Checking leap year – Working with Date and Time

67. Checking leap year

This problem become easy as long as we know what is a leap year. In a nutshell, a leap year is any year divisible by 4 (so, year % 4 == 0) that it is not a century (for instance, 100, 200, …, n00). However, if the year represents a century that it is divisible by 400 (so, year % 400 == 0) then it is a leap year. In this context, our code is just a simple chain of if statements as follows:

public static boolean isLeapYear(int year) {
  if (year % 4 != 0) {
    return false;
  } else if (year % 400 == 0) {
    return true;
  } else if (year % 100 == 0) {
    return false;
  }
  return true;
}

But, this code can be condensed using the GregorianCalendar as well:

public static boolean isLeapYear(int year) {
  return new GregorianCalendar(year, 1, 1).isLeapYear(year);
}

Or, starting with JDK 8, we can rely on the java.time.Year API as follows:

public static boolean isLeapYear(int year) {
  return Year.of(year).isLeap();      
}

In the bundled code, you can see more approaches.

68. Calculating the quarter of a given date

A year has 4 quarters (commonly denoted as Q1, Q2, Q3, and Q4) and each quarter has 3 months. If we consider that January is 0, February is 1, …, and December is 11, then we can observe that January/3 = 0, February/3 =0, March/3 = 0, and 0 can represent Q1. Next, 3/3 = 1, 4/3 = 1, 5/3 = 1, so 1 can represent Q2. Based on the same logic, 6/3 = 2, 7/3 = 2, 8/3 = 2, so 2 can represent Q3. Finally, 9/3 = 3, 10/3 = 3, 11/3 = 3, so 3 represents Q4.Based on this statement and the Calendar API, we can obtain the following code:

public static String quarter(Date date) {
  String[] quarters = {“Q1”, “Q2”, “Q3”, “Q4”};
  Calendar calendar = Calendar.getInstance();
  calendar.setTime(date);
  int quarter = calendar.get(Calendar.MONTH) / 3;
  return quarters[quarter];
}

But, starting with JDK 8, we can rely on java.time.temporal.IsoFields. This class contains fields (and units) that follow the calendar system based on the ISO-8601 standard. Among these artifacts, we have the week-based-year and the one that we are interested in, quarter-of-year. This time, let’s return the quarter as an integer:

public static int quarter(Date date) {
  LocalDate localDate = date.toInstant()
    .atZone(ZoneId.systemDefault()).toLocalDate();
  return localDate.get(IsoFields.QUARTER_OF_YEAR);
}

In the bundled code, you can see more examples including one that uses DateTimeFormatter.ofPattern(“QQQ”).