Task: Brownian tree
C#: using System;
using System.Drawing;

namespace BrownianTree
{
    class Program
    {
        static Bitmap BrownianTree(int size, int numparticles)
        {
            Bitmap bmp = new Bitmap(size, size);
            Rectangle bounds = new Rectangle { X = 0, Y = 0, Size = bmp.Size };
            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.Clear(Color.Black);
            }
            Random rnd = new Random();
            bmp.SetPixel(rnd.Next(size), rnd.Next(size), Color.White);
            Point pt = new Point(), newpt = new Point();
            for (int i = 0; i < numparticles; i++)
            {
                pt.X = rnd.Next(size);
                pt.Y = rnd.Next(size);
                do
                {
                    newpt.X = pt.X + rnd.Next(-1, 2);
                    newpt.Y = pt.Y + rnd.Next(-1, 2);
                    if (!bounds.Contains(newpt))
                    {
                        pt.X = rnd.Next(size);
                        pt.Y = rnd.Next(size);
                    }
                    else if (bmp.GetPixel(newpt.X, newpt.Y).R > 0)
                    {
                        bmp.SetPixel(pt.X, pt.Y, Color.White);
                        break;
                    }
                    else
                    {
                        pt = newpt;
                    }
                } while (true);
            }
            return bmp;
        }

        static void Main(string[] args)
        {
            BrownianTree(300, 3000).Save("browniantree.png");
        }
    }
}

Java: import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.util.*;
import javax.swing.JFrame;

public class BrownianTree extends JFrame implements Runnable {

    BufferedImage I;
    private List<Particle> particles;
    static Random rand = new Random();

    public BrownianTree() {
        super("Brownian Tree");
        setBounds(100, 100, 400, 300);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        I = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);
        I.setRGB(I.getWidth() / 2, I.getHeight() / 2, 0xff00);
        particles = new LinkedList<Particle>();
    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(I, 0, 0, this);
    }

    public void run() {
        for (int i = 0; i < 20000; i++) {
            particles.add(new Particle());
        }
        while (!particles.isEmpty()) {
            for (Iterator<Particle> it = particles.iterator(); it.hasNext();) {
                if (it.next().move()) {
                    it.remove();
                }
            }
            repaint();
        }
    }

    public static void main(String[] args) {
        BrownianTree b = new BrownianTree();
        b.setVisible(true);
        new Thread(b).start();
    }

    private class Particle {

        private int x, y;

        private Particle() {
            x = rand.nextInt(I.getWidth());
            y = rand.nextInt(I.getHeight());
        }

        
        private boolean move() {
            int dx = rand.nextInt(3) - 1;
            int dy = rand.nextInt(3) - 1;
            if ((x + dx < 0) || (y + dy < 0)
                    || (y + dy >= I.getHeight()) || (x + dx >= I.getWidth())) {
                return true;
            }
            x += dx;
            y += dy;
            if ((I.getRGB(x, y) & 0xff00) == 0xff00) {
                I.setRGB(x - dx, y - dy, 0xff00);
                return true;
            }
            return false;
        }
    }
}