THE CODE BELOW WILL SOLVE THE PROBLEM OF Your program crashing when a fish is eaten while it is not the current object in use

***************SCROLL DOWN TO SEE USAGE OF THE instanceof OPERATOR****************

add the code below to your Minnow
notice the beginning of the ACT method

    /** Checks whether this fish is in an environment.
     *  @return  <code>true</code> if the fish is in the environment
     *           (and at the correct location); <code>false</code> otherwise
     **/
    public boolean isInEnv()
    {
        return environment().objectAt(location()) == this;
    }

//     YOU ALREADY HAVE AN ACT METHOD... just add that top check to make sure 
// object is in environment  
    /** 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();
    }


//----------------------------------------------------------------------------------
// USES THE "instanceof" OPERATOR
protected boolean eat()
    {
        // Get list of neighboring locations.
        ArrayList nbrs = environment().neighborsOf(location());
        for ( int index = 0; index < nbrs.size(); index++ )
        {
            Location loc = (Location) nbrs.get(index);
            if (! environment().isEmpty(loc) )
            {
                Minnow prey = (Minnow)environment().objectAt(loc);
                if (! (prey instanceof HungryFish))
                {
                    prey.die();
                    changeLocation(loc);
                    itsReserve +=5;
                    return true;
                }
            }
        }
        return false;
    }