Multi-stream: demultiplex one feed into per-handler streams

The inverse of merging. groupingBy on the discriminator splits one mixed feed into the lists each downstream handler wants, and the group count tells you how many handlers were actually needed.

Code

record Event(String type, String id) {}

var events = List.of(new Event("order", "o1"), new Event("refund", "r1"),
                     new Event("order", "o2"), new Event("audit", "a1"),
                     new Event("refund", "r2"));

Map<String, List<String>> routed = events.stream()
        .collect(Collectors.groupingBy(Event::type, TreeMap::new,
                 Collectors.mapping(Event::id, Collectors.toList())));

routed.forEach((type, ids) -> System.out.println(type + " -> " + ids));
System.out.println("routed " + events.size() + " events to " + routed.size() + " handlers");

Output

audit -> [a1]
order -> [o1, o2]
refund -> [r1, r2]
routed 5 events to 3 handlers