Java Calendar adds a random number of milliseconds?

Question

Hi I have something weird happening. I am simply taking a calendar object, converting it to its individual parts, and putting it back into a calendar (with or without any change

Answer 1

In the documentation of Calendar.set, it is said :

Sets the values for the fields YEAR, MONTH, DAY_OF_MONTH, HOUR, MINUTE, and SECOND. Previous values of other fields are retained. If this is not desired, call clear() first.

The reason is that not all fields are set with this method, in you case, you don't have MILLISECOND set. So it keep the value when the instance was created.

The call of Calendar.clear will

Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this Calendar undefined.

A quick example :

    Calendar c = GregorianCalendar.getInstance();
    c.clear();
    c.set(2019, Calendar.NOVEMBER, 03, 16, 15, 03);

    System.out.println(c.getTime());
    System.out.println(c.getTimeInMillis());

Sun Nov 03 16:15:03 CET 2019
1572794103000

Milliseconds being undefined will give 0