mouseX and mouseY carry the current position of the mouse pointer.  When you do int x = mouseX at the start of your sketch you're setting x to whatever mouseX is at that time (which is actually undefined before setup has run).  x isn't continually updated with the value of mouseX (it's not a pointer if that makes any sense to you).  After that you never alter the value of x or y in your code.

What you need is more like this:

1. int x, y;
2. 
   
3. void setup() {
4.       size(400, 400);
5. }
6. 
   
7. void draw() {
8.       if (mousePressed == true) {
9.             x = mouseX;
10.             y = mouseY;
11.             println("x is now: " + x + " y is now: " + y);
12.       }
13. }

That will print a load of messages each time you press a mouse button because the button is actually pressed for several frames (unless you press really fast or your PC is very slow).  To avoid that you can add a mousePressed() function which will only fire at the point the button is pressed.