Java Date vs Calendar

Question

Could someone please advise the current  best practice  around Date and Calendar types   When writing new code  is it best to always favour Calendar over Date  or are there circumstances where Date is the more appropriate datatype

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With Java 8  the new java time package should be used   Objects are immutable  time zones and day light saving are taken into account   You can create a ZonedDateTime object from an old java util Date object like this       Date date   new Date        ZonedDateTime zonedDateTime   date toInstant   atZone ZoneId systemDefault

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Date and Calendar are really the same fundamental concept  both represent an instant in time and are wrappers around an underlying long value    One could argue that Calendar is actually even more broken than Date is  as it seems to offer concrete facts about things like day of the week and time of day  whereas if you change its timeZone property  the concrete turns into blancmange  Neither objects are really useful as a store of year-month-day or time-of-day for this reason  Use Calendar only as a calculator which  when given Date and TimeZone objects  will do calculations for you  Avoid its use for property typing in an application  Use SimpleDateFormat together with TimeZone and Date to generate display Strings  If you re feeling adventurous use Joda-Time  although it is unnecessarily complicated IMHO and is soon to be superceded by the JSR-310 date API in any event  I have answered before that it is not difficult to roll your own YearMonthDay class  which uses Calendar under the hood for date calculations  I was downvoted for the suggestion but I still believe it is a valid one because Joda-Time  and JSR-310  are really so over-complicated for most use-cases

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Date should be re-developed  Instead of being a long interger  it should hold year  month  date  hour  minute  second  as separate fields  It might be even good to store the calendar and time zone this date is associated with   In our natural conversation  if setup an appointment at Nov  1  2013 1pm NY Time  this is a DateTime  It is NOT a Calendar  So we should be able to converse like this in Java as well   When Date is stored as a long integer  of mili seconds since Jan 1 1970 or something   calculating its current date depends on the calendar  Different calendars will give different date  This is from the prospective of giving an absolute time  eg 1 trillion seconds after Big Bang   But often we also need a convenient way of conversation  like an object encapsulating year  month etc   I wonder if there are new advances in Java to reconcile these 2 objectives  Maybe my java knowledge is too old

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tl dr     advise the current  best practice  around Date and Calendar      is it best to always favour Calendar over Date   Avoid these legacy classes entirely  Use java time classes instead    For a moment in UTC  use Instant the modern equivalent of Date  For a moment in a particular time zone  use ZonedDateTime the modern equivalent of GregorianCalendar  For a moment in a particular offset-from-UTC  use OffsetDateTime no equivalent in legacy classes  For a date-time  not a moment  with unknown time zone or offset  use LocalDateTime no equivalent in legacy classes      Details  The Answer by Ortomala Lokni is right to suggest using the modern java time classes rather than the troublesome old legacy date-time classes  Date  Calendar  etc    But that Answer suggests the wrong class as equivalent  see my comment on that Answer     Using java time  The java time classes are a vast improvement over the legacy date-time classes  night-and-day difference  The old classes are poorly-designed  confusing  and troublesome  You should avoid the old classes whenever possible  But when you need to convert to from the old new  you can do so by calling new methods add to the old classes    For much more information on conversion  see my Answer and nifty diagram to another Question  Convert java util Date to what    java time    type    Searching Stack Overflow gives many hundreds of example Questions and Answers on using java time  But here is a quick synopsis   Instant  Get the current moment with an Instant  The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds  up to nine  9  digits of a decimal fraction     Instant instant   Instant now      ZonedDateTime  To see that same simultaneous moment through the lens of some particular region   s wall-clock time  apply a time zone  ZoneId  to get a ZonedDateTime   Time zone  Specify a proper time zone name in the format of continent region  such as America Montreal  Africa Casablanca  or Pacific Auckland  Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones  not standardized  and not even unique       ZoneId z   ZoneId of   America Montreal     ZonedDateTime zdt   instant atZone      Offset  A time zone is a region   s history of changes in its offset-from-UTC  But sometimes you are given only an offset without the full zone  In that case  use the OffsetDateTime class   ZoneOffset offset   ZoneOffset parse    05 30     OffsetDateTime odt   instant atOffset  offset      Use of a time zone is preferable over use of a mere offset   LocalDateTime  The    Local    in the Local    classes means any locality  not a particular locality  So the name can be counter-intuitive   LocalDateTime  LocalDate  and LocalTime purposely lack any information about offset or time zone  So they do not represent actual moments  they are not points on the timeline  When in doubt or in confusion  use ZonedDateTime rather than LocalDateTime  Search Stack Overflow for much more discussion   Strings  Do not conflate date-time objects with strings that represent their value  You can parse a string to get a date-time object  and you can generate a string from a date-time object  But the string is never the date-time itself    Learn about standard ISO 8601 formats  used by default in the java time classes     About java time  The java time framework is built into Java 8 and later  These classes supplant the troublesome old legacy date-time classes such as java util Date  Calendar   amp  SimpleDateFormat   The Joda-Time project  now in maintenance mode  advises migration to the java time classes   To learn more  see the Oracle Tutorial  And search Stack Overflow for many examples and explanations  Specification is JSR 310   Using a JDBC driver compliant with JDBC 4 2 or later  you may exchange java time objects directly with your database  No need for strings nor java sql   classes   Where to obtain the java time classes     Java SE 8  Java SE 9  and later   Built-in   Part of the standard Java API with a bundled implementation  Java 9 adds some minor features and fixes   Java SE 6 and Java SE 7   Much of the java time functionality is back-ported to Java 6  amp  7 in ThreeTen-Backport   Android   Later versions of Android bundle implementations of the java time classes  For earlier Android  the ThreeTenABP project adapts ThreeTen-Backport  mentioned above   See How to use ThreeTenABP          The ThreeTen-Extra project extends java time with additional classes  This project is a proving ground for possible future additions to java time  You may find some useful classes here such as Interval  YearWeek  YearQuarter  and more

