I've got a system compatibility problem with a program I made that gives a view of a virtual arcade joystick controled with the arrow keys. Here's the code :

2. Joystick stick;
3. 
   
4. void setup() {
5.   size(50, 50);
6.   frameRate(50);
7.   stick = new Joystick();
8. }
9. 
   
10. void draw() {
11.   background(0);
12.   stick.check_input();
13.   stick.draw_joystick();
14. }
15. 
    
16. class Joystick {
17.   //Variables de classes
18.   int a;
19.   int b;
20.   boolean[] keys;
21.   
22.   //Constructeur
23.   Joystick(){
24.     keys = new boolean[526];
25.   }
26.   
27.   //Méthodes de classes
28.   boolean checkKey(int k){
29.     if (keys.length >= k){
30.       return keys[k];
31.     }  
32.     return false;
33.   }
34.   
35.   void check_key_pressed(){
36.     keys[keyCode] = true;
37.   }
38.   
39.   void check_key_released(){
40.     keys[keyCode] = false;
41.   }
42.   
43.   void check_input() {
44.     a = 0;
45.     b = 0;
46.     if (checkKey(UP)) b++;
47.     if (checkKey(DOWN)) b--;
48.     if (checkKey(RIGHT)) a++;
49.     if (checkKey(LEFT)) a--;
50.   }
51.   
52.   void draw_joystick() {
53.     ellipse(width/2*(1 + a), height/2*(1 - b), 10, 10);
54.   }
55. }
56. 
    
57. void keyPressed() {
58.   stick.check_key_pressed();
59. }
60. 
    
61. void keyReleased() {
62.   stick.check_key_released();
63. }

When running on Linux (last Lubuntu release) it works perfectly but when running on windows seven, in the upper left corner the stick is stuck if I press a third arrow key. I really don't understand why and it's pretty annoying because I'd like to use it to program games for both systems.

And sorry if there's any spelling mistake, I'm french and also trying to improve my english writing skills.