One event occuring more than the other

Question

For my exam on Java (i'm almost there), I got a simulation with Foxes & Rabbits. I've added Hares to this. Foxes eat the rabbits and hares, but the rabbits and hares breed faster again. It's going fine so far, the only thing that I need to implement is that a Fox eats rabbits more often than hares, and I have no clue on how to implement it. What I've thought of is to make an integer for both the rabbits and the hares, with a sort of attractiveness-rating, but then I got stuck on how to implement it.
Anyway, here are the codes:
import java.util.List;
import java.util.Random;
public class Rabbit extends PreyAnimal
    // Characteristics shared by all rabbits (static fields).
    // The age at which a rabbit can start to breed.
    // private static final int BREEDING_AGE = 5;
    // The age to which a rabbit can live.
    // private static final int MAX_AGE = 50;
    // The likelihood of a rabbit breeding.
    // private static final double BREEDING_PROBABILITY = 0.15;
    // The maximum number of births.
    private static final int MAX_LITTER_SIZE = 5;
    // A shared random number generator to control breeding.
     private static final Random rand = new Random();
     public static final int PREFERENCE = 6;
    // Individual characteristics (instance fields).
     * Create a new rabbit. A rabbit may be created with age
     * zero (a new born) or with a random age.
     * @param randomAge If true, the rabbit will have a random age.
    public Rabbit(boolean randomAge)
        super.incrementAge();
        if(randomAge) {
            setAge(rand.nextInt(MAX_AGE));
     * This is what the rabbit does most of the time - it runs
     * around. Sometimes it will breed or die of old age.
     * @param currentField The field currently occupied.
     * @param updatedField The field to transfer to.
     * @param newAnimals A list to add newly born rabbits to.
    public void act(Field currentField, Field updatedField, List newAnimals)
        super.incrementAge();
        if(isAlive()) {
            int births = breed();
            for(int b = 0; b < births; b++) {
                Rabbit newRabbit = new Rabbit(false);
                newAnimals.add(newRabbit);
                newRabbit.setLocation(
                        updatedField.randomAdjacentLocation(getLocation()));
                updatedField.place(newRabbit);
            Location newLocation = updatedField.freeAdjacentLocation(getLocation());
            // Only transfer to the updated field if there was a free location
            if(newLocation != null) {
                setLocation(newLocation);
                updatedField.place(this);
            else {
                // can neither move nor stay - overcrowding - all locations taken
                setDead();
     * Generate a number representing the number of births,
     * if it can breed.
     * @return The number of births (may be zero).
    private int breed()
        int births = 0;
        if(canBreed() && rand.nextDouble() <= BREEDING_PROBABILITY) {
            births = rand.nextInt(MAX_LITTER_SIZE) + 1;
        return births;
    public int getPreference()
        return PREFERENCE;
     * @return A string representation of the rabbit.
    public String toString()
        return "Rabbit, age " + getAge();
}The class Hare is nearly the same ofcourse.
Here's the fox, on where I got to implement the preference:
import java.util.List;
import java.util.Iterator;
import java.util.Random;
public class Fox extends Animal
    // Characteristics shared by all foxes (static fields).
    // The age at which a fox can start to breed.
    private static final int BREEDING_AGE = 10;
    // The age to which a fox can live.
    private static final int MAX_AGE = 150;
    // The likelihood of a fox breeding.
    private static final double BREEDING_PROBABILITY = 0.09;
    // The maximum number of births.
    private static final int MAX_LITTER_SIZE = 3;
    // The food value of a single rabbit. In effect, this is the
    // number of steps a fox can go before it has to eat again.
    private static final int RABBIT_FOOD_VALUE = 6;
    private static final int HARE_FOOD_VALUE = 4;
    // A shared random number generator to control breeding.
    private static final Random rand = new Random();
    // Individual characteristics (instance fields).
    // The fox's food level, which is increased by eating rabbits.
    private int foodLevel;
     * Create a fox. A fox can be created as a new born (age zero
     * and not hungry) or with random age.
     * @param randomAge If true, the fox will have random age and hunger level.
    public Fox(boolean randomAge)
        super();
        if(randomAge) {
            setAge(rand.nextInt(MAX_AGE));
            foodLevel = rand.nextInt(RABBIT_FOOD_VALUE);
        else {
            // leave age at 0
            foodLevel = RABBIT_FOOD_VALUE;   
     * This is what the fox does most of the time: it hunts for
