Database DateTime milli and nano seconds are truncated by default if they are 0s, while using it in Java 11 using ZonedDateTime

Question

I am fetching datetime from an Oracle database and parsing in Java 11 using ZonedDateTime as below:

1. Oracle --> 1/19/2020 06:09:46.038631 PM

   Java ZonedDateTime output --> 2020-01-19T18:09:46.038631Z[UTC]

2. Oracle --> 1/19/2011 4:00:00.000000 AM

   Java ZonedDateTime output --> 2011-01-19T04:00Z[UTC] (So, here the 0s are truncated by default. However, my requirement is to have consistent fixed length output like #1.)

Expected Java ZonedDateTime output --> 2011-01-19T04:00:00.000000Z[UTC]

However, I didn’t find any date API methods to achieve above expected output. Instead of manipulating a string, is there a way to preserve the trailing 0s with fixed length?

We have consistent ZonedDateTime types in the application, so we do not prefer to change that.

Answer 1

We have consistent ZonedDateTime type in application, so we do not prefer to change that.

Why do you think 2011-01-19T04:00Z[UTC] is inconsistent? A date-time object is supposed to hold (and provide methods/functions to operate with) only the date, time, and time-zone information. It is not supposed to store any formatting information; otherwise, it will violate the Single-responsibility principle. The formatting should be handled by a formating class e.g. DateTimeFormatter (for modern date-time API), DateFormat (for legacy java.util date-time API) etc.

Every class is supposed to override the toString() function; otherwise, Object#toString will be returned when its object will be printed. A ZonedDateTime has date, time and time-zone information. Given below is how its toString() for time-part has been implemented:

@Override
public String toString() {
    StringBuilder buf = new StringBuilder(18);
    int hourValue = hour;
    int minuteValue = minute;
    int secondValue = second;
    int nanoValue = nano;
    buf.append(hourValue < 10 ? "0" : "").append(hourValue)
        .append(minuteValue < 10 ? ":0" : ":").append(minuteValue);
    if (secondValue > 0 || nanoValue > 0) {
        buf.append(secondValue < 10 ? ":0" : ":").append(secondValue);
        if (nanoValue > 0) {
            buf.append('.');
            if (nanoValue % 1000_000 == 0) {
                buf.append(Integer.toString((nanoValue / 1000_000) + 1000).substring(1));
            } else if (nanoValue % 1000 == 0) {
                buf.append(Integer.toString((nanoValue / 1000) + 1000_000).substring(1));
            } else {
                buf.append(Integer.toString((nanoValue) + 1000_000_000).substring(1));
            }
        }
    }
    return buf.toString();
}

As you can see, the second and nano parts are included in the returned string only when they are greater than 0. It means that you need to use a formatting class if you want these (second and nano) zeros in the output string. Given below is an example:

    import java.time.LocalDateTime;
    import java.time.ZoneOffset;
    import java.time.ZonedDateTime;
    import java.time.format.DateTimeFormatter;
    import java.time.format.DateTimeFormatterBuilder;
    import java.util.Locale;

    public class Main {
        public static void main(String[] args) {
            String input = "1/19/2011 4:00:00.000000 AM";

            // Formatter for input string
            DateTimeFormatter inputFormatter = new DateTimeFormatterBuilder()
                                                .parseCaseInsensitive()
                                                .appendPattern("M/d/u H:m:s.n a")
                                                .toFormatter(Locale.ENGLISH);

            ZonedDateTime zdt = LocalDateTime.parse(input, inputFormatter).atZone(ZoneOffset.UTC);

            // Print `zdt` in default format i.e. the string returned by `zdt.toString()`
            System.out.println(zdt);

            // Formatter for input string
            DateTimeFormatter outputFormatter = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.nnnnnnz");
            String output = zdt.format(outputFormatter);
            System.out.println(output);
        }
    }

Output:

2011-01-19T04:00Z
2011-01-19T04:00:00.000000Z

Food for thought:

public class Main {
    public static void main(String[] args) {
        double d = 5.0000;
        System.out.println(d);
    }
}

What output do you expect from the code given above? Does 5.0 represent a value different from 5.0000? How will you print 5.0000? [Hint: Check String#format, NumberFormat, BigDecimal etc.]