Lab 15: HugLife


In this lab, you'll create a package named creature that will implement two creatures (or more, if you'd like) that will inhabit a world simulated by the huglife package. Along the way we'll learn how to debug small pieces of a much larger system, even if those small pieces happen to live inside another package.


First, pull the Lab 15 starter files:

git pull skeleton master

Start the lab by booting up the HugLife simulator. To do this, import the project into IntelliJ and run the huglife.HugLife class with a single command line argument samplesolo.

Or alternately, use the following commands from the command line (make sure you're in the lab15 directory):

$ javac -g huglife/*.java creatures/*.java
$ java huglife.HugLife samplesolo

This starts up a world called samplesolo. You should see a little red square wandering around randomly.

The creatures you'll create in this assignment will go in the creatures/ directory, in these two files:

Eventually these two types of creatures will also inhabit the world, and unlike this red guy, they actually do something interesting.

These classes will extend the huglife.Creature class, which provides a template that all creatures should follow.

How the simulator works

Creatures live on an NxN grid, with no wraparound. Each square may be empty, impassible, or contain exactly one creature. At each tic (timestep), exactly one creature takes a single action. Creatures choose actions in a round-robin fashion.

There is a global queue of all creatures in the world, waiting their turn to take an action. When a creature is at the front of the queue, the world simulator tells that creature who its four neighbors are and requests a choice of action from the creature. More specifically, the world simulator calls the creature's chooseAction() method, which takes as an argument a collection of all four neighbors. Based on the identity of the four neighbors, the creature then chooses one of exactly five actions: MOVE, REPLICATE, ATTACK, STAY, or DIE.

MOVE, REPLICATE, and ATTACK are directional actions, and STAY and DIE are stationary actions. If a creature takes a directional action, it must specify either a direction or a location. For example, if the acting creature sees a creature to its LEFT that it can eat, it might choose to ATTACK LEFT.

One of your main tasks for this lab is to write the code that makes Creature decisions. Actions are returned as objects of type Action, which are fully described in huglife/

After a creature chooses an Action, the simulator enacts the changes to the world grid. You'll be responsible for writing the code that tracks the state of each creature. For example, after the acting creature eats another creature, the acting Creature might become stronger, die, change colors, etc.

This will be accomplished by a callback to the creature, which is required to provide move(), replicate(), attack(), and stay() methods that describe how the state of the acting creature will evolve after each of these respective actions.

For example, if your creature chooses to replicate upwards by returning new Action(Action.ActionType.REPLICATE, Direction.TOP), then the game simulator is guaranteed to later call the replicate() method of the creature that made this choice.

Experimenting with the Sample Creature

Open up,, and, which you'll find in the huglife/ directory.

Make some changes to the sample creature and observe how they affect the HugLife simulation. As one of your experiments, you might have the SampleCreature react in some observable way when it sees a wall. You can do this by requesting a list of all neighbors of type "impassible" from the getNeighborsOfType() method.

Hint: After you're done experimenting, you can use git checkout to revert your lab directory to its original state. Consult the documentation for Git if you don't know how.

Creating a Plip

Now it's time to add a new type of creature to the world. Go into the creatures/ directory, and you'll see a file named there, waiting to be filled out.

Basic Plip functionality

Plips will be lazy (but motile) photosynthesizing creatures. They mostly just stand around and grow and replicate, but they'll flee if they happen to see their mortal enemy, the Clorus.

Let's start with just a few of the properties that we'll eventually need for our Plip class.

We could test our Plip class by sticking a bunch of Plips on a HugLife world grid and watching what they do as they run amok. However, it would be hard to determine whether everything was working correctly. Instead, let's perform testing on the Plip class directly.

Note on testing: It's not necessarily desirable to test everything! Use tests only when you think they might reveal something useful, i.e. there is some chance you'll get something wrong. Figuring out what to test is a bit of an art!

Open, which is also in the creatures/ directory. You'll see that we've provided a test file creatures.TestPlip that you can run to test your Plip class.

Try it out and you'll see that the testBasics test fails.

