Defining a day period – Working with Date and Time

63. Defining a day period

Let’s imagine that we want to say hello to a friend from another country (different time zone) via a message such as Good morning, Good afternoon, and so on based on his local time. So, having access to AM/PM flags is not enough, because we consider that a day (24 hours) can be represented by the following periods:

21:00 PM – 6:00 AM – night

6:00 AM – 12:00 AM – morning

12:00 AM – 18:00 PM – afternoon

18:00 PM – 21:00 PM – evening

Before JDK 16

Next, we have to obtain the time corresponding to our friend’s time zone. For this, we can start from our local time given as a java.util.Date, java.time.LocalTime, and so on. If we start from a java.util.Date then we can obtain the time in our friend’s time zone as follows:

LocalTime lt = date.toInstant().atZone(zoneId).toLocalTime();

Here, date is a new Date() and zoneId is java.time.ZoneId. Of course, we can pass the zone ID as a String and use the ZoneId.of(String zoneId) method to get the ZoneId instance.If we prefer to start from LocalTime.now() then we can obtain the time in our friend’s time zone as follows:

LocalTime lt = LocalTime.now(zoneId);

Next, we can define the day periods as a bunch of LocalTime and add some conditions to determine the current period. The following code exemplifies this statement:

public static String toDayPeriod(Date date, ZoneId zoneId) {
 LocalTime lt = date.toInstant().atZone(zoneId).toLocalTime();
 LocalTime night = LocalTime.of(21, 0, 0);
 LocalTime morning = LocalTime.of(6, 0, 0);
 LocalTime afternoon = LocalTime.of(12, 0, 0);
 LocalTime evening = LocalTime.of(18, 0, 0);
 LocalTime almostMidnight = LocalTime.of(23, 59, 59);
 LocalTime midnight = LocalTime.of(0, 0, 0);
 if((lt.isAfter(night) && lt.isBefore(almostMidnight))
  || lt.isAfter(midnight) && (lt.isBefore(morning))) {
   return “night”;
  } else if(lt.isAfter(morning) && lt.isBefore(afternoon)) {
   return “morning”;
  } else if(lt.isAfter(afternoon) && lt.isBefore(evening)) {
   return “afternoon”;
  } else if(lt.isAfter(evening) && lt.isBefore(night)) {
   return “evening”;
  }
  return “day”;
}

Now, let’s see how we can do this in JDK 16+.

JDK 16+

Starting with JDK 16+, we can go beyond AM/PM flags via the following strings: in the morning, in the afternoon, in the evening, and at night.These friendly outputs are available via the new pattern, B. This pattern is available starting with JDK 16+ via DateTimeFormatter and DateTimeFormatterBuilder (you should be familiar with these APIs from Chapter 1, Problem 15).So, the following code uses the DateTimeFormatter to exemplify the usage of pattern B, representing a period of the day:

public static String toDayPeriod(Date date, ZoneId zoneId) {
 ZonedDateTime zdt = date.toInstant().atZone(zoneId);
 DateTimeFormatter formatter
    = DateTimeFormatter.ofPattern(“yyyy-MMM-dd [B]”);
 return zdt.withZoneSameInstant(zoneId).format(formatter);
}

Here is an output for Australia/Melbourne:

2023-Feb-04 at night

You can see more examples in the bundled code. Feel free to challenge yourself to adjust this code to reproduce the result from the first example.