OOP is a good choice. That would leed to more understandable code :

1. Shape shape;
2. 
   
3. void noteOn(Note note, int device, int channel){
4.   shape = new Shape(note.getPitch());
   
5. }
6. 
   
7. void noteOff(Note note, int device, int channel){
8.   if (shape != null) {
9.     shape.startFading();
10.   }
    
11. }
12. 
    
13. void draw() {
14.   background(255);
15.   if (shape != null) {
16.     shape.draw();
17.   }
    
18. }
    

i have now a mini test class made like this