you could make it invisible outside the area using cursor() and noCursor()

I think
http://wiki.processing.org/w/Robot_class

could do it but runs not in browser

           

the mouseMove method from Robot works, thanks !

the only problem is that it moves the mouse to absolute coordinates of the screen, not relative to the applet... unless the applet works in fullscreen (it would be quite difficult to implement a resolution for my game), I won't be able to move it in the right area of the applet...

...because I don't know the position of the window in the screen. is there a way to know it ?

You can access the container in which the Processing sketch is currently running, normally the application frame. Or in some cases the inet browser. (There's actually a 'frame' object available I just noticed, so you don't even have to declare it... it's already there)

Here's an example to get you started. Use the "wasd" keys.

1. import java.awt.AWTException;
2. import java.awt.Robot;
3. import java.awt.Rectangle;
4. Robot robot;
5. int x, y;
6. void setup() {
7.   size(400, 400);
8.   try {
9.     robot = new Robot();
10.   }
11.   catch (AWTException e) {
12.     e.printStackTrace();
13.   }
14. }
15. void draw() {
16. }
17. void getTop() {
18.   Rectangle r = frame.getBounds();
19.   x = r.x + r.width/2;
20.   y = r.y;
21. }
22. void getBottom() {
23.   Rectangle r = frame.getBounds();
24.   x = r.x + r.width/2;
25.   y = r.y + r.height;
26. }
27. void getLeft() {
28.   Rectangle r = frame.getBounds();
29.   x = r.x;
30.   y = r.y + r.height/2;
31. }
32. void getRight() {
33.   Rectangle r = frame.getBounds();
34.   x = r.x + r.width;
35.   y = r.y + r.height/2;
36. }
37. void keyPressed() {
38.   switch( key ) {
39.   case 'w':
40.     getTop();
41.     break;
42.   case 'a':
43.     getLeft();
44.     break;
45.   case 'd':
46.     getRight();
47.     break;
48.   case 's':
49.     getBottom();
50.     break;
51.   }
52.   robot.mouseMove( x, y );
53. }

thanks :)