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I use Calendar when I need some specific operations over the dates like moving in time  but Date I find it helpful when you need to format the date to adapt your needs  recently I discovered that Locale has a lot of useful operations and methods So I m using Locale right now

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Date is a simpler class and is mainly there for backward compatibility reasons  If you need to set particular dates or do date arithmetic  use a Calendar  Calendars also handle localization  The previous date manipulation functions of Date have since been deprecated   Personally I tend to use either time in milliseconds as a long  or Long  as appropriate  or Calendar when there is a choice   Both Date and Calendar are mutable  which tends to present issues when using either in an API

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Dates should be used as immutable points in time  Calendars are mutable  and can be passed around and modified if you need to collaborate with other classes to come up with a final date   Consider them analogous to String and StringBuilder and you ll understand how I consider they should be used    And yes  I know Date isn t actually technically immutable  but the intention is that it should not be mutable  and if nothing calls the deprecated methods then it is so

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The best way for new code  if your policy allows third-party code  is to use the Joda Time library   Both  Date and Calendar  have so many design problems that neither are good solutions for new code

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Btw  date  is usually tagged as  obsolete   deprecated   I dont know exactly why  - something about it is wrote there  Java  Why is the Date constructor deprecated  and what do I use instead   It looks like it s a problem of the constructor only- way via new Date int year  int month  int day   recommended way is via Calendar and set params separately     Calendar cal   Calendar getInstance

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I always advocate Joda-time  Here s why    the API is consistent and intuitive  Unlike the java util Date Calendar APIs it doesn t suffer from threading issues  unlike java text SimpleDateFormat etc   I ve seen numerous client issues relating to not realising that the standard date time formatting is not thread-safe  it s the basis of the new Java date time APIs  JSR310  scheduled for Java 8  So you ll be using APIs that will become core Java APIs    EDIT  The Java date time classes introduced with Java 8 are now the preferred solution  if you can migrate to Java 8

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I generally use Date if possible  Although it is mutable  the mutators are actually deprecated   In the end it basically wraps a long that would represent the date time   Conversely  I would use Calendars if I have to manipulate the values   You can think of it this way  you only use StringBuffer only when you need to have Strings that you can easily manipulate and then convert them into Strings using toString   method   In the same way  I only use Calendar if I need to manipulate temporal data   For best practice  I tend to use immutable objects as much as possible outside of the domain model   It significantly reduces the chances of any side effects and it is done for you by the compiler  rather than a JUnit test   You use this technique by creating private final fields in your class   And coming back to the StringBuffer analogy   Here is some code that shows you how to convert between Calendar and Date  String s    someString      immutable string StringBuffer buf   new StringBuffer s      mutable  string  via StringBuffer buf append  x    assertEquals  someStringx   buf toString        convert to immutable String     immutable date with hard coded format   If you are hard    coding the format  best practice is to hard code the locale    of the format string  otherwise people in some parts of Europe    are going to be mad at you  Date date       new SimpleDateFormat  yyyy-MM-dd   Locale ENGLISH  parse  2001-01-02        Convert Date to a Calendar Calendar cal   Calendar getInstance    cal setTime date       mutate the value cal add Calendar YEAR  1       convert back to Date Date newDate   cal getTime         assertEquals new SimpleDateFormat  yyyy-MM-dd   Locale ENGLISH  parse  2002-01-02    newDate

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Date is best for storing a date object  It is the persisted one  the Serialized one      Calendar is best for manipulating Dates      Note  we also sometimes favor java lang Long over Date  because Date is mutable and therefore not thread-safe  On a Date object  use setTime   and getTime   to switch between the two  For example  a constant Date in the application  examples  the zero 1970 01 01  or an applicative END OF TIME that you set to 2099 12 31   those are very useful to replace null values as start time and end time  especially when you persist them in the database  as SQL is so peculiar with nulls

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A little bit late at party  but Java has a new Date Time API in JDK 8  You may want to upgrade your JDK version and embrace the standard  No more messy date calendar  no more 3rd party jars

[java] Java Date vs Calendar

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