与等效的Calendar代码相比，结果java.time代码具有更多的代码语句吗？

Question

Recently, I try to port one of our old code base, from Calendar to java.time , as we need quite a number of arithmetic functionalities, which is only found in java.time .

If we use Calendar in our current code base, we need to perform a lot of conversion back-and-forth (From Calendar to Instant , From Instant back to Calendar ), in the middle of our code.

To avoid such cumbersome conversion, we decide to eliminate usage of Calendar , and port them to equivalent java.time code.

I'm a bit skeptical on my port. As compared with Calendar code, it seems to

• Create more temporary object instances within the while loop.
• Requires more code statements.

Calendar code

// reminderCal is Calendar object.

long startTimestamp = getStartTimestamp();
reminderCal.setTimeInMillis(startTimestamp);

while (startTimestamp <= maxTimestamp) {
    resultList.add(startTimestamp);

    reminderCal.add(Calendar.DAY_OF_MONTH, 1);

    startTimestamp = reminderCal.getTimeInMillis();
}

return resultList;

java.time code

// Epoch timestamp loopTs as initial input.

long startTimestamp = getStartTimestamp();
final ZoneId zoneId = ZoneId.systemDefault();

while (startTimestamp <= maxTimestamp) {
    resultList.add(startTimestamp);

    // More code, more temporary instances required compared
    // with Calendar's version. Not sure we're doing the right
    // way.
    Instant instant = Instant.ofEpochMilli(startTimestamp);
    LocalDateTime time = LocalDateTime.ofInstant(instant, zoneId);
    time = time.plus(1, ChronoUnit.DAYS);

    startTimestamp = time.atZone(zoneId).toInstant().toEpochMilli();
}

return resultList;

---

For the above code, I was wondering, are we doing the port correctly and optimized? Is there any room we can improve in our java.time 's port?

Answer 1

Since you want date manipulations between times in a given time zone, you shouldn't use milliseconds nor LocalDateTime, but ZonedDateTime. And I would argue that your List should contain Instants instead of longs, but let's keep it that way for now:

long startTimestamp = getStartTimestamp();
ZoneId zoneId = ZoneId.systemDefault();

ZonedDateTime maxDateTime = Instant.ofEpochMilli(maxTimestamp).atZone(zoneId);
ZonedDateTime loopDateTime = Instant.ofEpochMilli(loopTs).atZone(zoneId);

while (!loopDateTime.isAfter(maxDateTime)) {
    tsList.add(loopDateTime.toInstant().toEpochMilli());
    loopDateTime = loopDateTime.plusDays(1);
}

This is more concise, but also more readable. And all the Instant.ofEpochMilli() and toEpochMilli() calls wouldn't be needed if you wroked with Instants instead of longs.