Well here's one way:

1. max(min(mouseX,width),0);  //for mouseX
   max(min(mouseY,height),0); //for mouseY

so your code would be:

1. void setup() {
     size(600,400);
   }
   void draw() {
     println(max(min(mouseX,width),0)+":"+max(min(mouseY,height),0));
   }

or just use a if-statement ?

sure, but doesnt work in my case. i really need it not to send the coordinates.

wait... what do you actually want mouseX and mouseY to return when being outside the screen ?

should behave the same way when pressed as when mousekey is not pressed.
so if you leave the screen on the left side. x should stay 0; and not become negative.

nope, like i said, doesn't cause i am not using mouseX and mouseY directly in my code.

but... im super confused right now, you asked for "a simple line of code to prevent the mouse to return mouseX and mouseY values from outside the sketch" and you also said "so if you leave the screen on the left side. x should stay 0; and not become negative." and my code does exactly that.. And it shouldn't matter that you aren't using mouseX and mouseY directly because somewhere in your code you're obviously extracting the x and y values somewhere using mouseX and mouseY, right? (and if you're not then you really need to post some code to show what you're doing) And then it's just a matter of replacing every mouseX in your code with max(min(mouseX,width),0); and the same for y. Actually i can't think of any possible scenario where using that line of code (or a modified version of it) couldn't solve your problem so you really need to post some code sir!

perfect, thx adam. that solved my problem.