ArrayListStockExchange.java

public class ArrayListStockExchange implements StockExchange {

    private List<Trade> trades = new ArrayList<>();

    @Override
    public void order(int ticket, int amount, int price, boolean buy) {
        trades.add(new Trade(ticket, amount, price, buy));
    }

    @Override
    public double dayBalance() {
        double balance = 0;
        for (Trade trade : trades) {
            balance += trade.amount * trade.price * (trade.buy ? 1 : -1);
        }
        return balance;
    }
}