How do I calculate someone's age in Java?

java

exception-handling

date-time

unit-testing

byAlex Kataev·Dec 9, 2024

Punch the age out of Java using LocalDate and Period:

import java.time.LocalDate;
import java.time.Period;

public static int calculateAge(LocalDate birthDate) {
    if (birthDate != null) {
        // Because every year you survive is a victory!!
        return Period.between(birthDate, LocalDate.now()).getYears();
    } else {
        // Nobody knows the age of ghosts!
        return 0; 
    }
}

To get the age, call magic wand calculateAge with a birthdate like LocalDate.of(1990, 5, 24).

Error and edge cases

Code error? Blame the hardware or the user. But mature programmers know handling exceptions is what makes a software resilient:

Time zone: Remember to note the time difference when partying in different time zones. Account for the system's time zone when using LocalDate.now().
Null inputs: What if Voldemort uses your code? He does not have a birthdate! A simple check at the start of the method will return 0 for such Dark-Lords.
Future birthdates: Is your user a time traveler? For birthdates in the future, an IllegalArgumentException should be thrown. No, you can't be born tomorrow!

Many roads lead to Rome

There are myriad ways to calculate age , but let's get you lost in fewer of them:

ChronoUnit: Who needs a time machine when you have ChronoUnit.YEARS.between(birthDate, LocalDate.now())? It's another easy way to calculate the age.
Joda-Time: If you miss old timers, then Joda-Time is for you. However, cross-validate results, or you might land in an alternate timeline.
Calendar approach: If you want to go vintage, you can use Calendar.getInstance(). However, it's like riding a horse to work. Just throw it out!

Ironclad robustness

How strong is your age calculating code? Only testing will tell :

Unit tests: Ensure your calculateAge method doesn't cheat at hide & seek. Test it with various date inputs, including the tricksy leap years.
Leap year handling: A year can sometimes pack an extra day! Ensure that your calculation includes leap years or face the wrath of February 29th.
Exception handling: A good piece of code anticipates the storm. Brace your code for potential exceptions like DateTimeException on invalid dates.

Polishing your age calculator

Scrub until it shines:

Months and days: When someone says they are 24, are they 24 years, 8 months, and 16 days old? For precise age, include months and days.
Current date: While using LocalDate.now(), remember that the earth spins! If your method runs over midnight, the date might change causing discrepancies.

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