Add up BigDecimals in Map by matching with keys from another Set

Question

I have a Map with sales numbers divided by year:

Map<Integer, BigDecimal> sales_by_year = new TreeMap<>();
sales_by_year.put(2012, BigDecimal.valueOf(19283));
sales_by_year.put(2013, BigDecimal.valueOf(24832));
sales_by_year.put(2014, BigDecimal.valueOf(19562));
sales_by_year.put(2015, BigDecimal.valueOf(21879));
sales_by_year.put(2016, BigDecimal.valueOf(23587));
sales_by_year.put(2017, BigDecimal.valueOf(28756));

and a list of which years I want to add up these sales:

Set<Integer> years = new HashSet<>(Arrays.asList(new Integer[] {2012, 2013, 2014}));

and I want to write a lambda to combine these years into one BigDecimal. I wrote this:

BigDecimal sales_for_timeframe = sales_by_year.entrySet().stream()
        .filter(a -> years.contains(a.getKey()))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))
        .values().stream()
        .reduce(BigDecimal.ZERO, BigDecimal::add);

System.out.println(sales_for_timeframe);

and it works. But is this the most efficient way? This solution:

1. Converts the map into a stream
2. Filters the map
3. Collects it back into a new map
4. Converts the new map's valueSet into a stream
5. Reduces the values to a single BigDecimal

Is there a way to reduce the number of steps? Will doing so increase efficiency? Or is this the best solution?

Answer 1

You could shorten it slightly:

BigDecimal sum = years.stream()
    .map(y -> sales_by_year.getOrDefault(y, BigDecimal.ZERO))  
    .reduce(BigDecimal.ZERO, BigDecimal::add);