calculate leap year in java

[Violet_82]

Hi all, I was reading a few things on the net about how to calculate a leap year in java (and not only for that matter). So something like ...if( year % 400 == 0 || ( year % 4 == 0 && year % 100 != 0 ) )...

So on wikipedia I have found that:

if year is divisible by 400 then
   is_leap_year
else if year is divisible by 100 then
   not_leap_year
else if year is divisible by 4 then
   is_leap_year
else
   not_leap_year

so what's that year % 4 == 0 about? the above if should only say if( year % 400 == 0 && year % 100 != 0 ) ) because if these 2 conditions are satisfied then the year is leap year
Or am I getting it wrong?
thanks

[kRod]

As found on GOOGLE

public static boolean isLeapYear(int year) {
    assert year >= 1583; // not valid before this date.
    return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}

[JamesCherrill]

so what's that year % 4 == 0 about? the above if should only say if( year % 400 == 0 && year % 100 != 0 ) ) because if these 2 conditions are satisfied then the year is leap year
Or am I getting it wrong?

Yes, you're getting it wrong! A year is a leap year if it's divisible by 4 ( ie year % 4 == 0), with special cases for years divisble by 100 or 400. If you dont have the year % 4 == 0 then your answer is correct for the special cases, but not for the basic ordinary case.

[ddanbe]

The year 2012 was a leap year as will be the year 2016. Both years are not divisible by 100 or 400.

[Violet_82]

ah ok, sorry this is getting more confusing. Wikipedia above said that if a year is divisible by 100 then it's not a leap year hence the year % 100 != 0 condition, so if ((the year is divisible by 4) and (not by 100) or (it is divisible by 400)) then it's a leap year.

[JamesCherrill]

extra () for absolute clarity...
if ( ( (the year is divisible by 4) and (not by 100) ) or (it is divisible by 400) ) then it's a leap year

[JamesCherrill]

... or maybe this is clearer still:

boolean isLeapYear(year)
   if year divisible by 400 return true
   if year divisible by 100 return false
   if year divisible by 4 return true
   return false

[Violet_82]

boolean isLeapYear(year)
   if year divisible by 400 return true
   if year divisible by 100 return false
   if year divisible by 4 return true
   return false

Ok, why is it returning false at the very end?

[bguild]

Ok, why is it returning false at the very end?

Because most years are not leap years. For example, what would isLeapYear(2013) return if not false?

[Violet_82]

ok got it, thanks!