public boolean executeBid(int userId, String stockCode, int quantity, double maxPrice)
			throws UserException, StockException, BalanceException {
		User user = users.get(userId);
        if(user==null) {throw new UserException();}
        Stock s = stocks.get(stockCode);
        if(s==null) {throw new StockException();}
        if (user.getBalance()<maxPrice*quantity){throw new BalanceException();}
        user.addToBid(s, quantity);
        List<Stock> selling = users.values().stream().flatMap(p->p.getProposal().entrySet().stream())
        		.filter(p->p.getKey().getCode().equals(stockCode))
        		.sorted(Comparator.comparing(p->p.getKey().getPrice())).map(p->p.getKey()).collect(Collectors.toList());
        		if(selling.size()!=0) {
        selling.get(0).getUser().executeBid(quantity, maxPrice, s);
       user.reduceAmount(maxPrice*quantity);
       user.addStock(s, quantity);
		return true;}
       return false;
	}