I have converted data into functions, objects and a class, but I don't understand where the mouseClicked function fits into this class. It seems to belong to two objects. Can someone show me? thanks.

Assembly assembly;

boolean button = false;

void setup() {
size(200,200);
assembly = new Assembly(20,20,100,20,30);
}

void draw() {
background(255);
assembly.display_urect();
assembly.display_bdoor();
assembly.display_brect();
assembly.move_door();
assembly.mouseClicked();
}

class Assembly {
 
 int x,y,w,h,d;
 
 Assembly(int tempx, int tempy, int tempw, int temph, int tempd) {
   x = tempx;
   y = tempy;
   w = tempw;
   h = temph;
   d = tempd;
 }
 
 void display_urect() {
 stroke(0);
 fill (140,160,220);
 if (mouseX > x && mouseX < x+w && mouseY > y && mouseY < y+h)
 fill(150,190,250);
 rect(x,y,w,h);
 }
 
 void display_bdoor() {
   fill(60);
   rect(80,140,30,50);
}

 void display_brect() {
 if (button) {
 stroke(0);
 fill(180,190,250);
 rect(20,60,160,60);
 }
 }
 
 void move_door() {
 if (button) {
 fill(100,150,100);
 rect(80,140,d,50);
 d = 5;
 }
}

void mouseClicked() {
if (mouseX > x && mouseX < x+w && mouseY > y && mouseY < y+h    ) {
button = !button;
}
}
}

Thanks for your response.

I am so new at this it is not clear to me what your suggesting.

It sounds like your saying keep mouseClicked() at its current location and call this method from assembly.mouseclicked() in draw().

Is that not what I am already doing. I think maybe I don't understand.

Maybe this confuses the issue further, but Quark posted a really neat example of implementing mouse controls in a class.

(As a sidenote I ran into trouble when trying to registerDraw() on two different classes, but haven't had time to look into a solution to that yet.  Turns out it's just a matter of explicitly declaring classes as 'public'.)

The P of PApplet means Processing...
A PApplet is actually your sketch, sorry for using obscure terms.
A more concrete example:
Code:

Assembly assembly;

boolean button = false;

void setup() {
size(200,200);
assembly = new Assembly(20,20,100,20,30);
}

void draw() {
background(255);
assembly.display_urect();
assembly.display_bdoor();
assembly.display_brect();
assembly.move_door();
}

void mouseClicked() // <-- That's what I meant
{
assembly.mouseClicked();
}
 

I mostly learned by looking at reference, and even more looking at examples, those of Processing, and those on this forum.
Beside, yes, I have a good experience of programming, it helps... :-P

How my code works? mousePressed() is a somehow "magical" function, like setup() and draw(): these functions are automatically called by Processing (the PApplet) when needed: setup() at the start, draw() every n milliseconds (depending on frameRate), mousePressed and similar when user does a physical action. These are also called "callback" functions, because the application call them back when an event occurs.

So Processing's mousePressed() is called (it knows only this one, unless you use the register trick above), and assembly.mouseClicked() delegates this event to your class. If you have several class instances, you have to call them all in a loop. Or, again, to register them (and Processing will loop itself to do this job).