Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 178

The program's first bug is in the method invocation cal.set(1999, 12, 31) . When months are
represented numerically, convention dictates that the first month be assigned the number 1.
Unfortunately, Date represents January as 0, and Calendar perpetuates this mistake. Therefore,
this method invocation sets the calendar to the thirty-first day of the thirteenth month of 1999. But
the standard (Gregorian) calendar has only 12 months; surely this method invocation should cause
an IllegalArgumentException ? It should, but it doesn't. The Calendar class silently substitutes the
first month of the next year, in this case, 2000. This explains the first number printed by our
program ( 2000 ).
There are two ways to fix this problem. You could change the second parameter of the cal.set
invocation from 12 to 11 , but that would be confusing. The number 11 would suggest November to
readers. It would be better to use the constant that Calendar declares for this purpose, which is
Calendar.DECEMBER .
What about the second number printed by the program? The cal.set invocation clearly indicates
that the calendar is set to the thirty-first day of the month. The Date instance d represents the same
point in time as the Calendar , so its getday method should return 31, but the program prints 1 .
What is going on?
To find out, you have to read the documentation, which says that Date.getDay returns the day of
the week represented by the Date instance, not the day of the month. The returned value is 0-
based, starting at Sunday, so the 1 printed by the program indicates that January 31, 2000, fell on a
Monday. Note that the corresponding Calendar method, get(Calendar.DAY_OF_WEEK) ,
inexplicably returns a day-of-the-week value that is 1-based, not 0-based like the value returned by
its Date counterpart.
There are also two ways to fix this problem. You could call the confusingly named Date.date
method, which returns the day of the month. Like most Date methods, however, it is deprecated, so
you would be better off dispensing with Date entirely and calling the Calendar method
get(Calendar.DAY_OF_MONTH) . With both problems fixed, the program prints 1999 31 as expected:
public class DatingGame {
public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
cal.set(1999, Calendar.DECEMBER, 31);
System.out.print(cal.get(Calendar.YEAR) + " ");
System.out.println(cal.get(Calendar.DAY_OF_MONTH));
}
}