I am making a simple paint program in java, I can draw but when I minimize the window or resize what I painted goes away. How do I get them to stay?

Question

Here is what I have so far, I tried creating the draw function just to test if that would work. The palette is just another gui window that will have color options once I figure this saving problem out first

import csc260final.Palette;
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JOptionPane;


/**
*
* @author Jared Hughes
*/
public class Canvas extends javax.swing.JFrame {

/**
 * Creates new form Canvas
 */


Draw line;
private int curX, curY, oldX, oldY;
public Graphics2D g2;
Palette palette;



public Canvas() {

    initComponents();
    palette = new Palette();
    palette.setVisible(true);



}
public void draw(){

     g2 = (Graphics2D) jPanel1.getGraphics();
     g2.setColor(Color.yellow);
     g2.setStroke(new BasicStroke(10));
     g2.drawLine(oldX, oldY, curX, curY);

}

public void paint(Graphics g){

    Graphics2D g2 = (Graphics2D)g;
    super.paint(g);




}



/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    jPanel1 = new javax.swing.JPanel();
    jButton1 = new javax.swing.JButton();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    jPanel1.setBackground(new java.awt.Color(255, 255, 255));
    jPanel1.setPreferredSize(new java.awt.Dimension(1130, 550));
    jPanel1.setRequestFocusEnabled(false);
    jPanel1.addMouseMotionListener(new java.awt.event.MouseMotionAdapter() {
        public void mouseDragged(java.awt.event.MouseEvent evt) {
            jPanel1MouseDragged(evt);
        }
        public void mouseMoved(java.awt.event.MouseEvent evt) {
            jPanel1MouseMoved(evt);
        }
    });
    jPanel1.addMouseListener(new java.awt.event.MouseAdapter() {
        public void mouseClicked(java.awt.event.MouseEvent evt) {
            jPanel1MouseClicked(evt);
        }
        public void mouseEntered(java.awt.event.MouseEvent evt) {
            jPanel1MouseEntered(evt);
        }
        public void mouseExited(java.awt.event.MouseEvent evt) {
            jPanel1MouseExited(evt);
        }
        public void mousePressed(java.awt.event.MouseEvent evt) {
            jPanel1MousePressed(evt);
        }
        public void mouseReleased(java.awt.event.MouseEvent evt) {
            jPanel1MouseReleased(evt);
        }
    });

    javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
    jPanel1.setLayout(jPanel1Layout);
    jPanel1Layout.setHorizontalGroup(
        jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 1130, Short.MAX_VALUE)
    );
    jPanel1Layout.setVerticalGroup(
        jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 550, Short.MAX_VALUE)
    );

    jButton1.setText("jButton1");

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
            .addComponent(jButton1)
            .addGap(94, 94, 94))
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 28, Short.MAX_VALUE)
            .addComponent(jButton1)
            .addContainerGap())
    );

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

private void jPanel1MousePressed(java.awt.event.MouseEvent evt) {                                     
    // TODO add your handling code here:
    // save x and y when mouse is pressed        
     curX = evt.getX();
     curY = evt.getY();
     oldX = curX;
     oldY = curY; 


}                                    

private void jPanel1MouseDragged(java.awt.event.MouseEvent evt) {                                     
    // TODO add your handling code here:
    // get x and y when mouse is dragged

     curX = evt.getX();
     curY = evt.getY();

     g2 = (Graphics2D) jPanel1.getGraphics();
     g2.setColor(Color.yellow);
     g2.setStroke(new BasicStroke(10));
     g2.drawLine(oldX, oldY, curX, curY);

     oldX = curX;
     oldY = curY; 



}                                    

private void jPanel1MouseClicked(java.awt.event.MouseEvent evt) {                                     
    // TODO add your handling code here:      


}                                    

private void jPanel1MouseEntered(java.awt.event.MouseEvent evt) {                                     
    // TODO add your handling code here:
}                                    

private void jPanel1MouseMoved(java.awt.event.MouseEvent evt) {                                   
    // TODO add your handling code here:
}                                  

private void jPanel1MouseExited(java.awt.event.MouseEvent evt) {                                    
    // TODO add your handling code here:
}                                   

private void jPanel1MouseReleased(java.awt.event.MouseEvent evt) {                                      
    // TODO add your handling code here:


