CSE 142 Programming Assignment #9: Critters


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This assignment focuses on classes and objects. Turn in Ant.java, Bird.java, Hippo.java, Vulture.java, and
Husky.java. Download supporting files on the course web site. Run CritterMain.java to start the simulation.
Program Behavior:
You are provided with several client program classes that implement a
graphical simulation of a 2D world of animals. You will write classes that
define the behavior of those animals. Animals move and behave in different
ways. Your classes will define the unique behaviors for each animal.
The critter world is divided into cells with integer coordinates. The world is
60 cells wide and 50 cells tall. The upper-left cell has coordinates (0, 0); x
increases to the right and y increases downward.
On each round of the simulation, the simulator asks each critter object which direction it wants to move. Each round a critter
can move one square north, south, east, west, or stay at its current location (center). The world has a finite size, but it wraps
around in all four directions (for example, moving east from the right edge brings you back to the left edge).
This program will be confusing at first, because you do not write its main method; your code is not in control of the overall
execution. Instead, your objects are part of a larger system. You might want your critters make several moves at once using
a loop, but you can’t. The only way a critter can move is to wait for the simulator to ask it for a single move and return that
move. This experience can be frustrating, but it is a good introduction to object-oriented programming.
As the simulation runs, animals may collide by moving onto the same location. When two animals collide, if they are from
different species, they fight. The winning animal survives and the losing animal is killed and removed from the game. Each
animal chooses one of Attack.ROAR, Attack.POUNCE, or Attack.SCRATCH. Each attack is strong against one other (e.g.
roar beats scratch) and weak against another (roar loses to pounce). The following table summarizes the choices and which
animal will win in each case. To remember which beats which, notice that the starting letters of “Roar, Pounce, Scratch” match
those of “Rock, Paper, Scissors.” If the animals make the same choice, the winner is chosen at random.
Critter #2
Attack.ROAR Attack.POUNCE Attack.SCRATCH
Critter #1 Attack.ROAR random winner #2 wins #1 wins
Attack.POUNCE #1 wins random winner #2 wins
Attack.SCRATCH #2 wins #1 wins random winner
If two animals of the same species collide, they mate to produce a baby. Animals are vulnerable to attack while mating: any
other animal that collides with them will defeat them. An animal can mate only once during its lifetime.
The simulation world also contains food (represented by the period character, “.”) for the animals to eat. There are pieces of
food on the world initially, and new food is gradually added into the world over time. As an animal moves, it may encounter
food, in which case the simulator will ask your animal whether it wants to eat it. Different kinds of animals have different
eating behavior; some always eat, and others only eat under certain conditions.
Every time one class of animals eats a few pieces of food, that animal will be put to sleep by the simulator for a small amount
of time. While asleep, animals cannot move, and if they enter a fight with another animal, they will always lose.
The simulator keeps a score for each class of animal, shown on the right side of the screen. A class’s score is based on how
many animals of that class are alive, how much food they have eaten, and how many other animals they have killed.
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Provided Files:
Each class you write will extend a superclass named Critter. This is an example of inheritance, as discussed in Ch. 9 of
the textbook. Inheritance makes it easier for our code to talk to your critter classes, and it helps us be sure that all your critter
classes will implement all the methods we need. Your class headers should indicate the inheritance by using the extends
keyword, such as:
public class Ant extends Critter { …
The Critter class contains the following methods, some or all of which you will write in each of your classes:
 public boolean eat()
When your animal encounters food, our code calls this on it to ask whether it wants to eat (true) or not (false).
 public Attack fight(String opponent)
When two animals move onto the same square of the grid, they fight. When they collide, our code calls this on each
animal to ask it what kind of attack it wants to use in a fight with the given opponent.
 public Color getColor()
Every time the board updates, our code calls this on your animal to ask it what color it wants to be drawn with.
 public Direction getMove()
Every time the board updates, our code calls this on your animal to ask it which way it wants to move.
 public String toString()
Every time the board updates, our code calls this on your animal to ask what letter it should be drawn as.
By declaring that your class extends Critter as shown above, you receive a default version of these methods. The default
behavior is to never eat, to always forfeit fights, to use the color black, to always stand still (a move of Direction.CENTER),
and to display as a “?”. If you don’t want these defaults, rewrite (override) the necessary methods in your class with your own
For example, below is a critter class Stone. A Stone is displayed with the letter S, is gray in color, never moves or eats, and
always roars in a fight. Your classes will look like this class, except with fields, a constructor, and more sophisticated code.
