:08

2

Whenever you do any custom painting outside of the usually system calls to your paint() or paintComponent() methods, you must call invalidate() on any components you wish to repaint. So for your erasePanel() method, I would suggest setting some flag then calling panel.invalidate(). Then inside your panel's paintComponent() method, you can check the flag to see if you need to draw the introductory picture or just leave the panel blank.

answered Aug 02 '12 at 22:08

Code-Apprentice

81,660
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Thanks, I fixed a by making my own panel class. – Paul Koning Aug 02 '12 at 22:32
An upvote would be nice. ;-) You should have enough reputation now. – Code-Apprentice Aug 02 '12 at 22:33
I don't understand how `invalidate()` would be useful in this context. – trashgod Aug 02 '12 at 23:24

score 1 · Answer 4 · answered Aug 02 '12 at 22:29

Ok, it seems that the problem was that my coding was just bad. I wanted to change a panel object by a method from another class and that's not the way to do it. I rewrote my code and made a panel class in which the painting is done using paintcomponent. I now use objects of this panel class and it shows the graphics I want at startup.

Thanks for the help!

How to change/edit a JPanel object at startup/init

4 Answers4