javaで花火作成

javaアプレットで花火を作ろうと考えています！
調べたところ、サンプルコードはいくつか出てきたのですが、詳しい解説がないため、理解できない部分がありました。

コード
public class hanabi2 extends Applet implements Runnable, MouseListener{

    int MaxRocketNumber=2;
    int MaxRocketExplosionEnergy=800;
    int MaxRocketPatchNumber=80;
    int MaxRocketPatchLength=80;
    int Gravity=40;
    Rocket rocket[];
    Thread launcher=null;
    boolean fg = true;
    int mx, my;
    int speed = 50;
    AudioClip don;

    public void init() {
       

        mx=450;//getSize().width;
        my= 350;//getSize().height;
       
        don = getAudioClip(getDocumentBase(), "don.au");
        rocket = new Rocket[MaxRocketNumber];
        for (int i = 0; i < MaxRocketNumber; i++)
            rocket[i] = new Rocket(mx, my, Gravity);
        addMouseListener(this);
    }
    

    public void start() {

        if(launcher ==null){
            fg = true;
            launcher =new Thread(this);
            launcher.start();
        }
    }

   

    public void run() {
        int     i,e=0, p=0, l=0;
        long    s =0;
        int mg = 0;
        Graphics g =getGraphics();

        
        while (fg == true) {
            try {
                launcher.sleep(speed);
            } catch(InterruptedException x) {
            	
            }
            e = (int)(Math.random()*MaxRocketExplosionEnergy*3/4)+
                                    MaxRocketExplosionEnergy / 4 + 1;
            p = (int)(Math.sqrt(Math.random())*MaxRocketPatchNumber*3/4)+MaxRocketPatchNumber/4+1;
            l = (int)(Math.random()*MaxRocketPatchLength*3/4)+
                    MaxRocketPatchLength/4+1;
            s = (long)(Math.random()*1);
            
            for (i=0;i<MaxRocketNumber;++i) {
               if (rocket[i].sleep && Math.random()*Math.random()*
                                            MaxRocketNumber*l<1 && fg){
                    if(++mg % 7 == 0)repaint();
                    rocket[i].init(e, p, l, s, my);
                    rocket[i].start();
                }
                rocket[i].show(fg, g, don);
            }
        }
    }



class Rocket
{
    public  boolean sleep = true,fire=false;
    private int energy,patch,length;
    private int wx,hy,x,y,ox,oy,vx[],vy[];
    private int t,h,red,blue,green;
    private int a,b,gravity;
    private int ly;
    private int time;
    private Random random;
    private AudioClip don;
    Thread  launcher;
    private double cosA;
    private double sinA;
    public Rocket(int a,int b,int g) {
        wx=a;
        hy=b;
        gravity=g;
    }

    public void init(int e,int p,int l,long seed, int my) {
        int i;
        ly=my-50;
        double qx,qy,qz,ux,uy,uz;
        energy=e;
        patch=p;
        length=l;
        h=0;
        random=new Random(seed);
        vx =new int[patch];
        vy =new int[patch];
        red =(int)(random.nextDouble() * 128) + 128;
        blue =(int)(random.nextDouble() * 128) + 128;
        green =(int)(random.nextDouble() * 128) + 128;
        ox=(int)(Math.random()*wx/2)+wx/4;
        oy=(int)(Math.random()*hy/2)+hy/4;
        for(i=0;i<patch;++i) {
            qx=Math.random();
            qy=Math.random();
            ux=qx-0.5;
            uy=qy-0.5;
            if((ux*ux+uy*uy)<0.25){
                vx[i]=(int)(qx*energy)-energy/2;
                vy[i]=(int)(qy*energy*7/8)-energy/8;
            }else --i;
        }
    }

    public void start() {
        t=0;
        sleep=false;
    }

    public void line(Graphics g){
       if(h<=(ly-oy)){
            g.setColor(new Color(0,0,0));
            g.drawLine(ox,(ly-h)-3,ox,(ly-h)+5);
            if (h <(ly-oy)){
                g.setColor(new Color(160,180,255));
                g.drawLine(ox,(ly-h),ox,(ly-h)-3);
            }
        }
        h=h+4;
        if(h>=(ly-oy))  fire=true;
        else             fire=false;
    }

    public synchronized void show(boolean fg,Graphics g,AudioClip don) {
      if (!sleep) {
         if(fg){
          line(g);
          if(fire){
            if (t<length*4/5) {
                int i,c;
                int tb =6;
                if(length  < tb) tb =length ;
                if(t == tb) don.play();
                double s;
                Color color;
                c =(int)(random.nextDouble()*64)-32+red;
                if (c >= 0 && c <256) red=c;
                c =(int)(random.nextDouble()*64)-32+blue;
                if (c >= 0 && c <256) blue=c;
                c =(int)(random.nextDouble()*64)-32+green;
                if (c >= 0 && c <256) green=c;
                color = new Color(red,blue,green);
                for (i=0;i<patch;++i) {
                    s = (double)t/100;
                    x = (int)(vx[i]*s);
                    y = (int)(vy[i]*s-gravity*s*s);
                    g.setColor(color);
                    g.drawLine(ox+x,oy-y,ox+x,oy-y);
                    if (t>=length/2) {
                        for (int j=0;j<2;++j){
                            s = (double)((t-length/2)*2+j)/100;
                            x = (int)(vx[i]*s);
                            y = (int)(vy[i]*s-gravity*s*s);
                            g.setColor(new Color(0,0,0));
                            g.drawLine(ox+x,oy-y,ox+x,oy-y);

                        }
                    }
                }
                ++t;
            }
            else sleep=true;
         }
        }
      }
   }
}