Java GregorianCalendar change TimeZone
date, java, timezone
Solution
The `GregorianCalendar` class inherits its `get` method from `Calendar`, which has the following side effect:
In lenient mode, all calendar fields are normalized.
This means that the `time` value and all fields are recomputed when `get` is called on a `Calendar` object. This can lead to some unpredictable behavior, particularly when coupled with `setTimeZone`, which has some documented buggy behavior of its own.
Problem
I'm trying to set HOUR_OF_DAY field and change Timezone of the GregorianCalendar date object. ``` GregorianCalendar date = new GregorianCalendar(TimeZone.getTimeZone("GMT+10")); System.out.println("HOUR: " + date.get(Calendar.HOUR_OF_DAY)); date.set(Calendar.HOUR_OF_DAY, 23); //date.get(Calendar.HOUR_OF_DAY); date.setTimeZone(TimeZone.getTimeZone("GMT")); System.out.println("HOUR: " + date.get(Calendar.HOUR_OF_DAY)); ``` Output: ``` HOUR: 16 HOUR: 23 ``` For some reason value of HOUR_OF_DAY does not change after setting different timezone. But if I uncomment date.get for HOUR_OF_DAY, everything works exactly as it should ``` GregorianCalendar date = new GregorianCalendar(TimeZone.getTimeZone("GMT+10")); System.out.println("HOUR: " + date.get(Calendar.HOUR_OF_DAY)); date.set(Calendar.HOUR_OF_DAY, 23); date.get(Calendar.HOUR_OF_DAY); // uncommenting this line will is changing the output date.setTimeZone(TimeZone.getTimeZone("GMT")); System.out.println("HOUR: " + date.get(Calendar.HOUR_OF_DAY)); ``` Output: ``` HOUR: 16 HOUR: 13 ``` How is this possible? Why .get method is changing object behaviour?