Why Java util. The value of the calendar object will change after calling get (int)

I try to debug problems in some legacy code.

I have narrowed the problem down to the following methods:

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();
    }

The cal parameter in the second line of debugging is the future date (2015-01-06T00：00：00.000Z)

Use the first cal After get executes line 3 (the value of cal param has been changed (to 2014-12-12t00:00:00.000z)

Why / how did this happen?

Here is where you created your calendar:

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 xmlregionancalendar

Solution

This is because the call to get normalizes calendar in loose mode and verifies it in strict mode:

It looks like your calendar object is in loose mode, so perform normalization Normalization depends on calendar instances