Add 1 Week to a Date, which way is preferred?

Question

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

Answer 1

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.

Answer 2

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;

Answer 3

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.

Answer 4

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).

Answer 5

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);
    }

Answer 6

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!