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