Agent-based simulation example in Java

README.md

This program is a simple simulation of four agents giving each other credits.

This example shows you some of the basic Java concepts such as public & private member variables, passing this as an argument, and the difference between this.variable and variable.

You can compile it using javac Agent.java Simulation.java and run it using java Simulation.

Agent.java

class Agent
{
	// Like a struct, every agent object has a credits and a reference to the simulation
	
	private String name; // Name of the agent

	private int credits; // Number of credits the agent has

	private Simulation sim; // Simulation the agent lives in

	public Agent(Simulation sim, String name, int credits)
	{
		// here I was stupid enough to name the parameters with the
		// same name as the member variables (credits, name and sim)
		// Thus I have to use this, to make sure that Java knows I mean
		// the instance-specific variables.
		this.sim = sim;
		this.name = name;
		this.credits = credits;
	}

	public String getName()
	{
		return this.name;
	}

	public int getCredits()
	{
		return this.credits;
	}

	public void act()
	{
		// Initialize the variable in which I'll store the poorest agent. But for now, I'll initialize it
		// to 'null', meaning empty. (Very similar to a C pointer pointing to location 0.)
		Agent poorestAgent = null;

		// Find the poorest agent
		// Note that I can access the simulation through this.sim here, and access the list of agents
		// because that is a public member variable (thus accessible and editable by anyone)
		for (Agent agent : sim.agents) {
			// First, lets check if the agent is not me (because I'm one of the agents of the simulation!)
			if (agent != this) {
				// Then, if there is no poorest agent yet, or there is one but the current agent
				// is poorer than the currently poorest agent, save that one as the poorest agent
				if (poorestAgent == null || agent.getCredits() < poorestAgent.getCredits()) {
					poorestAgent = agent;
				}
			}
		}

		// Calculate how much I can give away and already remove it from my own count.
		// Because the receiving agent will ask me for how much I have as soon as I give
		// them credits (see the receiveCredits method).
		int creditsToGive = credits / 2;
		credits = credits - creditsToGive;

		// Now (given that there are more agents than only me in the sim.agents array) we can give the
		// agent some of our credits, and tell them it was ours so they can thank us
		// I pass on 'this' as the contributor, because it was I who gave the other agent credits!
		poorestAgent.receiveCredits(this, creditsToGive);
	}

	public void receiveCredits(Agent contributor, int amountOfCredits)
	{
		// Update our amount of credits
		this.credits += amountOfCredits;

		// Print out a thank you!
		System.out.println(this.getName() + " says:");
		System.out.println(contributor.getName() + " just gave me " + Integer.toString(amountOfCredits) + " credits.");
		System.out.println("Now I have " + Integer.toString(this.credits) + " and they have " + Integer.toString(contributor.getCredits()));

		// If I now have more credits than the contributing agent gave me, thank them especially
		if (credits > contributor.getCredits()) {
			System.out.println("I now have more than my contributor! Thanks!");
		}
	}
}

Simulation.java

import java.util.ArrayList;
import java.util.Random;

class Simulation
{
	public ArrayList<Agent> agents;

	public Simulation()
	{
		// This method is called when a new simulation instance is created

		// Create an empty list where we can put agents in
		agents = new ArrayList<Agent>();

		// But that could also be written as:
		this.agents = new ArrayList<Agent>();
	}

	public void step()
	{	
		// Call Agent.act method on every agent in the agents list
		// Again, you could also write this.agents here.
		for (Agent agent : agents) {
			agent.act();
		}
	}

	public void printAgentCredits()
	{
		for (Agent agent : agents) {
			System.out.println(agent.getName() + ":\t" + Integer.toString(agent.getCredits()));
		}
	}

	static public void main(String[] args)
	{
		// This (static) method is called when you run `java Simulation`
		// static means that it is not

		Random random = new Random(); // Source of random numbers

		Simulation sim = new Simulation();
	
		for (int i = 0; i < 4; ++i) {
			String name = "Agent " + Integer.toString(i + 1);
			int credits = random.nextInt(10); // int between 0 and 10
			Agent agent = new Agent(sim, name, credits);
			sim.agents.add(agent);
		}

		// Print the amount of credits all agents have at the start
		sim.printAgentCredits();
		
		// Simulate 3 epochs
		for (int epoch = 0; epoch < 3; ++epoch) {
			System.out.println("----- Epoch " + Integer.toString(epoch + 1) + " -----");
			sim.step();
		}

		// Print the final results
		sim.printAgentCredits();
	}
}