Sierpinski's Arrowhead Curve Generation

This assignment is about implementing an L-system, specifically Sierpinski's Arrowhead Curve, using an AnimatedTurtle to draw the generated string. The L-system has a predefined alphabet and initial generation, along with production rules for transforming one generation into another. The goal is to create a Java program that implements these rules without using recursion.

Background
In the late 1960's the biologist and botanist, Aristid Lindenmayer, created L-systems (Links to an
external site.) to describe and model plant growth. He published a book entitled The Algorithimic
Beauty of Plants, that shows the use of these systems (see link to PDF in modules on Canvas).
An L-system consists of an alphabet which is simply a String containing the characters used, a initial
(or axiom) String consisting of characters from the alphabet, and then a collection of production rules
for transforming one generation into another.
On this exam, you will only be dealing with one particular L-system -- Sierpinski's Arrowhead Curve.
So in this case the alphabet consists of "AB+-". The initial generation is "A". The production rules
are (A → +B−A−B+), (B → −A+B+A−). This means that when nextGen is called with "A", the only
rule that applies is the first production rule and so a new String is created that contains only
"+B−A−B+". If nextGen were called on that, the process is the same -- namely for each character
we look at the production rules to see if one applies and then replace that in the result string with the
production.
So here is a full look at nextGen for "+B−A−B+":
Character Production
rule Result so far
+ none +
B B → −A+B+A− +−A+B+A−
- none +−A+B+A−-
A A → +B−A−B+ +−A+B+A−-+B−A−B+
- none +−A+B+A−-+B−A−B+-
B B → −A+B+A− +−A+B+A−-+B−A−B+-−A+B+A−
+ none +−A+B+A−-+B−A−B+-−A+B+A−+
So the result of nextGen("+B−A−B+") is "+−A+B+A−-+B−A−B+-−A+B+A−+".
Given a String that represents a generation, you can use an AnimatedTurtle to draw that String by
following the rules that A and B mean move forward and + means turn left 60 degrees and - means
turn right 60 degrees.
Requirements
Download, unzip, and open the starter project in IntelliJ. You must fill in the methods in the
ArrowheadCurve.java file.
 Your program must not use recursion.
 You must implement each of the methods in ArrowheadCurve.java that have TODO comments.
What to Submit
Submit only your ArrowheadCurve.java file.