The Stone does not need an eat or getMove method; it uses the default behavior for those operations.
import java.awt.*; // for Color
public class Stone extends Critter {
public Attack fight(String opponent) {
return Attack.ROAR;

public Color getColor() {
return Color.GRAY;

public String toString() {
return “S”;
NOTE: You will not necessarily write extends Critter on every single critter class you write. If you find that two critter
classes are very similar to each other, you should have one extend the other to reduce redundancy.
Running the Simulator:
When you press the Go button, it begins a series of turns. On each turn, the simulator does the following for each animal:
 move the animal once (calling its getMove method), in random order
 if the animal has moved onto an occupied square, fight! (call both animals’ fight methods) or mate
 if the animal has moved onto food, ask it if it wants to eat (call the animal’s eat method)
After moving all animals, the simulator redraws the screen, asking each animal for its toString and getColor values. It
can be difficult to test and debug with many animals. We suggest adjusting the initial settings to use a smaller world and fewer
animals. There is also a Debug checkbox that, when checked, prints console output about the game behavior.
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Critter Classes:
The following are the five critter classes to implement. Each has one constructor that should accepts exactly the parameter(s)
in the table. For random moves, each choice should be equally likely; use a Random object or the Math.random method.
1. Ant
constructor public Ant(boolean walkSouth)
color red
eating behavior always eats
fighting behavior always scratches
movement if the Ant was constructed with a walkSouth value of true, then
alternates between south and east in a zigzag (S, E, S, E, …); otherwise,
if the Ant was constructed with a walkSouth value of false, then
alternates between north and east in a zigzag (N, E, N, E, …)
toString “%” (percent)
2. Bird
constructor public Bird()
color blue
eating behavior never eats (always returns false)
fighting behavior roars if the opponent looks like an Ant (“%”); otherwise pounces
movement a clockwise square: first goes north 3 times, then east 3 times,
then south 3 times, then west 3 times, then repeats
toString “^” (caret) if the bird’s last move was north or it has not moved;
“>” (greater-than) if the bird’s last move was east;
“V” (uppercase letter v) if the bird’s last move was south;
“<" (less-than) if the bird's last move was west 3. Hippo constructor public Hippo(int hunger) color gray if the hippo is still hungry (would still eat food); otherwise white eating behavior eats the first hunger times it is called; never eats after that fighting behavior if this Hippo is hungry (would still eat food), then scratches; otherwise pounces movement moves 5 steps in a random direction (north, south, east, or west), then chooses a new random direction and repeats toString the number of pieces of food this Hippo still wants to eat, as a String The Hippo constructor accepts a parameter for the maximum amount of food that Hippo will eat in its lifetime (the number of times it will return true from a call to eat). For example, a Hippo constructed with a parameter value of 8 will return true the first 8 times eat is called and false after that. Assume that the value passed is non-negative. The toString method for a Hippo returns the number of times that a call to eat that would return true for that Hippo. For example, if a new Hippo(4) is constructed, initially its toString return "4". After eat has been called on it once, calls to toString return "3", and so on, until the Hippo is no longer hungry, after which all calls to toString return "0". You can convert a number to a string by concatenating it with an empty string. For example, "" + 7 makes "7". 4 of 5 4. Vulture constructor public Vulture() color black eating behavior Eats if vulture is hungry. A vulture is initially hungry, and remains hungry until it eats once. After eating, it will become non-hungry until he gets into a fight. After one or more fights, it will be hungry again. (see below) fighting behavior roars if the opponent looks like an Ant ("%"); otherwise pounces movement a clockwise square: first goes north 3 times, then east 3 times, then south 3 times, then west 3 times, then repeats toString "^" (caret) if the vulture's last move was north or has not moved; ">” (greater-than) if the vulture’s last move was east;
“V” (uppercase letter v) if the vulture’s last move was south;
“<" (less-than) if the vulture's last move was west A Vulture is a specific sub-category of bird with some changes. Think of the Vulture as having a "hunger" that is enabled when it is first born and also by fighting. Initially the Vulture is hungry (so eat would return true from a single call). Once the Vulture eats a single piece of food, it becomes non-hungry (so future calls to eat would return false). But if the Vulture gets into a fight or a series of fights (if fight is called on it one or more times), it becomes hungry again. When a Vulture is hungry, the next call to eat should return true. Eating once causes the Vulture to become "full" again so that future calls to eat will return false, until the Vulture's next fight or series of fights. 