Java Stream how to replace first item that meets criteria with the next item that meets criteria

Question

I have a list of schedules that I need to update when one expires with the next one of the same name so I can maintain the order. Example:

List<Schedule> scheduleList = new ArrayList<>(List.of(
        new SchedA("a", 1),
        new SchedB("b", 2),
        new SchedC("c", 3),
        new SchedA("a", 4),
        new SchedB("b", 5),
        new SchedC("c", 6),
        new SchedA("a", 7),
        new SchedB("b", 8),
        new SchedC("c", 3),));

I think I could do this to remove the expired item:

scheduleList.stream().filter(p -> p.getScheduleId().equals("a")).findFirst().ifPresent(scheduleList::remove);

Is there something like this to replace it with the next item that has "a" as the scheduleId?

Answer 1

You should consider changing the data structure. A Map of Lists is better suited for this use case.

Map<String, Schedule> map = new HashMap<>();
map.put("a", List.of(new SchedA(1), new SchedA(4), new SchedA(7)));
map.put("b", List.of(new SchedB(2), new SchedB(5), new SchedB(8)));
map.put("c", List.of(new SchedC(3), new SchedC(6), new SchedC(9)));

Now, if you want to remove, all you have to do is:

map.get("a").remove(0);

And getting the next item would be:

map.get("a").get(0);

On a side note, it also seems redundant to have the parameter a in new SchedA("a", 1) as you already have different SchedA subclasses anyways.

---

If you really want to remove and replace, then streams won't be the ideal implementation:

int first = -1;
int second = -1;
for (int i = 0; i < scheduleList.size(); i++) {
    Schedule schedule = scheduleList.get(0);
    if (schedule.getScheduleId().equals("a")) {
        if (first == -1) {
            first = i;
        } else if (second == -1) {
            second = i;
            break;    // found first two items
        }
    }
}

if (first != -1 && second != -1) {
    // found two items
    scheduleList.set(first, scheduleList.get(second));
} else (first != -1) {
    // found one item only
    scheduleList.remove(first);
} else {
    // no items found, do nothing
}