I made a simple program with only one class Bullet....

do you want to see many bullets or would one at a time be enough?

to break the class Bullet up, you would to have parallel arrays again for x,y etc. ......

1. //much simplified!!!!!!!!!!!!!!!!!!!!!!
2. //
3. //
4. //
5. // overall structure:
6. // A player (only a white rectangle)
7. // shoots bullets (see class Bullet), when
8. // they hit the wall.
9. // The tower can only shoot upwards, follows mouse, click mouse.
10. //
11. // idea from
12. // http://forum.processing.org/topic/need-help-adding-shooting
13. // with help from
14. // http://forum.processing.org/topic/have-eyes-follow-mouse
15. //
16. // where is the gun / player
17. PVector position; // contains x and y...
18. PVector velocity; // contains speed for x and y...
19. ArrayList <Bullet> bullets;  
20. //
21. // bullets
22. final float bulletFlySpeed = 4.2;  // how fast bullet flies
23. final int fireSpeed=200; // how often you fire / distance between bullets
24. int lastFired=millis();  // timer to determine when next bullet starts
25. // ammo not in use
26. int ammoCount = 70;
27. boolean hit=false;
28. // ------------------------------------------------------------------
29. void setup() {
30.   size(600, 480);
31.   // Create an empty ArrayList
32.   bullets  = new ArrayList();  
33.   position = new PVector(333, 333);
34.   //  velocity = new PVector();
35.   //  velocity.x = 0;
36.   //  velocity.y = 0;
37.   cursor(CROSS);
38. }
39. void draw() {
40.   background(111);
41.   drawPlayerAndBullets ();
42.   // blue rect
43.   noStroke();
44.   if (hit)
45.     fill(0, 255, 0); // green
46.   else
47.     fill(0, 0, 255);
48.   rect (100, 100, 40, 40);
49. }
50. void mousePressed() {
51.   fire();
52. }
53. void fire() {  
54.   if (fireSpeed<=millis()-lastFired) {
55.     if (ammoCount>0) {
56.       // ammoCount--;
57.       lastFired=millis();
58.       float xSpeed ;
59.       float ySpeed ;
60.       float angle = update(mouseX, mouseY);
61.       xSpeed = cos(angle);
62.       ySpeed = sin(angle);
63.       xSpeed*= bulletFlySpeed;
64.       ySpeed*= bulletFlySpeed;
65.       if (ySpeed>0)
66.         ySpeed=0;
67.       bullets.add ( new Bullet(
68.       position.x+12, position.y-14,
69.       xSpeed, ySpeed,
70.       5 ));
71.     } // if
72.   } // if
73. } //
74. //
75. float update(int mx, int my) {
76.   //determines angle to mouse
77.   float angle = atan2(float(my)-(position.y-14), float(mx)-(position.x+12));
78.   return angle;
79. }
80. void drawPlayerAndBullets () {
81.   //
82.   // draw player & bullets
83.   //
84.   // draw player
85.   pushMatrix();
86.   translate(position.x, position.y);
87.   noStroke();
88.   fill(255);
89.   rect (0, 0, 20, 40);
90.   popMatrix(); // undoes all
91.   // draw bullets
92.   for (Bullet currentBullet : bullets ) {     // new1
93.     // println ("************************");
94.     currentBullet.move();
95.     currentBullet.display();
96.   }
97.   // With an array, we say length, with an ArrayList, we say size()
98.   // The length of an ArrayList is dynamic
99.   // Notice how we are looping through the ArrayList backwards
100.   // This is because we are deleting elements from the list 
101.   for (int i = bullets.size()-1; i >= 0; i--) {
102.     // An ArrayList
103.     Bullet currentBullet = bullets.get(i);
104.     if (currentBullet.finished()) {
105.       // Items can be deleted with remove()
106.       bullets.remove(i);
107.     }
108.   }
109. }
110. // ================================================================
111. // Bullet - Simple class for bullets
112. // http://www.processing.org/learning/topics/arraylistclass.html
113. class Bullet {
114.   float x;
115.   float y;
116.   float speedX ; 
117.   float speedY;
118.   float w;
119.   float life = 255;
120.   Bullet(float tempX, float tempY, // new2
121.   float tempSpeedX, float tempSpeedY,
122.   float tempW) {
123.     x = tempX;
124.     y = tempY;
125.     w = tempW;
126.     speedX = tempSpeedX; // 4.2; // new2
127.     speedY = tempSpeedY; // 0;
128.   }
129.   void move() {
130.     // Add speed to location
131.     x = x + speedX;
132.     y = y + speedY;
133.     //
134.     // kill bullet when outside screen
135.     if (x<4)
136.       life=-1;
137.     if (y<4)
138.       life=-1;
139.     if (x>width-4)
140.       life=-1;
141.     if (y>width-4)
142.       life=-1;
143.     // blue rect
144.     if (x>=100 && y>=100 && x<=140 && y<=140) {  
145.       life=-1;
146.       hit=true;
147.     }
148.     //
149.     if (life==-1) {
150.       //      explode () ;
151.     }
152.   } // method
153.   boolean finished() {
154.     // bullet dead?
155.     if (life < 0) {
156.       return true;
157.     }
158.     else {
159.       return false;
160.     }
161.   }
162.   void display() {
163.     // Display the circle / bullet
164.     fill(244, 2, 2);
165.     noStroke();
166.     //stroke(0,life);
167.     ellipse(x, y, w, w);
168.     // image( ammo_fired, x, y );
169.   }
170. } // class
171. //