Learn modernJava, not 2005’s.Real JUnit tests. Spring Boot. No videos.

// Sum only the settled payments. The grader runs real JUnit 5
// tests against every branch — including the ones tutorials skip.
sealed interface Payment permits Settled, Pending, Failed {}
record Settled(long cents) implements Payment {}
record Pending(long cents) implements Payment {}
record Failed(String reason) implements Payment {}

static long settledTotal(List<Payment> payments) {
    return payments.stream()
        .mapToLong(p -> switch (p) {
            case Settled s -> s.cents();
            case Pending pn -> 0;
            case Failed f -> 0;
        })
        .sum();
}