Can JPanel's paint() use different Graphics2D objects?

[Dave Elwood]

I've got NormPanel here :

class NormPanel extends JPanel {

        NormPanel() {}

        public void update(Graphics g) {
             paint(g);
        }

        public void paint(Graphics g) {

             setBackground(Color.WHITE);
     
             if (dblBuffClass.getGraphicsForOffScreen() == null) { // DblBuffClass not yet initiated

                    Graphics2D g2 = (Graphics2D)g;

                    g2.draw(line);  // standard normal boring shapes, not using double buffering
                    g2.draw(circle);
                    g2.draw(box);
             }
             else { // DblBuffClass is ready to roll

                    // let another class use double buffering to show different shapes
                    g2 = dblBuffClass.showDblBuffShapes();

                    g2.draw(line);  // add the standard shapes as well
                    g2.draw(circle);
                    g2.draw(box);
             }
        }
}

after I have called DblBuffClass.init() then DblBuffClass is ready to show it's own shapes

class DblBuffClass {

        private Graphics gForOffScreen;
        private Image imageForOffScreen;

        private JPanel panel; // reference to NormPanel

        public DblBuffClass() { }

        public void init(JPanel p) { //  now gForOffScreen is non-null so 
                                     //  NormPanel will use this DblBuffClass to show shapes
                panel = p;                
                imageForOffscreen = panel.createImage(panel.getWidth(), panel.getHeight());
                gForOffScreen = imageForOffscreen.getGraphics();
        }

        public Graphics2D showDblBuffShapes(Graphics g) {

                gForOffScreen.draw(dblBuffLine);
                gForOffScreen.draw(dblBuffCircle);
                gForOffScreen.draw(dblBuffBox);

                return (Graphics2D)gForOffScreen ;
        }
}

is it possible at all for paint() to optionally go over to another class and
have it populate the Graphics2D object with different things and then
have paint() add a few more things?

This thing now doesn't work

[Michael Dunn]

for Swing components you should be using paintComponent(..) instead of paint(..)

not sure exactly what you're trying to do, but doing stuff inside paintComponent()
is generally not good, due to the number of times the method is called, unless that's what you want.

to see how often it is called, run this, then drag the frame wider or smaller and watch the counter go.

import javax.swing.*;
import java.awt.*;
class Testing
{
  int x = 0;
  public void buildGUI()
  {
    JPanel p = new JPanel(new GridBagLayout()){
      protected void paintComponent(Graphics g){
        super.paintComponent(g);
        System.out.println("Counter = "+(x++));
      }
    };
    JFrame f = new JFrame();
    f.getContentPane().add(p);
    f.setSize(400,300);
    f.setLocationRelativeTo(null);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setVisible(true);
  }
  public static void main(String[] args)
  {
    SwingUtilities.invokeLater(new Runnable(){
      public void run(){
        new Testing().buildGUI();
      }
    });
  }
}

[Dave Elwood]

not sure exactly what you're trying to do

I'm trying to invoke double-buffering on a part-time basis on a JPanel.
I've succeeded in invoking additional painting on a part-time basis by doing this in my NormPanel :

    @Override
    public void paintComponent(Graphics g) 
    { 
        super.paintComponent(g);
        setBackground(Color.WHITE);
        
        if (blPaintBoom == true) { // used when I want 'boom' to take place
          
            blastPainter.paintBoom(g);               
        }

        standardPaint(g);
    }
/** standardPaint() : my rinky-dinky standard boring painting   
*
*/
    private Graphics standardPaint(Graphics g) {
        
        Graphics2D g2 = (Graphics2D)g;
        g2.setColor(Color.BLACK);
        g2.setStroke(thinStroke);
        g2.draw(line);
        return g2;
    }

if blPaintBoom is false then I haven't invoked my extra little animation
otherwise I invoke blastPainter.paintBoom(g), where I add some growing concentric circles (an explosion) with a loop
and repaint() routine and when I'm done I put blPaintBoom back to false

using paint() instead of paintComponent() along with the use of an update() override was suggested in a description of double buffering.
doing this hasn't caused any bad things to happen.

