Check leap year

/** determine if the given year is a leap year.<p>
 
     The Gregorian calendar rule states that a leap year occurs
     every fourth year, except every 100 years, except every 400
     years.<p>
 
     @param year The year to be tested.  Make sure this is a four digit year\!<p>
     @return true if "year" is a leap year.<p>
*/
boolean isLeapYear(int year) {
  boolean y4 = ( (year % 4) == 0 );
  boolean y100 = ( (year % 100) == 0 );
  boolean y400 = ( (year % 400) == 0 );
  return ( y400 || (y4 && ! y100) );
}