This file contains the code that should be added to Fish.java
for Chapter 3.  There are six code fragments.
            - the Modification History in the class documentation
            - the two new instance variables
            - the initialization of those variables in initialize
            - the act method
            - the breed and generateChild methods
            - the die method

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1.  Add the following lines to the documentation comments describing
    the Fish class (before the @author line).

 *
 *  <p>
 *  Modification History:
 *  - Modified to support a dynamic population in the environment:
 *    fish can now breed and die.
 *

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2.  Add the following lines immediately after the other instance variables
    in Fish.

// THE FOLLOWING TWO INSTANCE VARIABLES ARE NEW IN CHAPTER 3 !!!
    private double probOfBreeding;     // defines likelihood in each timestep
    private double probOfDying;        // defines likelihood in each timestep


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3.  Put the following lines in Fish.initialize after the line that sets myColor.

// THE FOLLOWING CODE IS NEW IN CHAPTER 3 !!!
        // For now, every fish is equally likely to breed or die in any given
        // timestep, although this could be individualized for each fish.
        probOfBreeding = 1.0/7.0;   // 1 in 7 chance in each timestep
        probOfDying = 1.0/5.0;      // 1 in 5 chance in each timestep


-----------------

4.  Replace Fish.act with the following.

// THE FOLLOWING METHOD IS MODIFIED FOR CHAPTER 3 !!!
//       (was originally a check for aliveness and a simple call to move)
    /** Acts for one step in the simulation.
     **/
    public void act()
    {
        // Make sure fish is alive and well in the environment -- fish
        // that have been removed from the environment shouldn't act.
        if ( ! isInEnv() )
            return;

        // Try to breed.
        if ( ! breed() )
            // Did not breed, so try to move.
            move();

        // Determine whether this fish will die in this timestep.
        Random randNumGen = RandNumGenerator.getInstance();
        if ( randNumGen.nextDouble() < probOfDying )
            die();
    }


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5.  Add the following two methods before the move method.

// THE FOLLOWING METHOD IS NEW FOR CHAPTER 3 !!!
    /** Attempts to breed into neighboring locations.
     *  @return    <code>true</code> if fish successfully breeds;
     *             <code>false</code> otherwise
     **/
    protected boolean breed()
    {
        // Determine whether this fish will try to breed in this
        // timestep.  If not, return immediately.
        Random randNumGen = RandNumGenerator.getInstance();
        if ( randNumGen.nextDouble() >= probOfBreeding )
            return false;

        // Get list of neighboring empty locations.
        ArrayList emptyNbrs = emptyNeighbors();
        Debug.print("Fish " + toString() + " attempting to breed.  ");
        Debug.println("Has neighboring locations: " + emptyNbrs.toString());

        // If there is nowhere to breed, then we're done.
        if ( emptyNbrs.size() == 0 )
        {
            Debug.println("  Did not breed.");
            return false;
        }

        // Breed to all of the empty neighboring locations.
        for ( int index = 0; index < emptyNbrs.size(); index++ )
        {
            Location loc = (Location) emptyNbrs.get(index);
            generateChild(loc);
        }

        return true;
    }

// THE FOLLOWING METHOD IS NEW FOR CHAPTER 3 !!!
    /** Creates a new fish with the color of its parent.
     *  @param loc    location of the new fish
     **/
    protected void generateChild(Location loc)
    {
        // Create new fish, which adds itself to the environment.
        Fish child = new Fish(environment(), loc,
                              environment().randomDirection(), color());
        Debug.println("  New Fish created: " + child.toString());
    }


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6.  Add the following method after the changeDirection method.

// THE FOLLOWING METHOD IS NEW FOR CHAPTER 3 !!!
    /** Removes this fish from the environment.
     **/
    protected void die()
    {
        Debug.println(toString() + " about to die.");
        environment().remove(this);
    }