Why do java.util.calendar object's values change after a call to get(int)

Question

I am trying to debug a problem in some legacy code.

I have narrowed the problem to the following method:

public String formatDateTimeFromCalendar (Calendar cal){
        StringBuffer sb = new StringBuffer();
        String hr = ""+cal.get(Calendar.HOUR_OF_DAY);
        sb.append(String.format("%02d", hr)); 
        sb.append(":");
        sb.append(String.format("%02d", cal.get(Calendar.MINUTE)));
        sb.append( " on ");
        sb.append(String.format("%02d", cal.get(Calendar.DAY_OF_MONTH)));
        sb.append( "/");
        sb.append(String.format("%02d", cal.get(Calendar.MONTH)+1));
        sb.append( "/");
        sb.append( cal.get(Calendar.YEAR));
        return sb.toString();
    }

Debugging Line 2 the cal parameter is a future date. (2015-01-06T00:00:00.000Z)

After execution of line 3 with the first cal.get( the cal param's values have changed (to 2014-12-12T00:00:00.000Z)

Why/how could this be?

Here is where the calendar is being created:

Calendar startDateAndTime = Calendar.getInstance();
        startDateAndTime.setTime(response.getStartDate().toGregorianCalendar().getTime());
        startDateAndTime.set(Calendar.HOUR_OF_DAY, response.getStartTime().getHour());
        startDateAndTime.set(Calendar.MINUTE, response.getStartTime().getMinute());
        startDateAndTime.set(Calendar.SECOND, response.getStartTime().getSecond());
        startDateAndTime.set(Calendar.MILLISECOND, response.getStartTime().getMillisecond());

response.getStartDate() returns an XMLGregorianCalendar

Answer 1

This is because calls of get normalize the Calendar in lenient mode, and validate it in strict mode:

returns the value of the given calendar field. In lenient mode, all calendar fields are normalized. In non-lenient mode, all calendar fields are validated and this method throws an exception if any calendar fields have out-of-range values. The normalization and validation are handled by the complete() method, which process is calendar system dependent.

It looks like your Calendar object is in lenient mode, hence the normalization is performed. Normalization is dependent on the calendar instance.

Answer 2

This is source code of java.util.Calendar for get() method.

public int get(int field)
    {
        complete();
        return internalGet(field);
    }

complete() method checks if all fields were set in instance and could normalize date and time.
This is default implementation of complete() method

protected void complete()
{
    if (!isTimeSet)
        updateTime();
    if (!areFieldsSet || !areAllFieldsSet) {
        computeFields(); // fills in unset fields
        areAllFieldsSet = areFieldsSet = true;
    }
}

But you shouldn't think that your fields are necessary invalid. This could happen if you manually edited calendar instance calling set() method, it unconditionally sets isTimeSet and areFieldsSet to false enforcing recomputation on subsequent obtaining of data.