Lab 9: Predator-Prey Simulation

Pre-lab Reading

In lab this week you and your partner (or partners) will be experimenting with a biological simulator. This is an exercise in using subtype polymorphism. It help you make a quicker start during the lab if you take a little bit of time to think about how the simulation is structured before you come to lab.

1. Introduction

You will be working with various implementations of the Agent interface. The subclasses of Agent represent the various species occupying the simulated world, which is represented by the SimulationGrid class. As the name suggests, the world is modelled as a rectangular grid of cells, where each cell can be occupied by at most one Agent.

On each tick of the clock, the simulation calls the act() method for each agent in the grid. That agent can examine the world around it (via methods on the grid), update its state, and, if appropriate, move to another cell.

This is similar to the ball-catching project. In that program, you are writing the Player class, which corresponds to an agent in this simulation. The constructor for a Player was passed its coordinates in the grid and the GridInfo for the grid it was in. The relationship between various agents and the grid here is similar: an agent can examine locations in the by calling three methods on SimulationGrid:

• boolean isOccupied(int xPos, int yPos)
• boolean isOpen(int xPos, int yPos)
• Agent getAgentAt(int xPos, int yPos)

Conveniently, these methods all do the right thing when passed a position that is outside the grid: both isOccupied() and isOpen() return false, and getAgentAt() returns null.

In the ball-catching game, the Player never updated the grid directly; the only way that it could interact was to return a new position to move to, and the simulation took care of everything else. In this program, an agent must update the grid using the method

• void setAgentAt(int xPos, int yPos, Agent a)

Movement requires calling this method twice: once to clear the cell where the agent previously resided (by setting the agent there to null) and then a second to occupy its new location. Because the grid allows only one agent to occupy a cell, setting the agent in a cell results in the demise of any agent that previously occupied that location.

2. What you start with

Your simulation starts out with three agent classes, implementing behaviors of Clover, Rabbit, and Fox. Even before you look at the code, you can imagine the strategies of each:

• Clover doesn't move, but after growing for a few turns will attempt to reproduce by creating a new instance in an adjacent cell that is open.
• A rabbit will try to move toward clover and away from foxes. When it moves onto a clover, it eats it and grows heavier. When it gets heavy enough, it will attempt to reproduce into an adjacent cell.
• A fox will try to catch a rabbit, so that it can eat it and (eventually) reproduce.

Before you look at the code, you might want to think about how you might write each of these behaviors. When you come to lab, you'll have a chance to see how they have been implemented.

3. During the lab

When you come for the lab, remember to bring your printed copy of the agent classes. You and your partner(s) will do several things:

1. You will need to read the code for the various agents to understand how each implements its strategy (and to understand the fine points of that implementation).
2. You should run a series of experiments with various initial populations to see how they turn out. (Do any species go extinct? Are there any peculiar behaviors that need to be explained?)
3. You will make some changes to the simulation. For example, you might add one or more new species to the simulation. You also might do something like impose a maximum lifetime on a particular species, or you could model effort (movement) as reducing weight, so that a particular instance could die of starvation. This is a chance for some imagination.