Multi-stream: flatMap fans an order list out to its line items

Aggregates live at a grain. flatMap is how you move from the order grain down to the line grain, which is the only level where a revenue sum is correct.

Code

record Line(String sku, int qty, double price) {}
record Order(String ref, List<Line> lines) {}

var orders = List.of(
        new Order("A-1", List.of(new Line("S1", 2, 50.0), new Line("S2", 1, 150.0))),
        new Order("A-2", List.of(new Line("S1", 1, 50.0))));

double revenue = orders.stream()
        .flatMap(o -> o.lines().stream())
        .mapToDouble(l -> l.qty() * l.price())
        .sum();

Map<String, Integer> unitsBySku = orders.stream()
        .flatMap(o -> o.lines().stream())
        .collect(Collectors.groupingBy(Line::sku, TreeMap::new, Collectors.summingInt(Line::qty)));

System.out.println("revenue: " + revenue);
System.out.println("units  : " + unitsBySku);

Output

revenue: 300.0
units  : {S1=3, S2=1}