Is there a way to pass a copy of the object rather than reference to Java method

Question

Maybe there is a smarter way of resolving my issue so my question might not be pointing you into the right direction of thinking.

However, my issue is that I want to initialize my Calendar instance only once at the beginning of the program, so I work with only that date and time.

Then I want to pass that calendar to a method which adds/subtracts some months/days from it and uses that datetime for an API call. The thing is, I want to reuse the initial date all over in different places of my application, but that method keeps changing my Calendar instance.

CODE:

Initialization of the Calendar on the beginning of the main thread:

Calendar cal = Calendar.getInstance();

Method using the instance:

private static String getDateUntil(int i, Calendar cal) {
    int month = 0 - i;
    //Calendar cal = Calendar.getInstance();
    cal.add(Calendar.MONTH, month);
    Date result = cal.getTime();
    String dateUntil = new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ss").format(result);

    return dateUntil;
}

After calling this method, my cal is changed and I can't reuse the initial datetime. Any ideas?

Answer 1

Use clone in your methods:

private static String getDateUntil(int i, Calendar cal) {
    int month = 0 - i;
    Calendar mycal = (Calendar)cal.clone();  // Make a copy of the initial instance
    mycal.add(Calendar.MONTH, month);
    Date result = mycal.getTime();
    String dateUntil = new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ss").format(result);
    return dateUntil;
}

Answer 2

Use calendar.clone() to obtain a copy.