Java Swing drawing disappears when resizing window

Question

i need your help badly because i cannot solve this problem on my own. I am trying to create a GUI and want to draw something in it after pressing a button, but i seem to ha

Answer 1

Try your hands on this code, and ask any questions that may arise, do painting inside the paintComponent(...) method of the JPanel. Instead of providing size everytime for the said JComponent you can simply override getPreferredSize(), of the said component. In order to call your paintComponent(...) you can simply write repaint() instead of explicitly making a call to paintComponent(...) from within your program, Swing will do that automatically.

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

public class PaintingExample
{
    private CustomPanel paintingPanel;
    private Timer timer;
    private int x = 1;
    private int y = 1;
    private ActionListener timerAction = new ActionListener()
    {
        public void actionPerformed(ActionEvent ae)
        {
            x++;
            y++;
            paintingPanel.setPosition(x, y);
        }
    };

    private void createAndDisplayGUI()
    {
        JFrame frame = new JFrame("Painting Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        paintingPanel = new CustomPanel();
        final JButton startStopButton = new JButton("STOP");
        startStopButton.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent ae)
            {
                if (timer.isRunning())
                {
                    startStopButton.setText("START");
                    timer.stop();
                }
                else if (!timer.isRunning())
                {
                    startStopButton.setText("STOP");
                    timer.start();
                }
            }
        });

        frame.add(paintingPanel, BorderLayout.CENTER);
        frame.add(startStopButton, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
        timer = new Timer(100, timerAction);
        timer.start();
    }

    public static void main(String... args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                new PaintingExample().createAndDisplayGUI();
            }
        });
    }
}

class CustomPanel extends JPanel
{
    private int x = 0;
    private int y = 0;

    @Override
    public Dimension getPreferredSize()
    {
        return (new Dimension(800, 600));
    }

    public void setPosition(int a, int b)
    {
        x = a;
        y = b;
        if (x <(getWidth() - 10) && y < (getHeight() - 10))
            repaint();
        else
            System.out.println("Nothing is happening...");
    }

    @Override
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.clearRect(0, 0, getWidth(), getHeight());
        g.setColor(Color.MAGENTA);
        g.fillOval(x, y, 10, 10);
    }
}