java,tips and tricks,date
Ways of comparing Date Objects in Java
Let's face it, compairing Date objects in Java is complex and as well as error-prone. This is the case in standard code, but this is also the case in testing code, where we regularly need to create Date objects that point at specific instant in time, to be our reference in comparison.
The good ol' deprecated wayIn test code, I've no qualms about using deprecated methods. So, I used the old Date constructor to initialize dates:
Date date = new Date(112, 5, 3);Pro: it's concise. Con: it really isn't intuitive and you need a good knowledge of the Java API to know the first parameter is the year minus 1900 and the second is the month count (beginning at 0, for January). It's almost a surprise to learn the last parameter is simply... the day of the month.
The canonical waySince Java 1.1, Calendar was introduced in the Java API to dissociate between an instant in time (the date) and its view in a specific referential (the calendar). The following snippet is a naive way to obtain the same result as above.
Calendar calendar = Calendar.getInstance(); calendar.set(YEAR, 2012); calendar.set(MONTH, JUNE); calendar.set(DAY_OF_MONTH, 3);It's not only more verbose, it's also a mistake: hours, minutes and the rest are not 0 (depending on the exact creation time of the calendar) so using equals() here will return false. Here's the correct code:
Calendar calendar = Calendar.getInstance(); calendar.set(YEAR, 2012); calendar.set(MONTH, JUNE); calendar.set(DAY_OF_MONTH, 3); calendar.set(HOUR_OF_DAY, 0); calendar.set(MINUTE, 0); calendar.set(SECOND, 0); calendar.set(MILLISECOND, 0);It defeats brevity purpose, to say the least ;-)
Apache Commons LangApache Commons provides since ages different utility libraries that help develop in Java. One of such library is Apache Commons Lang, which aims at providing code that would merit being part of the Java API. In our case, the DateUtils class let us shorten the previous code, while keeping its readibility:
Calendar calendar = Calendar.getInstance(); calendar.set(YEAR, 2012); calendar.set(MONTH, JUNE); calendar.set(DAY_OF_MONTH, 3); calendar = DateUtils.truncate(calendar, DAY_OF_MONTH);Even better, DateUtils let us work directly on Date objects, to make the following alternative also possible:
Date date = new Date(); date = DateUtils.setYears(date, 2012); date = DateUtils.setMonths(date, JUNE); date = DateUtils.setDays(date, 3); date = DateUtils.truncate(date, DAY_OF_MONTH);Note that it leaves parameters untouched, enforcing the immutability dear to Functional Programing proponents. Pro: we use the standard Java API. Con: none. And yet, wouldn't a full-fledged DSL feel somewhat more right?
Joda TimeThe final option is to use Joda Time, which aims at being a replacement for Date and Calendar. It also has spawned the JSR-310"a new and improved date and time API for Java", that should be part of Java 8 (it was originally scheduled for Java 7). A whole article (or even a mini-guide) could be dedicated to Joda Time. For our present concern, the following snippet can advantageously replace our original one:
DateMidnight dm = new DateMidnight(2012, 6, 3);Back to square one, it seems: clear and concise. And yet, the parameters self explanatory, no real need to regularly check the JavaDocs to see how to initialize the year. Besides, the semantics of the class name are clear. Finally, the toDate() method let us bridge to the standard Java API.