/** in the class BoomPainter separate from NormPanel
*/
public Graphics paintBoom(Graphics g) {
 
//      flush the screen       
        g.setColor(Color.WHITE);
        g.fillRect( 0, 0, normpanel.getWidth(), normpanel.getHeight());   
            
        Graphics2D g2 = (Graphics2D)g;
 
//      paint a concentric circle
        if (blastEllipse != null) {  
            g2.setStroke(fatStroke);
            g2.setColor(boomColor);            
            g2.draw(blastEllipse);
        }
     
        return g2;
    }

I'd like this snippet of code in paintBoom to employ double-buffering in order to
smooth out my animation. However getting it back into NormPanel.paintComponent() is
difficult since I'm not sure of the true nature of Graphics2D, Graphics and Image objects.

I'm afraid the code you showed me doesn't employ double-buffering at all.

[Randall Twede]

for what it's worth, and without reading your code.
you can have more than one Graphics in your paintComponent()
you can cast the one the method was given to a Graphics2D or create a new Graphics2D
as for double buffering, i am no expert, but i think swing does some of that now for you.

[Rob Camick]

Don't override the update() method. That is done in AWT painting, NOT Swing painting.

[Dave Elwood]

Thanks Rob Camick, Trying to imagine the difference between AWT and Swing is hard for me. I'm slow.

To Randall : >>you can have more than one Graphics in your paintComponent()

Do you mean I can do this :

Graphics2D g1 = new Graphics2D();
g1.draw(circle);
Graphics2D g2 = new Graphics2D();
g2.draw(rectangle);

and both will show up?

I'll try that out. Thanks for the tip.

[Rob Camick]

I'm not sure what you are trying to do. Swing is double buffered by default so there is no need to worry about double buffering.

You can't just create a Graphics objects and do painting with it. Generally you just use the Graphics objects that is passed to a paintCompnent() method. However if you want to change a lot of the properties of the Graphics object you can use the Graphics.create() method to get a copy of the original Graphics, just make sure you dispose of the new Graphics objects when you are finished with it.

[Michael Dunn]

if you're just trying to create a 'boom' effect, something like this might be all you need to do.

run it, then click the 'boom' button.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
class Testing
{
  boolean paintBoom = false;
  int radius = 25;
  int diameter = 50;
  javax.swing.Timer timer;
  public void buildGUI()
  {
    final JPanel p = new JPanel(){
      public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.drawLine(375,300,425,300);
        g.drawLine(400,275,400,325);

        if(paintBoom)
        {
          Graphics2D g2d = (Graphics2D) g;
          g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
          g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
          g2d.draw(new Ellipse2D.Double(400-radius,300-radius, diameter, diameter));
          g2d.dispose();
        }
      }
    };
    JButton btn = new JButton("Boom");
    p.setPreferredSize(new Dimension(800,600));
    JFrame f = new JFrame();
    f.getContentPane().add(p);
    f.getContentPane().add(btn,BorderLayout.SOUTH);
    f.pack();
    f.setLocationRelativeTo(null);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setResizable(false);
    f.setVisible(true);

    timer = new javax.swing.Timer(500,new ActionListener(){
      public void actionPerformed(ActionEvent ae){
        radius += 10;
        diameter = radius * 2;
        p.repaint();
        if(diameter > p.getHeight())
        {
          timer.stop();
          radius = 25;
          diameter = radius * 2;
          paintBoom = false;
          p.repaint();
        }
      }
    });
    btn.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent ae){
        paintBoom = true;
        if(timer.isRunning() == false) timer.start();
      }
    });
  }
  public static void main(String[] args)
  {
    SwingUtilities.invokeLater(new Runnable(){
      public void run(){
        new Testing().buildGUI();
      }
    });
  }
}

[Dave Elwood]

That's interesting. I like the idea of the anon inner class for the timer.

[Darryl Burke]

The anonymous inner class is an ActionListener passed to the Timer's constructor.

The Timer is just a javax.swing.Timer --not an anonymous inner class.