quadratic koch island in Processing, as in The Algorithmic Beauty of Plants.

main.pde

void setup() {
  size(500, 500);
  smooth();  
  background(255);
  stroke(0);
  strokeWeight(1);

  HashMap productions = new HashMap();
  productions.put('F', "F-F+F+FF-F-F+F");
  String axiom = "F-F-F-F";
  for (int i = 0;  i < 3; i++) {
    axiom = gen(axiom, productions);
  }
  
  HashMap commands = new HashMap();
  commands.put('F', F);
  commands.put('f', f);
  commands.put('-', minus);
  commands.put('+', plus);
  
  Turtle t = new Turtle(width * 0.25, height * 0.75, PI * 0.5);
  t.run(commands, axiom);
}

String gen(String input, HashMap productions) {
  StringBuffer buf = new StringBuffer();
  for (int i = 0; i < input.length(); i++) {
    char c = input.charAt(i);
    if (!productions.containsKey(c)) {
      buf.append(c);
      continue;
    }
    buf.append((String)productions.get(c));
  }
  return buf.toString();
}

turtle.pde

float D = 4.0;

class Turtle {
  PVector position;
  float heading;
  HashMap commands;
  
  Turtle(float x, float y, float heading) {
    this.position = new PVector(x, y);
    this.heading = heading;
  }
  
  void run(HashMap commands, String program) {
    for (int i = 0; i < program.length(); i++) {
      char c = program.charAt(i);
      if (!commands.containsKey(c)) continue;
      TurtleCommand cmd = (TurtleCommand)commands.get(c);
      cmd.run(this);
    }
  }
}

abstract class TurtleCommand {
  public abstract void run(Turtle t);
}

TurtleCommand F = new TurtleCommand() {
  public void run(Turtle t) {
    float x = t.position.x + D * cos(t.heading);
    float y = t.position.y - D * sin(t.heading);
    line(t.position.x, t.position.y, x, y);
    t.position.x = x;
    t.position.y = y; 
    println("move forward and draw");
  }
};

TurtleCommand f = new TurtleCommand() {
  public void run(Turtle t) {
    t.position.x += D * cos(t.heading);
    t.position.y -= D * sin(t.heading);
    println("move forward");
  }
};

TurtleCommand plus = new TurtleCommand() {
  public void run(Turtle t) {
    t.heading += PI * 0.5;
    println("turn left");
  }
};

TurtleCommand minus = new TurtleCommand() {
  public void run(Turtle t) {
    t.heading -= PI * 0.5;
    println("turn right");
  }
};