The Way of the Java/The Slate Class

The Slate Class
verbatim 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);

verbatim