Multi-stream: count matched and unmatched in a single pass

partitioningBy always returns both keys, so a reconciliation summary can never blow up on an empty side - and a downstream collector shapes each half without a second traversal.

Code

record Txn(String id, String account) {}

var txns = List.of(new Txn("t1", "AC1"), new Txn("t2", "AC9"),
                   new Txn("t3", "AC2"), new Txn("t4", "AC9"));
var known = Set.of("AC1", "AC2");

Map<Boolean, List<String>> split = txns.stream()
        .collect(Collectors.partitioningBy(t -> known.contains(t.account()),
                                           Collectors.mapping(Txn::id, Collectors.toList())));

System.out.println("matched  : " + split.get(true));
System.out.println("unmatched: " + split.get(false));

Output

matched  : [t1, t3]
unmatched: [t2, t4]