Now modify the Plip class according to the specifications above until all tests pass. When you're done, you'll be well on your way to having a fully functional Plip!

The Plip replicate method

Now we'll work on adding the correct replication property to our Plips, namely:

You'll be filling out the replicate() method in Take a look at that now.

Before your start, write an appropriate test in the testReplicate() method. Be sure to check that the returned Plip is not the same Plip as the Plip whose replicate() method was called. You can use the JUnit assertNotSame() method for this purpose. (Do not confuse assertNotEquals() with assertNotSame(). See the JUnit documentation if the distinction is unclear!)

The Plip chooseAction() method

All that's left is giving the Plip a brain. To do this, you'll be filling out the chooseAction() method.

The Plip should obey the following behavioral rules, in order of preference:

  1. If there are no empty spaces, the Plip should STAY.
  2. Otherwise, if the Plip has energy greater than 1.0, it should replicate to an available space.
  3. Otherwise, if it sees a neighbor with name() equal to "clorus", it will move to any available empty square with probability 50%. It should choose the empty square randomly. As an example, if it sees a Clorus to the LEFT and to the BOTTOM, and "empty" to the TOP and RIGHT, there is a 50% chance it will move (due to fear of Cloruses), and if it does move, it will pick randomly between RIGHT and TOP.
  4. Otherwise, it will stay.

These rules must be obeyed in this strict order! Example: If it has energy greater than 1, it will always replicate, even if there are neighboring Cloruses.

Writing tests for chooseAction()

Before you start on chooseAction(), uncomment the @Test annotation tag for the testChoose() method in This will allow the testChoose test to run. The existing test checks the first rule, namely that if there are no empty spaces next to the Plip, then it should stay.

Add some more tests to testChoose(). You might find it useful to look at the code for the Action class to see the various constructors for Actions.

Don't worry (yet) about testing the 50% rule if a Clorus is nearby. This isn't possible since you haven't created a Clorus class yet, and thus you won't be able to create a HashMap that involves Cloruses.

Later, once you write the Clorus class, you might find it interesting to come back and try to write a randomness test. One possibility is to simply test that both choices are possible by making many calls and ensuring that each happens at least once. Performing a statistical test is probably a bit too much for lab today (though you're welcome to try).

Writing chooseAction()

After you're happy with the tests you've written, edit the Plip class so that it makes the right choices. You'll want to look carefully at the SampleCreature class as a guide.

Simulating Plips

Assuming your tests worked, you can now see how your Plips fare in the real HugLife world. Run huglife.Huglife with the command line argument sampleplip, or use the following commands from the command line:

$ javac huglife/*.java creatures/*.java
$ java huglife.HugLife sampleplip

You should see your Plips happily growing along. If something goes wrong, it's probably because your tests are not thorough enough. If this is the case, using the error messages, add new tests to until something finally breaks.

Introducing the Clorus

Now we'll implement the Clorus, a fierce blue-colored predator that enjoys nothing more than snacking on hapless Plips.

To begin, create and in the creatures package. Unlike before, you'll be writing these classes from scratch.

The Clorus should obey the following rules exactly:

As before, write a TestClorus class first. You probably don't need to test the move(), stay(), or color() methods, but you're welcome to. You should include at least one test for each type of action.

Once you're done writing tests, write the Clorus class itself.

After you've written and tested the class thoroughly, go into and uncomment the lines in readWorld().


We did it.

Now it's time to watch Cloruses and Plips battling it out. Use the following command to kick off a Manichaean struggle that will end either in eternal oscillations or in lonely immortals wandering the wastes forever. Run huglife.Huglife with the command line argument strugggz, or use the following commands from the command line:

$ javac huglife/*.java creatures/*.java
$ java huglife.HugLife strugggz

If you did everything right, it should hopefully look cool. You might consider tweaking the HugLife simulator parameters, including the world size and the pause time between simulation steps. Be warned that world sizes above 50x50 are probably going to run fairly slowly.


The autograder for this lab is very basic. If your HugLife simulation looks mostly right—that is, if it resembles the animation from the introduction—you probably did everything correctly.