Appendix C

The Slate Class

import java.awt.*;

public class Example {

// demonstrate simple use of the Slate class

public static void main (String[] args) {
int width = 500;
int height = 500;

Slate slate = Slate.makeSlate (width, height);
Graphics g = Slate.getGraphics (slate);

g.setColor (Color.blue);
draw (g, 0, 0, width, height);

anim (slate, 0, 0, width, height);
}

// draw is taken from Section 4.14 of the book

public static void draw (Graphics g, int x, int y, int width, int height) {
if (height < 3) return;

g.drawOval (x, y, width, height);

draw (g, x, y+height/2, width/2, height/2);
draw (g, x+width/2, y+height/2, width/2, height/2);
}

// anim demonstrates a simple animation

public static void anim (Slate slate, int x, int y, int width, int height) {
Graphics g = slate.image.getGraphics ();
g.setColor (Color.red);

for (int i=-100; i<500; i+=8) {
g.drawOval (i, 100, 100, 100);
slate.repaint ();
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
}
}
}

class Slate extends Frame {

// image is a buffer: when Slate users draw things, they
// draw on the buffer. When the Slate gets painted, we
// copy the image onto the screen.
Image image;

public static Slate makeSlate (int width, int height) {
Slate s = new Slate ();
s.setSize (width, height);
s.setBackground (Color.white);
s.setVisible (true);
s.image = s.createImage (width, height);
return s;
}

// when a Slate user asks for a Graphics object, we give
// them one from the off-screen buffer.

public static Graphics getGraphics (Slate s) {
return s.image.getGraphics ();
}

// normally update erases the screen and invokes paint, but
// since we are overwriting the whole screen anyway, it is
// slightly faster to override update and avoid clearing the
// screen

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

// paint copies the off-screen buffer onto the screen

public void paint (Graphics g) {
g.drawImage (image, 0, 0, null);
}
}