http://codegolf.stackexchange.com/q/36515/194

Controller.java

import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class Controller {
	private static final File workingDir = new File("resources/players").getAbsoluteFile();

	public static final char BUY = 'B';
	public static final char SELL = 'S';

	public static void main(String[] args){
		try {
			BufferedReader br1 = new BufferedReader(new InputStreamReader(new FileInputStream("resources/config")));

			int numRounds = Integer.parseInt(br1.readLine());
			int turnsPerRound = Integer.parseInt(br1.readLine());

			List<Player> players = new ArrayList<Player>();
			Map<Player, BigInteger> totals = new HashMap<Player, BigInteger>();
			String line1 = null;
			while ((line1 = br1.readLine()) != null) {
				Player p = new Player(line1);
				players.add(p);
				totals.put(p, BigInteger.ZERO);
			}
			br1.close();

			//Begin processing
			for(int round = 0; round < numRounds; round++){
				//Create players' shares and currency array
				Map<Player, Possessions> vals = new HashMap<Player, Possessions>();
				for (Player player : players) vals.put(player, new Possessions());

				BigInteger marketValue = BigInteger.valueOf(getRandInt(10,150));
				for (int turn = 0; turn < turnsPerRound; turn++){
					BigInteger newValue = marketValue;
					for (Player player : players) {
						Possessions position = vals.get(player);

						String line = "Err";
						try {
							Process proc = player.run(marketValue, position);
							BufferedReader br = new BufferedReader(new InputStreamReader(proc.getInputStream()));
							line = br.readLine();
							br.close();
							proc.destroy();

							switch (line == null ? '\u0000' : line.charAt(0)) {
								case BUY: {
									BigInteger bought = new BigInteger(line.substring(1).trim());
									if (bought.compareTo(BigInteger.ZERO) < 0) throw new IllegalArgumentException("Can't buy negative shares");
									BigInteger cost = bought.multiply(marketValue);
									if (cost.compareTo(position.cash) <= 0) {
										position.cash = position.cash.subtract(cost);
										position.shares = position.shares.add(bought);

										newValue = newValue.add(bought.multiply(BigInteger.valueOf(getRandInt(0,2))));
										if (newValue.compareTo(BigInteger.valueOf(Integer.MAX_VALUE)) > 0) newValue = BigInteger.valueOf(Integer.MAX_VALUE - 1);
									}
									break;
								}
								case SELL: {
									BigInteger sold = new BigInteger(line.substring(1).trim());
									if (sold.compareTo(BigInteger.ZERO) < 0) throw new IllegalArgumentException("Can't sell negative shares");
									if (sold.compareTo(position.shares) <= 0) {
										position.cash = position.cash.add(sold.multiply(marketValue));
										position.shares = position.shares.subtract(sold);

										newValue = newValue.subtract(sold.multiply(BigInteger.valueOf(getRandInt(5,10))));
										if (newValue.compareTo(BigInteger.ONE) < 0) newValue = BigInteger.ONE;
									}
									break;
								}
								default:
									break;
							}
						} catch (Exception e) {
							System.err.println("[" + player + "] threw error:");
							e.printStackTrace();
						}
					}

					marketValue = newValue;
					System.out.println("Turn " + turn + " over: " + marketValue);
				}
				System.out.println("End of round market value is: " + marketValue);
				for (Player player : players) {
					Possessions position = vals.get(player);
					totals.put(player, totals.get(player).add(position.cash).add(position.shares.multiply(marketValue)));
				}
			}
			for (Player player : players) {
				System.out.println(player + ":\t$" + totals.get(player).divide(BigInteger.valueOf(numRounds)));
			}
		} catch (Exception e) {
			System.err.println("An exception occured while running the controller.");
			e.printStackTrace();
		}
	}
	public static Random r = new Random(new Date().getTime());
	public static int getRandInt(int min, int max){
		return r.nextInt(max - min) + min;
	}

	private static class Possessions {
		public BigInteger cash;
		public BigInteger shares;

		public Possessions() {
			cash = BigInteger.valueOf(5000);
			shares = BigInteger.valueOf(getRandInt(20, 30)); // TODO Fix this random bias
		}
	}

	private static class Player {
		private List<String> cmd;

		public Player(String configLine) {
			String[] parts = configLine.split("&");
			cmd = new ArrayList<String>();
			for (int i = 0; i < parts.length; i++) cmd.add(parts[i]);
		}

		public Process run(BigInteger stockPrice, Possessions position) throws IOException {
			List<String> fullCmd = new ArrayList<String>(cmd);
			fullCmd.add(stockPrice.toString());
			fullCmd.add(position.cash.toString());
			fullCmd.add(position.shares.toString());
			return new ProcessBuilder(fullCmd).directory(workingDir).start();
		}

		@Override
		public String toString() {
			return cmd.get(cmd.size() - 1);
		}
	}
}