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 have some kind of refresh/revalidate or threading issue. The drawing is painted, but when I resize the window, the painting disappears. Also, when moving the window very quickly, parts of the drawing disappear. I have tried many things, but I can't get this problem to work, maybe you can help me. I was instructed not to write my own code to begin with, but use the NetBeans Design functions to generate Buttons and Panels etc. Maybe this impairs the drawing process/functions, but I don't know. I'll post you the relevant code and would be very thankful for suggestions (the outcommented stuff are just artefacts from what I've tried before, so don't mind it):

public class NewJFrame extends JFrame  {
  public NewJFrame() { initComponents(); }

  @SuppressWarnings("unchecked")
  private void initComponents() {
    jButton1 = new javax.swing.JButton();
    jPanel1 = new javax.swing.JPanel();
    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
    setMinimumSize(new java.awt.Dimension(1200, 1000));
    jButton1.setText("Draw");
    jButton1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        jButton1ActionPerformed(evt);
      }
    });
    GroupLayout jPanel1Layout = new GroupLayout(jPanel1);
    jPanel1.setLayout(jPanel1Layout);
    jPanel1Layout.setHorizontalGroup(
        jPanel1Layout.createParallelGroup(GroupLayout.Alignment.LEADING)
        .addGap(0, 1000, Short.MAX_VALUE)
    );
    jPanel1Layout.setVerticalGroup(
        jPanel1Layout.createParallelGroup(GroupLayout.Alignment.LEADING)
        .addGap(0, 0, Short.MAX_VALUE)
    );
    GroupLayout layout = new GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(jPanel1, GroupLayout.DEFAULT_SIZE, 1000, Short.MAX_VALUE)
            .addGap(18, 18, 18)
            .addComponent(jButton1)
            .addGap(33, 33, 33))
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addGroup(layout.createParallelGroup(GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                    .addGap(0, 745, Short.MAX_VALUE)
                    .addComponent(jButton1)
                    .addGap(237, 237, 237))
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jPanel1, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)
                        .addContainerGap())))
    );

    pack();
  }// </editor-fold>

  private void jButton1ActionPerformed(ActionEvent evt) {
    Graphics g = jPanel1.getGraphics();
    draw(jPanel1, g);
  }

  protected void paintComponent(Graphics g){
    jPanel1.setSize(1000, 1000);
    Dimension d = jPanel1.getSize();
    g.setColor(Color.BLACK);
    for (int i=0; i<=1000;i++){
      if (i%100==0){
        g.setColor(Color.RED);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
      else if(i%50==0 && i%100!=0){
        g.setColor(Color.BLUE);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
      else {
        g.setColor(Color.BLACK);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
    }
    g.setColor(Color.green);
    g.drawLine(0, d.height / 2, d.width, d.height / 2);
  }

  public void draw(JPanel Jpanel1, Graphics g) {
    System.out.println("wuffkowski");
    Jpanel1.setSize(1000,1000);
    Dimension d = Jpanel1.getSize();
    g.setColor(Color.BLACK);
    for (int i=0; i<=1000;i++){
      if (i%100==0){
        g.setColor(Color.RED);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
      else if(i%50==0 && i%100!=0){
        g.setColor(Color.BLUE);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
      else {
        g.setColor(Color.BLACK);
        g.drawLine(i, d.height/2, i, (d.height/2)+100);
      }
    }
    g.setColor(Color.green);
    g.drawLine(0, d.height / 2, d.width, d.height / 2);
    Jpanel1.paintComponents(g);
  }

  public static void lala () {
    try {
      for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
        if ("Nimbus".equals(info.getName())) {
          javax.swing.UIManager.setLookAndFeel(info.getClassName());
          break;
        }
      }
    } catch (ClassNotFoundException ex) {
      java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
      java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
      java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
      java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    java.awt.EventQueue.invokeLater(new Runnable() {
      public void run() {
        NewJFrame JF =new NewJFrame();
        JF.setVisible(true);
      }
    });
  }
  private javax.swing.JButton jButton1;
  private javax.swing.JPanel jPanel1;
}

Again thanks a lot for your precious time.

Answer 1

Java doesn't remember drawing commands for you; your graphic is rendered once when you click the button because that's the only time when draw() is called.

If you want to refresh the rendering after a resize, override paint(Graphics) and call draw() in there.

If that is related to the button click, you must add fields to your class in which you remember everything that you need in draw() including the fact whether it should draw anything:

private boolean drawAtAll = false;

private void jButton1ActionPerformed(ActionEvent evt) {
  drawAtAll = true; // ok to draw now
  draw();
}

@Override
public void paint(Graphics g) {
   super.paint(g);
   draw();
}

public void draw() {
  if( !drawAtAll ) return;

  Graphics g = jPanel1.getGraphics();
  ...
}

Further reading:

• Lesson: Performing Custom Painting
• Painting in AWT and Swing

Answer 2

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);
    }
}