I want to convert my Processing program to the full Java mode.
In the documentation, this example is given:

public class MyDemo extends PApplet {

 void setup() {
   size(200, 200);
   rectMode(CENTER);
   noStroke();
   fill(0, 102, 153, 204);
 }

 void draw() {
   background(255);
   rect(width-mouseX, height-mouseY, 50, 50);
   rect(mouseX, mouseY, 50, 50);
 }

}

When I try this it on my working program, it is no longer working, cause it can no longer find my other classes.

Should there not be a
public static void main(String[] args) {} ?

Where should setup() and draw() go?
Inside main or outside?

Is the extends PApplet necessary? sufficient?

What about my other classes in my other tabs?