5. Husky constructor ? public Husky() (must not accept any parameters) all behavior you decide (see below) You will decide the behavior of your Husky class. Part of your grade will be based upon writing creative and non-trivial Husky behavior. The following are some guidelines and hints about how to write an interesting Husky. Your Husky's fighting behavior may want to utilize the opponent parameter to the fight method, which tells you what kind of critter you are fighting against (such as "%" if you are fighting against an Ant). Your Husky can return any text you like from toString (besides null) and any color from getColor. Each critter's getColor and toString are called on each simulation round, so you can have a Husky that displays differently over time. The toString text is also passed to other animals when they fight you; you may want to try to fool other animals. Unlike on most assignments, your Husky can use advanced material you happen to know in Java (with some restrictions). If you wish to use particularly complex material in your Husky, contact your TA or instructor to make sure it will be compatible with our system. Each critter class has some additional methods that it receives by inheritance from Critter. Your Husky may want to use these methods to guide its behavior. (None of the methods below are needed for Ant, Bird, Hippo, or Vulture.)  public int getX() public int getY() Returns your critter's current x and y coordinates. For example, to check whether your critter's x-coordinate is greater than 10, you would write code such as: if (getX() > 10) {
 public int getWidth() public int getHeight()
Returns the width and height of the simulation grid.
 public String getNeighbor(Direction direction)
Returns a String representing what is next to your critter in the given direction. ” ” means an empty square.
For example, to check if your neighbor to the west is a “Q”, you could write this in your getMove method:
if (getNeighbor(Direction.WEST).equals(“Q”)) {
 public void win() public void sleep() public void mate() public void reset()
public void lose() public void wakeup() public void mateEnd()
The simulator calls these methods on your critter to notify you when you have won/lost a fight, when you have been
put to sleep/woken up, when you have started/ended mating, or when the game world has reset, respectively.
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Development Strategy:
The simulator runs even if you haven’t completed all the critters. The classes increase in difficulty from Ant to Bird to Hippo
to Vulture. We suggest doing Ant first. Look at Stone.java and the lecture/section examples to see the general structure.
It will be impossible to implement each behavior if you don’t have the right state in your object. As you start writing each
class, spend some time thinking about what state will be needed to achieve the desired behavior.
One thing that students in the past have found particularly difficult to understand is the various constructors for each type of
animal. Some of the constructors accept parameters that guide the behavior of later methods of the animal. It is your job to
store data from these parameters into fields of the animal as appropriate, so that it will “remember” the proper information
and will be able to use it later when the animal’s other methods are called by the simulator.
Test your code incrementally. A critter class will compile even if you have not written all of the methods (the unwritten ones
will use default behavior). So add one method, run the simulator to see that it works, then add another.
Style Guidelines:
Since this assignment is largely about classes and objects, much of the style grading will be about how well you follow proper
object-oriented programming style. You should encapsulate the data inside your objects, and you should not declare
unnecessary data fields to store information that isn’t vital to the state of the object. Style points will also be awarded for
expressing each critter’s behavior elegantly.
Another aspect of the style of this program is inheritance. Your critter classes should properly extend a superclass as described.
Inheritance can also be used to remove redundancy between classes that are similar, and you should make use of this concept
in your classes as appropriate. In other words, if two of your critter classes A and B are very much alike, you should have B
extend A rather than having both simply extend Critter.
Some of the style points for this assignment will be awarded on the basis of how much energy and creativity you put into
defining an interesting Husky class. These points allow us to reward the students who spend time writing an interesting critter
definition. Your Husky’s behavior should not be trivial or closely match that of an existing animal shown in class. A single
point of extra credit will be awarded to exceptionally creative or impressive Husky implementations. Note that this point will
be awarded very sparingly, and that writing complex code or using advanced material is neither necessary nor sufficient to earn
this point.
Follow past style guidelines about indentation, spacing, identifiers, and localizing variables. Place comments at the beginning
of each class documenting that critter’s behavior, as well as on the top of each method and on any complex code. Your critters
should not produce any console output. For reference, our Ant, Bird, Hippo, and Vulture together occupy just under 199
lines including blank lines and comments (117 “substantive” lines, according to our Indenter tool).
The Husky is not graded on internal correctness at all. Its code does not need to be commented, can be redundant, and can
use advanced material, so long as it works properly and obeys the other constraints described previously.