Multi-stream: roll line items straight up to the customer grain

Two hops - line to order, order to customer - do not need two passes. Join on the way into groupingBy and the classifier does the hop for you.

Code

record Order(String ref, String customer) {}
record Line(String orderRef, String sku, double amount) {}

var orders = List.of(new Order("A-1", "Ava"), new Order("A-2", "Ben"), new Order("A-3", "Ava"));
var lines = List.of(new Line("A-1", "S1", 100.0), new Line("A-1", "S2", 150.0),
                    new Line("A-2", "S1", 50.0), new Line("A-3", "S3", 40.0));

Map<String, String> customerOf = orders.stream()
        .collect(Collectors.toMap(Order::ref, Order::customer));

Map<String, Double> byCustomer = lines.stream()
        .collect(Collectors.groupingBy(l -> customerOf.get(l.orderRef()),
                                       TreeMap::new,
                                       Collectors.summingDouble(Line::amount)));

System.out.println(byCustomer);

Output

{Ava=290.0, Ben=50.0}