I am reviewing some code at work and came across an inconsistency in how the code handles adding 1 week to the current time and was wondering if there was any reason why one should really be preferred over the other:

The first was a utility method:

public static Date addDaysToDate(final Date date, int noOfDays) {
    Date newDate = new Date(date.getTime());

    GregorianCalendar calendar = new GregorianCalendar();
    calendar.setTime(newDate);
    calendar.add(Calendar.DATE, noOfDays);
    newDate.setTime(calendar.getTime().getTime());

    return newDate;
}

And the second used simple millisecond arithmetic:

long theFuture = System.currentTimeMillis() + (86400 * 7 * 1000);
Date nextWe开发者_开发知识库ek = new Date(theFuture);

The second method obviously uses 'magic numbers' to define a week, but this could be moved to a constant MILLISECONDS_IN_ONE_WEEK = 86400 * 7 * 1000 So other than that, is there any reasons why one of these methods should be preferred over the other?

Basically I want to change the code to be consistent throughout, but I'm not entirely sure which one to remove. So any arguments one way or the other would be useful.

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The two methods will behave differently on daylight savings boundaries. The first method will continue returning the same time of the day, regardless of daylight savings status. The second method will return times which vary an hour in each direction as daylight savings time starts and stops.

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The below can be done in java 8, Java 8 rocks !!

public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println("Current date: " + today);

        //add 1 week to the current date
        LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
        System.out.println("Next week: " + nextWeek);
    }

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Be very careful about using method two which caught me out the other day. Consider

private static long ONE_YEAR_AS_MILLISECONDS = 365*24*60*60*1000;

This looks innocent enough, but in fact will not produce what is expected as the multiplication uses integers which, when multiplied by each the other integers, causes a numeric overflow and will yield an unexpected result. This is because the max int value in Java is 2,147,483,647 and yet there are 31,536,000,000 ms in a year. On my machine the above code produces 1,471,228,928 which is obviously not correct.

Instead you need to do this:

private static long ONE_YEAR_AS_MILLISECONDS = 365L*24L*60L*60L*1000L;

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First and foremost, I would argue that you replace it with JodaTime. http://joda-time.sourceforge.net/ It is a very nice time library. You'll want to look at this page to see how easy it is to add days or weeks to a particular point in time: http://joda-time.sourceforge.net/key_period.html Can't do this, mobile device with incompatible JVM. Bummer.

Your first example is easier to read and will be easier to use by your developers. It also uses the Calendar classes which is the generally accepted way to manipulate dates in Java. What makes it better is that it has a clear method name that sets the expectation for what it does.

So if you refactor your system to consistently use com.DaveJ.util.date.DateUtils.addDaysToDate(final Date date, int noOfDays) you can then do whatever you want inside that method, be it Calendar or millis or Joda, and be consistent within your application. Don't forget to write some unit tests for it!

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The more general, the more useful it will be. If you always add a week, I'd chose the second method. If you have different adds, I'd chose the first one, perhaps replacing days for seconds or even millis if needed.

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The first will be slower, so if performance is an issue, the second one is better.

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It depends! Because of leap seconds, DST and other calendar oddities, the two are not always equivalent.

For business and every day use, always use the first method and the performance are not bad at all. It will handle those things for you.

For scientific needs, often you have to use a monotone clock (here the second one).

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The two methods might give different results when a change in daylight saving time is involved. Imagine the current time is 23:50 and at 02:00 the clock jumps to 03:00. When you just add 7 days in milliseconds the time would be 00:50 on the following day. Addings 7 days, the resulting time would still be 23:50.

To make the confusion complete, you could also try add(Calendar.WEEK_OF_YEAR, 1), not sure how that would differ though.

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Since you're using it on the mobile device, the first method is preferable. The reason is that your code should be independent of specific calendar, DST and other problems, such as seconds overrun(leap seconds).

You have to remove dependence on GregorianCalendar and create it using Calendar.getInstance().

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For a date-only value, see the Answer by javaHelper. The LocalDate class purposely has no time-of-day and no time zone.

ZonedDateTime

For a date-time value, use ZonedDateTime to represent a date and a time-of-day along with a time zone to account for anomalies such as Daylight Saving Time (DST).

ZoneId zoneId = ZoneId.of( "America/Montreal" );
ZonedDateTime now = ZonedDateTime.now( zoneId );
ZonedDateTime weekLater = now.plusWeeks( 1 );

Or add some arbitrary number of days.

ZonedDateTime later = now.plusDays( someNumberOfDays );

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the old troublesome date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP.

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time.