     * rabbits. In the process, it might breed, die of hunger,
     * or die of old age.
     * @param currentField The field currently occupied.
     * @param updatedField The field to transfer to.
     * @param newAnimals A list to add newly born foxes to.
    public void act(Field currentField, Field updatedField, List newAnimals)
        incrementAge();
        incrementHunger();
        if(isAlive()) {
            // New foxes are born into adjacent locations.
            int births = breed();
            for(int b = 0; b < births; b++) {
                Fox newFox = new Fox(false);
                newAnimals.add(newFox);
                newFox.setLocation(
                        updatedField.randomAdjacentLocation(getLocation()));
                updatedField.place(newFox);
            // Move towards the source of food if found.
            Location newLocation = findFood(currentField, getLocation());
            if(newLocation == null) {  // no food found - move randomly
                newLocation = updatedField.freeAdjacentLocation(getLocation());
            if(newLocation != null) {
                setLocation(newLocation);
                updatedField.place(this);  // sets location
            else {
                // can neither move nor stay - overcrowding - all locations taken
                setDead();
     * Increase the age. This could result in the fox's death.
    private void incrementAge()
        setAge(getAge() + 1);
        if(getAge() > MAX_AGE) {
            setDead();
     * Make this fox more hungry. This could result in the fox's death.
    private void incrementHunger()
        foodLevel--;
        if(foodLevel <= 0) {
            setDead();
     * Tell the fox to look for rabbits adjacent to its current location.
     * @param field The field in which it must look.
     * @param location Where in the field it is located.
     * @return Where food was found, or null if it wasn't.
    private Location findFood(Field field, Location location)
        Iterator adjacentLocations =
                          field.adjacentLocations(location);
        while(adjacentLocations.hasNext()) {
            Location where = (Location) adjacentLocations.next();
            Object animal = field.getObjectAt(where);
            if(animal instanceof Rabbit) {
                Rabbit rabbit = (Rabbit) animal;
                if(rabbit.isAlive()) {
                    rabbit.setDead();
                    foodLevel = RABBIT_FOOD_VALUE;
                    return where;
            else if(animal instanceof Hare) {
            Hare hare = (Hare) animal;
            if(hare.isAlive()) {
                hare.setDead();
                foodLevel = HARE_FOOD_VALUE;
                return where;
    return null;
     * Generate a number representing the number of births,
     * if it can breed.
     * @return The number of births (may be zero).
    private int breed()
        int births = 0;
        if(canBreed() && rand.nextDouble() <= BREEDING_PROBABILITY) {
            births = rand.nextInt(MAX_LITTER_SIZE) + 1;
        return births;
     * @return A string representation of the fox.
    public String toString()
        return "Fox, age " + getAge();
     * A fox can breed if it has reached the breeding age.
    private boolean canBreed()
        return getAge() >= BREEDING_AGE;
}Is there anybody out there with a clue on how to do it? Or maybe with another solution than adding an integer? Would be much appreciated :)
Btw, the full code with all classes is availble @ http://members.home.nl/firehawk/S200.rar

Answer 1

dubwai offers a simple solution for you...this would be implemented in the findFood() method, i believe.
i would offer a suggest. implements to an interface...this will get rid of the instanceof checking.
example
public interface Animal {  // common to all animal (in your application)
    public void setDead();
    public boolean isAlive();
public abstract class Prey implements Animal{  // common to all animal that are a prey
    private boolean alive = true;
    private int foodValue = 0;
    public void setDead(){  alive = false; }  // probably better to name it die() instead of setDead()
    public boolean isAlive(){ return alive; }
    public abstract int getFoodLevel();
public class Rabbit extends Prey {
    private static final RABBIT_FOOD_LEVEL = 3;
    public int getFoodLevel(){ return RABBIT_FOD_LEVEL; }
private Location findFood(Field field, Location location){
    Iterator adjacentLocations = field.adjacentLocations(location);
    while(adjacentLocations.hasNext()) {
        Location where = (Location) adjacentLocations.next();
        Animal animal = (Animal) field.getObjectAt(where);
        if (animal.isAlive()){
            animal.setDead();
            foodLevel = animal.getFoodLevel();
            return where;
        return false;
}i would probably makes the Prey an interface instead of an abstract class.