Computing pregnancy due date – Working with Date and Time

71. Computing pregnancy due date

I’m not an expert in the field but let’s start with these two constants:

public static final int PREGNANCY_WEEKS = 40;
public static final int PREGNANCY_DAYS = PREGNANCY_WEEKS * 7;

Let’s consider the first day as a LocalDate and we want to write a calculator that prints the pregnancy due date, the number of remaining days, the number of passed days, and the current week.Basically, the pregnancy due date is obtained by adding the PREGNANCY_DAYS to the given first day. Further, the number of remaining days is the difference between today and the given first day, while the number of passed days is PREGNANCY_DAYS minus the number of remaining days. Finally, the current week is obtained as the number of passed days divided by 7 (since a week has 7 days). Based on these statements, the code speaks for itself:

public static void pregnancyCalculator(LocalDate firstDay) {
  firstDay = firstDay.plusDays(PREGNANCY_DAYS);      
  System.out.println(“Due date: ” + firstDay);
  LocalDate today = LocalDate.now();
  long betweenDays =  
    Math.abs(ChronoUnit.DAYS.between(firstDay, today));
  long diffDays = PREGNANCY_DAYS – betweenDays;
  long weekNr = diffDays / 7;
  long weekPart = diffDays % 7;
  String week = weekNr + ” | ” + weekPart;
  System.out.println(“Days remaining: ” + betweenDays);
  System.out.println(“Days in: ” + diffDays);
  System.out.println(“Week: ” + week);      
}

Well, ladies, I wish you to have an easy and timely birth.

72. Implementing a stopwatch

A classical implementation for a stopwatch relies on System.nanoTime(), System.currentTimeMillis() or on Instant.now(). In all cases, we have to provide support for starting and stopping the stopwatch, and some helpers to obtain the measured time in different time units.While the solutions based on System.nanoTime() and currentTimeMillis() is available in the bundled code, here is the one based on Instant.now():

public final class NanoStopwatch {
  private Instant startTime;
  private Instant stopTime;
  private boolean running;
  public void start() {
    this.startTime = Instant.now();
    this.running = true;
  }
  public void stop() {
    this.stopTime = Instant.now();
    this.running = false;
  }
  //elaspsed time in nanoseconds
  public long getElapsedTime() {
    if (running) {
      return ChronoUnit.NANOS.between(
        startTime, Instant.now());          
    } else {
      return ChronoUnit.NANOS.between(startTime, stopTime);          
    }      
  }
}

If you need to return the measured time in milliseconds or seconds as well, then simply add the following two helpers:

//elaspsed time in millisecods
public long elapsedTimeToMillis(long nanotime) {              
  return TimeUnit.MILLISECONDS.convert(
    nanotime, TimeUnit.NANOSECONDS);
}
//elaspsed time in seconds
public long elapsedTimeToSeconds(long nanotime) {              
  return TimeUnit.SECONDS.convert(
    nanotime, TimeUnit.NANOSECONDS);
}

Done!