How do I calculate someone's age in Java?

Question

I want to return an age in years as an int in a Java method. What I have now is the following where getBirthDate() returns a Date object (with the birth date ;-)):

public int getAge() {
 long ageInMillis = new Date().getTime() - getBirthDate().getTime();

 Date age = new Date(ageInMillis);

 return age.getYear();
}

But since getYear() is deprecated I'm wondering if there is a better way to do this? I'm not even sure this works correctly, since I have no unit tests in place (yet).

Answer 1

Your example is NOT the way to do it.

Take a look at this.

The JODA datetime library has some nice interval calculations. Try that.

Answer 2

Calendar now = Calendar.getInstance();
Calendar dob = Calendar.getInstance();
dob.setTime(...);
if (dob.after(now)) {
  throw new IllegalArgumentException("Can't be born in the future");
}
int year1 = now.get(Calendar.YEAR);
int year2 = dob.get(Calendar.YEAR);
int age = year1 - year2;
int month1 = now.get(Calendar.MONTH);
int month2 = dob.get(Calendar.MONTH);
if (month2 > month1) {
  age--;
} else if (month1 == month2) {
  int day1 = now.get(Calendar.DAY_OF_MONTH);
  int day2 = dob.get(Calendar.DAY_OF_MONTH);
  if (day2 > day1) {
    age--;
  }
}
// age is now correct

Answer 3

Check out Joda, which simplifies date/time calculations (Joda is also the basis of the new standard Java date/time apis, so you'll be learning a soon-to-be-standard API).

e.g.

DateMidnight birthdate = new DateMidnight(1970, 1, 20);
DateTime now = new DateTime();
Years age = Years.betweenYears(birthdate, now);

which is as simple as you could want. The current Java stuff is (as you've identified) somewhat unintuitive.