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 ;-)):

Answer 1

I appreciate all correct answers but this is the kotlin answer for the same question

I hope would be helpful to kotlin developers

fun calculateAge(birthDate: Date): Int {
        val now = Date()
        val timeBetween = now.getTime() - birthDate.getTime();
        val yearsBetween = timeBetween / 3.15576e+10;
        return Math.floor(yearsBetween).toInt()
    }

Answer 2

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).

EDIT: Java 8 has something very similar and is worth checking out.

e.g.

LocalDate birthdate = new LocalDate (1970, 1, 20);
LocalDate now = new LocalDate();
Years age = Years.yearsBetween(birthdate, now);

which is as simple as you could want. The pre-Java 8 stuff is (as you've identified) somewhat unintuitive.

Answer 3

This is an improved version of the one above... considering that you want age to be an 'int'. because sometimes you don't want to fill your program with a bunch of libraries.

public int getAge(Date dateOfBirth) {
    int age = 0;
    Calendar born = Calendar.getInstance();
    Calendar now = Calendar.getInstance();
    if(dateOfBirth!= null) {
        now.setTime(new Date());
        born.setTime(dateOfBirth);  
        if(born.after(now)) {
            throw new IllegalArgumentException("Can't be born in the future");
        }
        age = now.get(Calendar.YEAR) - born.get(Calendar.YEAR);             
        if(now.get(Calendar.DAY_OF_YEAR) < born.get(Calendar.DAY_OF_YEAR))  {
            age-=1;
        }
    }  
    return age;
}

Answer 4

import java.io.*;

class AgeCalculator
{
    public static void main(String args[])
    {
        InputStreamReader ins=new InputStreamReader(System.in);
        BufferedReader hey=new BufferedReader(ins);

        try
        {
            System.out.println("Please enter your name: ");
            String name=hey.readLine();

            System.out.println("Please enter your birth date: ");
            String date=hey.readLine();

            System.out.println("please enter your birth month:");
            String month=hey.readLine();

            System.out.println("please enter your birth year:");
            String year=hey.readLine();

            System.out.println("please enter current year:");
            String cYear=hey.readLine();

            int bDate = Integer.parseInt(date);
            int bMonth = Integer.parseInt(month);
            int bYear = Integer.parseInt(year);
            int ccYear=Integer.parseInt(cYear);

            int age;

            age = ccYear-bYear;
            int totalMonth=12;
            int yourMonth=totalMonth-bMonth;

            System.out.println(" Hi " + name + " your are " + age + " years " + yourMonth + " months old ");
        }
        catch(IOException err)
        {
            System.out.println("");
        }
    }
}

Answer 5

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 6

JDK 8 makes this easy and elegant:

public class AgeCalculator {

    public static int calculateAge(LocalDate birthDate, LocalDate currentDate) {
        if ((birthDate != null) && (currentDate != null)) {
            return Period.between(birthDate, currentDate).getYears();
        } else {
            return 0;
        }
    }
}

A JUnit test to demonstrate its use:

public class AgeCalculatorTest {

    @Test
    public void testCalculateAge_Success() {
        // setup
        LocalDate birthDate = LocalDate.of(1961, 5, 17);
        // exercise
        int actual = AgeCalculator.calculateAge(birthDate, LocalDate.of(2016, 7, 12));
        // assert
        Assert.assertEquals(55, actual);
    }
}

Everyone should be using JDK 8 by now. All earlier versions have passed the end of their support lives.