java 21 lines · 5 steps

Backfilling gaps in a daily time series in Java

Turn a sparse list of dated samples into a continuous day-by-day series by synthesizing entries for every missing date.

Explained by highlit

Intermediate streams time-series treemap data-cleaning gap-filling

1public List<DailySample> backfillMissingDates(List<DailySample> samples, double fillValue) {
2    if (samples.isEmpty()) {
3        return List.of();
4    }
5 
6    Map<LocalDate, DailySample> byDate = samples.stream()
7            .collect(Collectors.toMap(
8                    DailySample::date,
9                    Function.identity(),
10                    (a, b) -> b,
11                    TreeMap::new));
12 
13    LocalDate start = ((TreeMap<LocalDate, DailySample>) byDate).firstKey();
14    LocalDate end = ((TreeMap<LocalDate, DailySample>) byDate).lastKey();
15 
16    return start.datesUntil(end.plusDays(1))
17            .map(date -> byDate.getOrDefault(date, new DailySample(date, fillValue)))
18            .collect(Collectors.toList());
19}
20 
21record DailySample(LocalDate date, double value) {}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1A sorted map lets you derive the full date range and do O(1) lookups in one structure.
2datesUntil generates a continuous sequence so you can materialize entries that never existed in the input.
3Guarding the empty case up front keeps later range calculations from throwing on missing keys.

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