explain SilyCanvas design

svn: r7094

collects/htdch/draw/World.java

@@ -1,7 +1,7 @@
 package draw;
 
 public abstract class World {
-    protected Canvas theCanvas = new SillyCanvas(600,600); // can I do better here? 
+    protected Canvas theCanvas = new SillyCanvas(600,600); // can I do better than null here? 
     
     /** 
      *@author Matthias Felleisen, Kathy Gray 

collects/htdch/draw/design-note.txt

@@ -0,0 +1,44 @@
+				[Mon Aug 13 20:03:56 EDT 2007]
+				 
+
+
+1. SillyClass exists to get around the NULL problem in World.java. 
+
+   I create a SillyCanvas, which is a 600 x 600 canvas so that
+   theCanvas doesn't have to be initialized to NULL. 
+
+   A SillyCanvas is a Canvas that displays a warning for every 
+   draw method that is invoked. So students can show the canvas 
+   before they call bigBang and test the draw method in World, 
+   but it is pretty silly. 
+
+ Rationale: 
+   The reason I don't want NULL in this draw.* library is that 
+   it is used with ProfessorJ's Beginner and Intermediate 
+   language levels and they don't include NULL. It turns out, 
+   however, that this particular NULL for theCanvas could "leak 
+   out" if a student did this: 
+   
+	new SampleWorld().theCanvas.show()
+
+   or something less direct than that. 
+
+2. Of course, the rationale also suggests that I may have made 
+   a mistake in the design of World. Instead of creating World
+   via a no-argument constructor, I add a constructor of two 
+   arguments, width and height, and force students to perform 
+   a super call: 
+
+     World(int width, int height) { 
+        this.theCanvas = new Canvas(width,height); 
+     }
+
+     World() { 
+         throw RuntimeException("can't create a world without size arguments"); 
+     }
+
+   PRO: And voli`a, the problem would go away. 
+   CONS: Students must fix the size of the canvas at World creation time.
+   	 With my existing design, they can choose different sizes when 
+	 they run bigBang. Is it worth it? Probably not. 
+