}                                     

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {
    /* Set the Nimbus look and feel */
    //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
    /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */
    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(Canvas.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(Canvas.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(Canvas.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(Canvas.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>


    /* Create and display the form */
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new Canvas().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
private javax.swing.JButton jButton1;
private javax.swing.JPanel jPanel1;
// End of variables declaration                   

}

Answer 1

Your Swing drawing is being done incorrectly. You should not draw with a Graphics object obtained by calling getGraphics() on a component. This will return a Graphics object that is short lived, risking disappearing graphics or worse, a NullPointerException. Instead, draw in the JPanel's paintComponent(...) method either directly, or indirectly by drawing on a BufferedImage (yes, you can get its Graphics object via getGraphics() ) and then drawing the BufferedImage to the GUI within the paintComponent method.

Useful Java Tutorials:

• Lesson: Performing Custom Painting : introductory tutorial to Swing graphics
• Painting in AWT and Swing : advanced tutorial on Swing graphics

Small nitpicks:

• Avoid giving your classes names that clash with core Java classes, such as Canvas . This will help prevent confusing others and your future self.
• When posting code here, we greatly appreciate it if you strive to format it well, including judicious use of empty lines. Your code above has several large empty regions that make following it hard. A single empty line here or there is OK, but a lot, not so much.

For example:

import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Stroke;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import javax.swing.*;

@SuppressWarnings("serial")
public class TestDrawingCanvas extends JPanel {
    private DrawingCanvas drawingCanvas = new DrawingCanvas();

    public TestDrawingCanvas() {
        JPanel btnPanel = new JPanel(new FlowLayout(FlowLayout.TRAILING));
        btnPanel.add(new JButton(new ClearAction()));

        setLayout(new BorderLayout());
        add(drawingCanvas, BorderLayout.CENTER);
        add(btnPanel, BorderLayout.PAGE_END);
    }

    private class ClearAction extends AbstractAction {
        public ClearAction() {
            super("Clear Canvas");
            putValue(MNEMONIC_KEY, KeyEvent.VK_C);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            drawingCanvas.clear();
        }
    }

    private static void createAndShowGui() {
        TestDrawingCanvas mainPanel = new TestDrawingCanvas();

        JFrame frame = new JFrame("Drawing Canvas");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

@SuppressWarnings("serial")
class DrawingCanvas extends JPanel {
    private static final int PREF_W = 800;
    private static final int PREF_H = 600;
    public static final Color LINE_COLOR = Color.YELLOW;
    public static final Stroke IMG_STROKE = new BasicStroke(10f);
    private Color lineColor = LINE_COLOR;
    private BufferedImage image;

    public DrawingCanvas() {
        setBackground(Color.WHITE);
        image = new BufferedImage(PREF_W, PREF_H, BufferedImage.TYPE_INT_ARGB);
        MyMouse myMouse = new MyMouse();
        addMouseListener(myMouse);
        addMouseMotionListener(myMouse);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (image != null) {
            g.drawImage(image, 0, 0, this);
        }
    }

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

    public void setLineColor(Color lineColor) {
        this.lineColor = lineColor;
    }

    public void clear () {
        image = new BufferedImage(PREF_W, PREF_H, BufferedImage.TYPE_INT_ARGB);
        repaint();
    }

    private class MyMouse extends MouseAdapter {
        private Graphics2D imgG2d;
        private Point p1;

        @Override
        public void mousePressed(MouseEvent e) {
            if (e.getButton() != MouseEvent.BUTTON1) {
                return;
            }

            imgG2d = image.createGraphics();
            imgG2d.setColor(lineColor);
            imgG2d.setStroke(IMG_STROKE);
            p1 = e.getPoint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            drawLine(e);
            imgG2d.dispose();
            p1 = null;
            imgG2d = null;
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            drawLine(e);
            p1 = e.getPoint();
        }

        private void drawLine(MouseEvent e) {
            if (imgG2d == null || p1 == null) {
                return;
            }
            Point p2 = e.getPoint();
            int x1 = p1.x;
            int y1 = p1.y;
            int x2 = p2.x;
            int y2 = p2.y;
            imgG2d.drawLine(x1, y1, x2, y2);
            repaint();
        }
    }
}