Question

I’m implementing my own double-buffering for a BufferedImage in a JPanel so that I can display the mouse location in the BufferedImage without repainting every object back onto it on mousemovement. When a JMenu in the parent JFrame is open, the BufferedImage gets repainted on top of the JMenu.

This class is not complete, and has only the necessary methods,

    public class Foo extends JPanel implements ComponentListener {
        BufferedImage bufferedImage;
        long mousePosX;
        long mousePoxY;

        protected void paintComponent(Graphics g) {
            paintComponent(g, this.xform);
        }
        protected void paintComponent(Graphics graphics, XFormPixel xformIn) {
            bufferedImage = new BufferedImage(this.getWidth(),this.getHeight(),BufferedImage.TYPE_INT_RGB);
            Graphics g = bufferedImage.getGraphics();
            super.paintComponent(g);
            //Draw lots of stuff to graphics

            if(drawMouseLocation) {
                int width = this.getWidth();
                int height = this.getHeight();
                Color origColor = g.getColor();
                g.setColor(textColor);
                if (cursorLocation != null) {
                    g.drawString("X: "+mousePosX + " Y: " + mousePosY);
                }
            }

            g.setColor(origColor);
            graphics.drawImage(bufferedImage,0,0,null);
        }

        public void drawMouseLocation() {   

            int width = this.getWidth();
            int height = this.getHeight();
            Image image = bufferedImage;
            Graphics graphics = this.getGraphics();
            Graphics g = image.getGraphics();
            Color origColor = g.getColor();
            g.setColor(textColor);
            if (cursorLocation != null) {
                g.drawString("X: "+mousePosX + " Y: " + mousePosY);
            }
            g.setColor(origColor);
            graphics.drawImage(image,0,0,this);
        }
    }

Is there another way to do this?

Another possible dealbreaker is that when the Foo JPanel is initialized, it has a black border, but when the image is drawn to display the mouselocation, the border disappears. I’m assuming calling a repaint() or something on the parent will fix both issues, but it would also call a repaint on the child, which I am trying to avoid.

EDIT 1:
Here is the requested runnable code. When creating it, I could not get the double-buffering working correctly, so I’m also having trouble with the flickering of the mouse location when the mouse is moved.

    import java.awt.Dimension;
    import java.awt.event.ActionEvent;
    import javax.swing.AbstractAction;
    import javax.swing.Action;
    import javax.swing.JFrame;
    import javax.swing.JMenu;
    import javax.swing.JMenuBar;

    public class DrawingTestFrame extends JFrame {
        private static final long serialVersionUID = 1L;        

        public static void main(String[] args) {
                new DrawingTestFrame();
        }   
        public DrawingTestFrame() {
                init();
        }

        public void init() {
                JMenuBar menuBar = new JMenuBar();                  
                    setJMenuBar(menuBar);
                JMenu dropMenu = new JMenu("Drop This");
                dropMenu.add(needs);
                dropMenu.add(to);
                dropMenu.add(overlap);
                menuBar.add(dropMenu);          
                    DrawingTest test = new DrawingTest();
                setTitle("Drawing Test");
                add(test);
                setMinimumSize(new Dimension(550,270));
                pack();
                setVisible(true);
        }

            public static Action needs = new AbstractAction("Needs") {
                private static final long serialVersionUID = 1L;
                public void actionPerformed(ActionEvent ae) {}};    
            public static Action to = new AbstractAction("To") {
                private static final long serialVersionUID = 1L;
                public void actionPerformed(ActionEvent ae) {}};    
            public static Action overlap = new AbstractAction("Overlap") {
                private static final long serialVersionUID = 1L;
                public void actionPerformed(ActionEvent ae) {}};    
    }


    import java.awt.Color;
    import java.awt.Component;
    import java.awt.Dimension;
    import java.awt.Graphics;
    import java.awt.Graphics2D;
    import java.awt.MouseInfo;
    import java.awt.Point;
    import java.awt.event.MouseEvent;
    import java.awt.event.MouseListener;
    import java.awt.event.MouseMotionListener;
    import java.awt.image.BufferedImage;
    import javax.swing.BorderFactory;
    import javax.swing.JPanel;

    public class DrawingTest extends JPanel implements MouseListener  {
        private static final long serialVersionUID = 1L;
            public Component parent;
            private Point mouseLocation;
            private BufferedImage bufferedImage;
            public DrawingTest() {
                    init();
            }        
            public void init() {
                    this.setPreferredSize(new Dimension(100, 100));
                    this.setBorder(BorderFactory.createLineBorder(Color.BLACK));
                    this.addMouseListener(this);
                    this.addMouseMotionListener(new MouseMotionListener() {
                public void mouseDragged(MouseEvent e) {
                    mouseLocation = MouseInfo.getPointerInfo().getLocation();
                    DrawingTest.this.repaint();
                }

                public void mouseMoved(MouseEvent e) {
                    mouseLocation = MouseInfo.getPointerInfo().getLocation();
                    DrawingTest.this.drawLocation();
                }
                    });
                    this.setVisible(true);
            }
        public void mouseClicked(MouseEvent e) {
            mouseLocation = MouseInfo.getPointerInfo().getLocation();
            this.repaint();
        }
        public void mousePressed(MouseEvent e) {}
        public void mouseReleased(MouseEvent e) {}
        public void mouseEntered(MouseEvent e) {}
        public void mouseExited(MouseEvent e) {}

