The following program shows that Calendar.equals changes the observable state of the calendar. The output I got is:
Date with equals: Tue May 30 20:25:08 PDT 2000
Date without equals: Sun Apr 30 20:25:08 PDT 2000
Calling a.equals(b) should never, ever have any observable effect on a or b.


import java.util.Calendar;

public class CalEquals {

    public static void main(String[] args) {
        Calendar cal1 = makeCalendar();
        Calendar cal2 = makeCalendar();
        Calendar cal3 = makeCalendar();
        
        cal1.equals(cal2);
        cal1.set(Calendar.DAY_OF_MONTH, 30);
        cal3.set(Calendar.DAY_OF_MONTH, 30);
        System.out.println("Date with equals: " + cal1.getTime());
        System.out.println("Date without equals: " + cal3.getTime());
    }
    
    private static Calendar makeCalendar() {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.MONTH, Calendar.APRIL);
        cal.set(Calendar.DAY_OF_MONTH, 31);
        return cal;
    }
}


The Date class has the same problem since JDK1.0.
###@###.### 2002-08-05