Java Calendar Timezone Code They Write

Yet, I see many developers still using good old java.util.Date and java.util.Calendar classes in the code they write today.

But this does not mean we can not use new java.time API when writing new code or refactoring the old one.

Many of its methods were deprecated since Java 1.1 and ported to (abstract) java.util.Calendar and java.util.GregorianCalendar.

Its been badly abused by library authors, adding further to the confusion.

There are many workarounds in the Java world around this banal design decision, like handling years before 1900.

As a result, any time you want to give a date back (say, as an instance structure) you need to return a clone of that date instead of the date object itself (since otherwise, people can mutate your structure).

You have to always take care about that or it could lead to some unexpected behaviors.

One of the serious weaknesses of the existing formatters (like java.util.SimpleDateFormat ) in Java is that they arent thread-safe.

The new API models its domain very precisely with classes that represent different use cases for Date and Time closely.

The new API allows people to work with different non-ISO-8601 calendaring systems, like one used in Japan or Thailand.

Someone reused an instance of SimpleDateFormat in the XML exporting logic.

In development and test environments we did not notice any problem.

But in production environment, on a bit increased load, each 10th or so execution suffered from this issue, mixing up printed dates.

For every date-time manipulation, there is probably already implemented method to use.

The new java.time package contains all the classes for date, time, datetime, time zones, instants, duration, and clocks manipulation.

It makes very easy to compute the different between two time values.

Period, on the other hand, performs a date based comparison between two dates.

The DateTimeFormatter is used to specify the datetime pattern.

LocalDateTime dateTime LocalDateTime.of(2017, Month.APRIL, 8, 12, 30).

String formattedDateTime dateTime.format(formatter); 2017-04-08 12:30.

For example: Using DateTimeFormatter.ISODATETIME to format the LocalDateTime instance from above would result in the string 2017-04-08T12:30:00.