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

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