Multi-stream: format a joined report as an aligned table

The last step of a multi-list pipeline is usually a human reading it. printf with explicit widths and Locale.ROOT keeps the columns lined up and the decimal separator stable wherever the JVM runs.

Code

record Region(String code, String name) {}
record Sale(String regionCode, double amount) {}

var regions = List.of(new Region("E", "East"), new Region("W", "West"), new Region("N", "North"));
var sales = List.of(new Sale("E", 1200.0), new Sale("W", 850.5), new Sale("E", 300.0));

Map<String, Double> totals = sales.stream()
        .collect(Collectors.groupingBy(Sale::regionCode, TreeMap::new,
                                       Collectors.summingDouble(Sale::amount)));

System.out.printf(Locale.ROOT, "%-8s %10s%n", "REGION", "TOTAL");
regions.stream()
       .sorted(Comparator.comparing(Region::name))
       .forEach(r -> System.out.printf(Locale.ROOT, "%-8s %10.2f%n",
               r.name(), totals.getOrDefault(r.code(), 0.0)));

Output

REGION        TOTAL
East        1500.00
North          0.00
West         850.50