Task: Brownian tree
Go: package main

import (
    "fmt"
    "image"
    "image/color"
    "image/png"
    "math/rand"
    "os"
)

const w = 400     
const h = 300     
const n = 15000   
const frost = 255 

var g *image.Gray

func main() {
    g = image.NewGray(image.Rectangle{image.Point{0, 0}, image.Point{w, h}})
    
    g.SetGray(w/3, h/3, color.Gray{frost})
generate:
    for a := 0; a < n; {
        
        rp := image.Point{rand.Intn(w), rand.Intn(h)}
        if g.At(rp.X, rp.Y).(color.Gray).Y == frost {
            
            for {
                rp.X += rand.Intn(3) - 1
                rp.Y += rand.Intn(3) - 1
                
                if !rp.In(g.Rect) {
                    continue generate
                }
                if g.At(rp.X, rp.Y).(color.Gray).Y != frost {
                    break
                }
            }
        } else {
            
            
            for !hasNeighbor(rp) {
                rp.X += rand.Intn(3) - 1
                rp.Y += rand.Intn(3) - 1
                
                if !rp.In(g.Rect) {
                    continue generate
                }
            }
        }
        
        g.SetGray(rp.X, rp.Y, color.Gray{frost})
        a++
        
        if a%100 == 0 {
            fmt.Println(a, "of", n)
        }
    }
    f, err := os.Create("tree.png")
    if err != nil {
        fmt.Println(err)
        return
    }
    err = png.Encode(f, g)
    if err != nil {
        fmt.Println(err)
    }
    f.Close()
}

var n8 = []image.Point{
    {-1, -1}, {-1, 0}, {-1, 1},
    {0, -1}, {0, 1},
    {1, -1}, {1, 0}, {1, 1}}

func hasNeighbor(p image.Point) bool { 
    for _, n := range n8 {
        o := p.Add(n)
        if o.In(g.Rect) && g.At(o.X, o.Y).(color.Gray).Y == frost {
            return true
        }
    }
    return false
}

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;
        }
    }
}