Multi-stream: correlate three feeds into one order status

Orders, payments and shipments arrive separately and a status is a function of all three. Index the two secondary feeds, then walk the primary one - the state machine stays readable because each lookup is a plain Map get.

Code

record Order(String ref, double amount) {}
record Payment(String ref, double paid) {}
record Shipment(String ref, String carrier) {}

var orders = List.of(new Order("A-1", 250.0), new Order("A-2", 90.0), new Order("A-3", 40.0));
var payments = List.of(new Payment("A-1", 250.0), new Payment("A-2", 45.0));
var shipments = List.of(new Shipment("A-1", "BlueDart"));

Map<String, Double> paid = payments.stream()
        .collect(Collectors.toMap(Payment::ref, Payment::paid));
Map<String, String> shipped = shipments.stream()
        .collect(Collectors.toMap(Shipment::ref, Shipment::carrier));

orders.forEach(o -> {
    double p = paid.getOrDefault(o.ref(), 0.0);
    String state = p == 0.0 ? "UNPAID"
                 : p < o.amount() ? "PART-PAID"
                 : shipped.containsKey(o.ref()) ? "SHIPPED" : "READY";
    System.out.println(o.ref() + " " + state + " paid=" + p + "/" + o.amount());
});

Output

A-1 SHIPPED paid=250.0/250.0
A-2 PART-PAID paid=45.0/90.0
A-3 UNPAID paid=0.0/40.0