This Java application calculates the age of a user based on their birthdate. It also provides additional information such as the number of days until the user's next birthday and wishes them a happy birthday if today is their birthday.

AgeCalculator.java

import java.time.LocalDate;
import java.time.Period;
import java.util.Scanner;

public class AgeCalculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Prompt the user to enter their birth year
        System.out.println("Enter your birth year: ");
        int year = scanner.nextInt();
        
        System.out.println("Enter your birth month: ");
        int month = scanner.nextInt();
        
        System.out.println("Enter your birth day: ");
        int day = scanner.nextInt();
        
        LocalDate birthDate = LocalDate.of(year, month, day);
        LocalDate currentDate = LocalDate.now();
        
        if (birthDate.isAfter(currentDate)) {
            System.out.println("Birthdate cannot be in the future.");
        } else {
            Period age = Period.between(birthDate, currentDate);
            System.out.println("You are " + age.getYears() + " years, " + age.getMonths() + " months, and " + age.getDays() + " days old.");
            
            if (birthDate.getMonth() == currentDate.getMonth() && birthDate.getDayOfMonth() == currentDate.getDayOfMonth()) {
                System.out.println("Happy Birthday!");
            } else {
                LocalDate nextBirthday = birthDate.withYear(currentDate.getYear());
                if (nextBirthday.isBefore(currentDate) || nextBirthday.isEqual(currentDate)) {
                    nextBirthday = nextBirthday.plusYears(1);
                }
                long daysUntilNextBirthday = java.time.temporal.ChronoUnit.DAYS.between(currentDate, nextBirthday);
                System.out.println("Your next birthday is in " + daysUntilNextBirthday + " days.");
            }
        }
        
        scanner.close();
    }
}