University of Washington, CSE 142

Lab 9: Critters Practice

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

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

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• 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 much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

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 increasingly complex in terms of its behaviors. The LabTA class will serve as a model for how to solve the homework.

Exercise : Critters Setup

To work on the critters assignment, you must first download the classes that are used by the simulator. To begin:

• Download the Critters zipped folder.
• Unzip the folder by navigating to wherever the Critters zipped folder downloaded, right clicking the folder icon, and left clicking "Extract all".
• Go into jGRASP and open CritterMain.java from the (unzipped) Critters folder.
• Compile and run CritterMain.java. 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 critters in the world.

Exercise : LabTA Version 1

• In jGRASP, create the following Java class and save it in the Critters folder:

  public class LabTA extends Critter {
  
  }
  

• Compile the class. If you get errors, make sure that it is saved in the same folder as your other Critter files.
• 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.

Exrecise : Solution

public class LabTA extends Critter {
   private int count;
   
   public LabTA() {
      this.count = 0;
   }
   
   public Action getMove(CritterInfo info) {
      count++;
      return Action.HOP;
   }

   public Color getColor() {
      if (count % 2 == 0) {
         return Color.MAGENTA;
      } else {
         return Color.WHITE;
      }
   }

   ...
  

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 a different creature is in front of it, otherwise hops if it can (if nothing is in front of it), and then otherwise 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. Also be sure to still increase your count field by 1 as in the last exercise
• Test your new version by running CritterMain and observing the LabTA critters.

Exercise : Solution

public class LabTA extends Critter {

   ...

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

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 : Solution

import java.util.*;

public class LabTA extends Critter {
   private Random r;
   
   ...
  
   public Action getMove(CritterInfo info) {
      count++;
      if (info.getFront() == Neighbor.OTHER) {
         return Action.INFECT;
      } else if (info.getFront() == Neighbor.EMPTY) {
         return Action.HOP;
      } else {
         int choice = r.nextInt(2);
         if (choice == 0) {
            return Action.LEFT;
         } else {
            return Action.RIGHT;
         }
      }
   }

   ...

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

Exercise : Fish

Define a Critter class called Fish:

• Fish should be White in color
• Fish should appear as "<><" on even numbered moves, and as "><>" on odd numbered moves.
• Fish should always infect if a different critter is in front of them, then should otherwise hop if able, and should otherwise turn right.

Solve this program in jGRASP with CritterMain, and be sure to add your Fish to the critter world in the same way you added your LabTA class. Your Fish should end up "swimming" around the outside edge of the world.

Exercise : Eagle

Define a Critter class called Eagle:

• An Eagle should alternate in Color between Red, White, and Blue (in that order)
• An Eagle should appear as an "E"
• An Eagle should always infect if a different critter is in front of them, then should otherwise move in a circular pattern. An Eagle should hop three times, then turn left, then hop three times, then turn left, and so on so forth.

Solve this program in jGRASP with CritterMain, and be sure to add your Eagle to the Critter world in the same way you added your LabTA class. You should see your Eagles moving in small circles, occasianally infecting other critters, and changing between red, white, and blue.

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!