Task: Barnsley fern
C#: using System;
using System.Diagnostics;
using System.Drawing;

namespace RosettaBarnsleyFern
{
    class Program
    {
        static void Main(string[] args)
        {
            const int w = 600;
            const int h = 600;
            var bm = new Bitmap(w, h);
            var r = new Random();
            double x = 0;
            double y = 0;
            for (int count = 0; count < 100000; count++)
            {
                bm.SetPixel((int)(300 + 58 * x), (int)(58 * y), Color.ForestGreen);
                int roll = r.Next(100);
                double xp = x;
                if (roll < 1)
                {
                    x = 0;
                    y = 0.16 * y;
                } else if (roll < 86)
                {
                    x = 0.85 * x + 0.04 * y;
                    y = -0.04 * xp + 0.85 * y + 1.6;
                } else if (roll < 93)
                {
                    x = 0.2 * x - 0.26 * y;
                    y = 0.23 * xp + 0.22 * y + 1.6;
                } else
                {
                    x = -0.15 * x + 0.28 * y;
                    y = 0.26 * xp + 0.24 * y + 0.44;
                }
            }
            const string filename = "Fern.png";
            bm.Save(filename);
            Process.Start(filename);
        }
    }
}

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

public class BarnsleyFern extends JPanel {

    BufferedImage img;

    public BarnsleyFern() {
        final int dim = 640;
        setPreferredSize(new Dimension(dim, dim));
        setBackground(Color.white);
        img = new BufferedImage(dim, dim, BufferedImage.TYPE_INT_ARGB);
        createFern(dim, dim);
    }

    void createFern(int w, int h) {
        double x = 0;
        double y = 0;

        for (int i = 0; i < 200_000; i++) {
            double tmpx, tmpy;
            double r = Math.random();

            if (r <= 0.01) {
                tmpx = 0;
                tmpy = 0.16 * y;
            } else if (r <= 0.08) {
                tmpx = 0.2 * x - 0.26 * y;
                tmpy = 0.23 * x + 0.22 * y + 1.6;
            } else if (r <= 0.15) {
                tmpx = -0.15 * x + 0.28 * y;
                tmpy = 0.26 * x + 0.24 * y + 0.44;
            } else {
                tmpx = 0.85 * x + 0.04 * y;
                tmpy = -0.04 * x + 0.85 * y + 1.6;
            }
            x = tmpx;
            y = tmpy;

            img.setRGB((int) Math.round(w / 2 + x * w / 11),
                    (int) Math.round(h - y * h / 11), 0xFF32CD32);
        }
    }

    @Override
    public void paintComponent(Graphics gg) {
        super.paintComponent(gg);
        Graphics2D g = (Graphics2D) gg;
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);

        g.drawImage(img, 0, 0, null);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame f = new JFrame();
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.setTitle("Barnsley Fern");
            f.setResizable(false);
            f.add(new BarnsleyFern(), BorderLayout.CENTER);
            f.pack();
            f.setLocationRelativeTo(null);
            f.setVisible(true);
        });
    }
}