public List<Stock> average(Collection<Stock> stocks) {
        // TODO: use functional programming to count average stock price
        // HINT: use collect Collectors.groupingBy
        return stocks.stream()
        .collect(Collectors.groupingBy(Stock::getCode))
        .entrySet().stream()
        .map(entry -> new ImmutablePair<String, Float>(entry.getKey(),
                entry.getValue().stream()
                .map(Stock::getValue)
                .reduce((v1, v2) -> v1 + v2)
                .orElse(0f) / entry.getValue().size()
        ))
        .map(pair -> new Stock(pair.left, pair.right))
        .collect(Collectors.toList());
    }