laysakura opened a new issue, #24510:
URL: https://github.com/apache/beam/issues/24510
### What would you like to happen?
## Example dataset
Per-hour weather data.
```json
{"timestamp": "2022-08-01T00:00:00.000+00:00", "temperature [°C]": 0.0,
"rainfall [mm]": 0.0}
{"timestamp": "2022-08-01T01:00:00.000+00:00", "temperature [°C]": 0.0,
"rainfall [mm]": 0.0}
{"timestamp": "2022-08-01T02:00:00.000+00:00", "temperature [°C]": 0.0,
"rainfall [mm]": 0.0}
```
## Example code
```java
// 1-day session window
PCollection<Float> windowedTemperature = temperatureOver30.apply(
Window.<Float>into(
Sessions.withGapDuration(Duration.standardDays(1))));
PCollection<KV<Instant, Float>> windowedTemperatureWithDate =
windowedTemperature.apply(
ParDo.of(new DoFn<Float, KV<Instant, Float>>() {
@ProcessElement
public void processElement(
@Element Float temperature,
IntervalWindow window,
OutputReceiver<KV<Instant, Float>> out) {
Instant winEnd = window.maxTimestamp();
System.out.println(window); // PRINT HERE
out.output(KV.of(winEnd, temperature));
}
}));
```
See the full codes:
<https://gist.github.com/laysakura/175cf1137c8cb3356236ed7ab2443b71>
## Expected outputs from `println()`
```text
[2022-08-01T00:00:00.000Z..2022-08-02T02:00:00.000Z)
```
## Actual outputs from `println()`
```text
[2022-08-01T00:00:00.000Z..2022-08-02T00:00:00.000Z)
[2022-08-01T01:00:00.000Z..2022-08-02T01:00:00.000Z)
[2022-08-01T02:00:00.000Z..2022-08-02T02:00:00.000Z)
```
## Problem hypothesis and solution
`assignWindows()` has control to `IntervalWindow`:
<https://github.com/apache/beam/blob/ec91ea4f6b83e6a8e3bd2041057fa90d0c8f0dd0/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/windowing/WindowFn.java#L59-L60>
but `mergeWindows()` doesn't:
<https://github.com/apache/beam/blob/ec91ea4f6b83e6a8e3bd2041057fa90d0c8f0dd0/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/windowing/WindowFn.java#L88>.
So `mergeWindoews()` might have to modify `IntervalWindow.start` and
`IntervalWindow.end` after merges of windows happen.
### Issue Priority
Priority: 2
### Issue Component
Component: sdk-java-core
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