University of Washington, CSE 142

Lab 9: Critters Practice

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

Talk to your classmates for help. You can even work on the lab with a partner if you like.
You may want to bring your textbook to future labs to look up syntax and examples.
Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
Complete as many problems as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
Feel free to complete problems in any order.
Before you leave today, make sure to check in with one of the TAs in the lab to get credit for your work.

Today's lab

Goals for today:

Practice working with the critter classes and defining your own critter class.
You will spend most of the time in this lab developing a critter class known as LabTA. Through a series of exercises, you will make your LabTA critter more and more complex in terms of its behaviors. The LabTA class will serve as a model for how to solve the homework.
Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Critters Setup

To work on the critters assignment, you must first download the classes that are used by the simulator. There are eight different standard classes that you will need access to.

Open the file ass9.zip. Choose the option to "Open" or "Open with/Windows Explorer".
Extract the files in the ass9 folder (you will probably need to first click on "Extract" and then click on "Extract all").
Go into jGRASP and open CritterMain.java. It will be stored in the ass9 folder either in the My Documents folder or the Downloads folder.
Compile and run CritterMain. You should see a world that has 30 "Food" critters and 30 "FlyTrap" critters. You can click on "start" although nothing will happen with just these two kinds of critters in the world.

Exercise : LabTA Version 1

In jGRASP, create the following Java class and save it in the ass9 folder:
```
public class LabTA extends Critter {

}
```
Compile the class. If you get errors, make sure that it is saved in the ass9 folder.
Modify CritterMain to include this line of code:
```
        frame.add(30, LabTA.class);
```
Compile and run CritterMain. You should see 30 new critters that are displayed as black question marks.
Click on the "start" button to start the simulator running. Then click on the "debug" button to see the direction that critters are facing. You should see that the LabTA critters are turning in a counterclockwise direction because their default behavior is Action.LEFT.

Exercise : LabTA Version 2

Copy and Paste the following lines of code and add them to your LabTA class:

    public Action getMove(CritterInfo info) {
        return Action.HOP;
    }

    public Color getColor() {
        return Color.MAGENTA;
    }

    public String toString() {
        return ":-)";
    }

Add the following at the top of the class:
```
import java.awt.*;
```
Compile the LabTA class and correct any errors. Switch to the CritterMain window and give the run command. Now you should see 30 magenta colored critters that are displayed with the smiley emoticon.
Click on the start button. You should see the LabTA critters moving forward until they hit a wall or another critter. Some might be infected by a FlyTrap. Click on the debug button to watch in detail and try running it several times.

Exercise : LabTA Version 3

Our LabTA critters have very simple behaviors that don't change. They always hop forward and they always display as magenta emoticons.

For this exercise, we want to alter the behavior of the LabTA critter so that it changes color over time. In particular, we want them to alternate between the colors magenta and white. Initially they should be magenta, but after they make a first move, they should change their color to white. Then after another move they should change back to magenta. Then after another move they should change back to white.

Developing this kind of behavior is a central part of the critters homework. Think about how you can implement this. In particular, think about what kind of state information you could add to the LabTA critter that would allow it to do this. Remember that in defining critters, you get to decide what fields to introduce. A field allows an object to remember information about its state.

The next slide tells you how to do it, but it would be good to try to come up with your own ideas first.

Exercise : LabTA Version 3

To allow your LabTA critters to change colors on alternate moves, they need to keep track of where they are in the cycle. For example, you could introduce a boolean field that keeps track of whether it should be displaying itself as magenta or as white. You can then switch the boolean value each time that getMove is called.

We can solve this in an even more general way by introducing a move counter:

Add a field to your class called count to keep track of how many moves the critter has made. Remember that fields should be declared to be private.
Modify the getMove method to increment this counter each time it is called.
Use the counter to rewrite the getColor method to return magenta when the move counter is even and to return white when it is odd.
Test your new version by recompiling the LabTA class and running CritterMain.

Exercise : LabTA Version 4

Now we're going to work on the moves that the LabTA critters make.

Open the FlyTrap class to see how it is written. Notice its getMove method:

    public Action getMove(CritterInfo info) {
        if (info.getFront() == Neighbor.OTHER) {
            return Action.INFECT;
        } else {
            return Action.LEFT;
        }
    }

Copy and paste this code to replace the current version of getMove in the LabTA class. Then modify it so that it always infects when it can and otherwise it hops if it can and otherwise it turns left. This involves adding a new branch in the if/else. Remember that the constant Neighbor.EMPTY can be used to see if a position is empty and that you move forward with the Action.HOP move.
Test your new version by running CritterMain and observing the LabTA critters.

Exercise : LabTA Version 5

Now we're going to include a bit of randomness. The current version of the LabTA class always turns left when it can't infect or hop. We are going to change it so that it instead randomly chooses between turning left and turning right.

To solve this problem, we will need access to a Random object. We don't want to construct one every time we call getMove, so we will instead store it as a field.

Take your time to complete each of the following steps because the practice will be very valuable. Ask a TA or your neighbor for help if you don't know how to accomplish any of the tasks:

Add a field called r of type Random.
Introduce a constructor for the class that initializes the Random object.
Modify the getMove method so that it uses the Random object to randomly pick between turning left and turning right if it can't infect or hop.
Test your new version by running CritterMain and observing the LabTA critters.

Exercise : Cat Class

Define a Critter class called Cat:

It should always infect if an enemy is in front of it. Otherwise it should hop if there is a critter of another species either to its left, its right, or behind it. Otherwise it should turn right.
Its color should alternate between green and white on successive moves.
It should be displayed as a "C".

To test the Cat class behavior, include 30 cats and watch what happens (turning on the debug feature will be particularly helpful)

Exercise : Dog Class

Define a Critter class called Dog:

It should always infect if an enemy is in front of it. Otherwise it should turn left five times and then it should hop once.
It should be colored pink.
It should display how many left turns it has made since it last tried to hop (initially 0, then 1, then 2, ..., eventually becoming 5, then going back to 0, 1, 2, ...).

To test the Dog class behavior, include 30 dogs and watch what happens (turning on the debug feature will be particularly helpful).

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!