            protected void paintComponent(Graphics graphics) {
                    bufferedImage = new   
                            BufferedImage(this.getWidth(),this.getHeight(),BufferedImage.TYPE_INT_RGB);
                    Graphics2D g = bufferedImage.createGraphics();
                    super.paintComponent(g);
                    g.setColor(Color.red);
                    g.drawRect(10,10,110,110);
                    graphics.drawImage(bufferedImage,0,0,null);
                    if (mouseLocation != null) {
                        graphics.drawString("X: " + mouseLocation.getX() + 
                "  Y: " + mouseLocation.getY(), this.getWidth()/2 - 50, this.getHeight()-10);
                    }
            }
            protected void drawLocation() {
                    this.getGraphics().drawImage(bufferedImage, 0,0,null);
                    this.getGraphics().setColor(Color.green);
                    if (mouseLocation != null) {
                        this.getGraphics().drawString("X: " + mouseLocation.getX() + 
                "  Y: " + mouseLocation.getY(), this.getWidth()/2 - 50, this.getHeight()-10);
                    }
            }
    }

Thanks!

Answer 1

The problem is you’re getting your Graphics object by calling getGraphics() on a component and this you should not do. Why are you doing this home-cooked double buffering, and where have you read that this is the way to implement it? I’d like to see that tutorial. If this were my application, I’d either:

• Use a BufferedImage for my background images, and simply draw over it in paintComponent, perhaps limiting my painting to the rectangle of interest…
• or show my mouse position using a JLabel.

For example, this code shows both techniques:

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;

import javax.swing.*;

@SuppressWarnings("serial")
public class DrawingTestFrame2 {
   private static void createAndShowGui() {
      JFrame frame = new JFrame("DrawingTestFrame2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      DrawingTest2 drawingTest = new DrawingTest2();

      frame.getContentPane().add(drawingTest);
      frame.setJMenuBar(drawingTest.createMenuBar());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

class DrawingTest2 extends JPanel {

   private static final int PREF_W = 500;
   private static final int PREF_H = 300;
   private static final int RECT_W = 100;
   private static final int RECT_H = 20;
   public static final Rectangle BOTTOM_RECT = new Rectangle(PREF_W/2 - RECT_W/2, PREF_H - RECT_H, 
         RECT_W, RECT_H);
   private String[] menuItemStrings = {"One", "Two", "Three"};
   public Point mouseLocation;
   private BufferedImage bufferedImage = new BufferedImage(PREF_W, PREF_H, 
         BufferedImage.TYPE_INT_ARGB);
   private JLabel mousePositionLabel = new JLabel("", SwingConstants.RIGHT);

   public DrawingTest2() {
      MouseAdapter mouseAdapter = new MyMouseAdapter();
      addMouseListener(mouseAdapter);
      addMouseMotionListener(mouseAdapter);

      mousePositionLabel.setForeground(Color.gray);
      setLayout(new GridBagLayout());
      GridBagConstraints gbc = new GridBagConstraints();
      gbc.gridx = 1;
      gbc.gridy = 1;
      gbc.gridwidth = 1;
      gbc.gridheight = 1;
      gbc.weightx = 1.0;
      gbc.weighty = 1.0;
      gbc.anchor = GridBagConstraints.SOUTHEAST;
      add(mousePositionLabel, gbc);

      Graphics2D g2 = bufferedImage.createGraphics();
      g2.setPaint(new GradientPaint(0, 0, Color.yellow, 40, 40, Color.green, true));
      g2.fillRect(0, 0, PREF_W, PREF_H);
      g2.dispose();
      g2 = bufferedImage.createGraphics();
      g2.setColor(Color.red);
      g2.setStroke(new BasicStroke(3f));
      g2.drawRect(10, 10, 110, 110);
      g2.dispose();
   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (bufferedImage != null) {
         g.drawImage(bufferedImage, 0, 0, this);
      }
      if (mouseLocation != null) {
         g.drawString("X: " + mouseLocation.getX() + "  Y: "
               + mouseLocation.getY(), this.getWidth() / 2 - 50,
               this.getHeight() - 10);
      }
   }

   public JMenuBar createMenuBar() {
      JMenuBar menuBar = new JMenuBar();
      JMenu menu = new JMenu("Menu");
      for (int i = 0; i < menuItemStrings .length; i++) {
         menu.add(new JMenuItem(menuItemStrings[i]));
      }
      menuBar.add(menu );
      return menuBar;
   }

   class MyMouseAdapter extends MouseAdapter {
      @Override
      public void mouseMoved(MouseEvent mEvt) {
         mouseLocation = mEvt.getLocationOnScreen();
         repaint(BOTTOM_RECT);

         String mousePosStr = String.format("x:%d y:%d", mouseLocation.x, mouseLocation.y);
         mousePositionLabel.setText(mousePosStr);
      }
   }
}