Multi-stream: join two lists on a key with a Map index

The building block of every multi-list pipeline. Index the smaller list once into a Map, then stream the larger one and look each row up - O(n+m) instead of the O(n*m) a nested filter costs.

Code

record Customer(int id, String name, String city) {}
record Order(String ref, int customerId, double amount) {}

var customers = List.of(new Customer(1, "Ava", "Pune"),
                        new Customer(2, "Ben", "Kochi"),
                        new Customer(3, "Cara", "Delhi"));
var orders = List.of(new Order("A-1", 1, 250.0),
                     new Order("A-2", 3, 90.0),
                     new Order("A-3", 1, 40.0));

// Build the index ONCE, outside the pipeline.
Map<Integer, Customer> byId = customers.stream()
        .collect(Collectors.toMap(Customer::id, c -> c));

orders.stream()
      .map(o -> byId.get(o.customerId()).name() + " " + o.ref() + " " + o.amount())
      .forEach(System.out::println);

Output

Ava A-1 250.0
Cara A-2 90.0
Ava A-3 40.0