Multi-stream: running balance across two merged transaction feeds

A running total carries state between elements, which a stream deliberately does not. A one-element array is the usual escape hatch - and it is only correct because this pipeline is sequential and ordered.

Code

record Txn(int at, double delta) {}

var deposits = List.of(new Txn(1, 500.0), new Txn(5, 200.0));
var withdrawals = List.of(new Txn(3, -120.0), new Txn(7, -80.0));

double[] balance = {0.0};   // sequential + ordered, or this is a data race

Stream.concat(deposits.stream(), withdrawals.stream())
      .sorted(Comparator.comparingInt(Txn::at))
      .forEach(t -> {
          balance[0] += t.delta();
          System.out.println(t.at() + "s " + t.delta() + " -> balance " + balance[0]);
      });

Output

1s 500.0 -> balance 500.0
3s -120.0 -> balance 380.0
5s 200.0 -> balance 580.0
7s -80.0 -> balance 500.0