1. float proClock = 0;
2. ArrayList danmaku;
3. float zeroClock = 0;
4. void setup() {
5.   size(800, 800);
6.   danmaku = new ArrayList();
7.   danmaku.add(new Projectile(400, 200, 0, 0, 0, 5, 15, color(255, 0, 255), random(100)));
8. }
9. void draw() {
10.   background(0);
11.   zeroClock = zeroClock + 1;
12.   if (zeroClock%120 == 0 && zeroClock%240 !=0) {
13.     for (int i = 0; i < 20; i++)
14.       danmaku.add(new Projectile(400, 200, 360*float(i)/20, 2/(proClock+1), 0.2/(proClock+1), 5, 15, color(255, 0, 0), random(100)));
15.   }
16.   if (zeroClock%240 == 0) {
17.     for (int i = 0; i < 20; i++)
18.       danmaku.add(new Projectile(400, 200, 360*float(i)/20, 2/(proClock+1), -0.2/(proClock+1), 5, 15, color(0, 0, 255), random(100)));
19.   }
20.   for (int i = danmaku.size()-1; i > 0; i--) {
21.     Projectile danmaku = (Projectile) danmaku.get(i);
22.     danmaku.displace();
23.     danmaku.display();
24.   }
25. }
26. class Projectile {
27.   color col; // Color
28.   float dim; // Size
29.   float xpos; // Current horizontal position
30.   float ypos; // Current vertical position
31.   float radialSpeed; // Radial speed
32.   float angularSpeed; // Angular speed
33.   float hitBox; // Lethal range
34.   float x0; // Original horizontal position
35.   float y0; // Original verical position
36.   float initialAngle; // Original angle
37.   float identifier; // For transformations
38.   float proClock; // The projectile's clock
39.     // Constructor
40.   Projectile(float x, float y, float a0, float r, float a, float h, float s, color c, float i) {
41.     col = c;
42.     xpos = x;
43.     ypos = y;
44.     initialAngle = a0;
45.     radialSpeed = r;
46.     angularSpeed = a;
47.     hitBox = h;
48.     dim = s;
49.     x0 = x;
50.     y0 = y;
51.     identifier = i;
52.     proClock = 0;
53.   }
54.   // Display the projectile
55.   void display() {
56.     noStroke();
57.     fill(col);
58.     ellipseMode(CENTER);
59.     ellipse(xpos, ypos, dim, dim);
60.     fill(255);
61.     ellipse(xpos, ypos, 0.9*dim, 0.9*dim);
62.   }
63.   // Displace the projectile
64.   void displace() {
65.     proClock = proClock + 1;
66.     xpos = x0 + cos(radians(initialAngle + proClock*angularSpeed)) * radialSpeed * proClock;
67.     ypos = y0 + sin(radians(initialAngle + proClock*angularSpeed)) * radialSpeed * proClock;
68.   }
69.   // To implement: one less life
70.   /*
71.   void collide() {
72.    if (dist(charX, charY, xpos, ypos) < hitBox) {
73.    }
74.    }
75.    */
76.   // To implement: transformation into another projectile
77.   void transform() {
78.   }
79. }

A priori, the Projectile class works fine. However, when I run this code, Line 21: "The function get(int) does not exist."
I reckon it's a problem with my danmaku ArrayList... but, being a beginning coder and having Googled the problem, I didn't manage to debug this.

The only think I did find on Google was this. I haven't been able to figure out whether it was relevant.

EDIT:
Showed this to a friend, she debugged it.
The problem lied in

1. Projectile danmaku = danmaku.get(i);

(variable and ArrayList sharing the same name).