Java GregorianCalendar change TimeZone

Question

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?

Answer 1

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.