Fixing syntax errors so the demo can run.

java/examples/Topics/Simulate/MultipleParticleSystems/CrazyParticle.pde

@@ -21,25 +21,20 @@ class CrazyParticle extends Particle {
   void update() {
     super.update();
     // Increment rotation based on horizontal velocity
-    float theta_vel = (vel.x * vel.mag()) / 10.0f;
+    float theta_vel = (velocity.x * velocity.mag()) / 10.0f;
     theta += theta_vel;
   }
 
-  // Override timer
-  void timer() {
-    timer -= 0.5;
-  }
-  
   // Method to display
-  void render() {
+  void display() {
     // Render the ellipse just like in a regular particle
-    super.render();
+    super.display();
 
     // Then add a rotating line
     pushMatrix();
-    translate(loc.x,loc.y);
+    translate(location.x,location.y);
     rotate(theta);
-    stroke(255,timer);
+    stroke(255,lifespan);
     line(0,0,25,0);
     popMatrix();
   }

java/examples/Topics/Simulate/MultipleParticleSystems/ParticleSystem.pde

@@ -1,4 +1,6 @@
-// An ArrayList is used to manage the list of Particles 
+// An ArrayList is used to manage the list of Particles
+// An Iterator is used to remove "dead" particles while iterating over the list
+import java.util.Iterator;
 
 class ParticleSystem {
 
@@ -42,4 +44,4 @@ class ParticleSystem {
     }
   }
 
-}
+}