1. Java Dynamic Graphics

2. In Java Repaint event: Sub-class the component (typically JPanel)
Java Swing components catch repaint event, and call their paintComponent( ) method
Default paintComponent( ) method paints component
E.g. panel background color
Sub-class the component (typically JPanel)
Over-ride paintComponent( ) method
Custom graphics here
Can call repaint( ) to invoke paintComponent( )

3. Code Hello World public class MyPanel extends JPanel {
public void paintComponent(Graphics g)
super.paintComponent(g); // erases background
Graphics2D g2 = (Graphics2D)g; //cast for java2
// my graphics:
g2.setColor(new Color(255,150,0));
g2.fillRect(10,10,200,50);
g2.setColor(new Color(0,0,0));
g2.drawString("Hello World", 10, 10); }
Hello World

4. Typical program structure for Dynamic Graphics
Store window contents in data structure
E.g. user drawn picture in paint program
Repaint event:
Erase window (draw background color)
Draw window contents using data structure
Other event that alters window contents:
modify the data structure
send repaint event

5. Storing window contents
2 approaches:
Store logical contents in a data structure(s)
e.g. drawing program: lines, shapes, colors, …
Then draw contents of data structure
Store visual contents as an off-screen image (bitmap)
e.g. pixels
Then use g2.drawImage( ) in paintComponent( )

6. Problem: Flashing Ugly flashing when repaint: Paint background Erase
Redraw shapes

7. Solution: Double buffering

8. Solution: Double buffering
Double buffered repaint:
Draw all graphics in temporary off-screen image
Paint background color
Paint shapes
Then paint image to JPanel
Bonus: Swing does this for you!!!
Draw graphics on JPanel
JPanel maintains off-screen image

9. What Subwindows Do Directs mouse input to correct component
Determines repaint events
Own coordinate system
Don’t need repaint when moving
Clipping: hides drawing behind a subwindow
Some subwindows remember what’s under them:
Popup menus