HUE_03_BirthdaysFile/src/Geburtstag.java

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

public class Geburtstag {
    String name;
    int jahr;
    int monat;
    int tag;
    LocalDate datum;

    public Geburtstag(LocalDate datum) {
        this.datum = datum;
    }

    public Geburtstag(String name, int jahr, int monat, int tag) {
        this.name = name;
        this.datum = LocalDate.of(jahr, monat, tag);
    }

    public LocalDate naechsteGeburtstagsfeier() {
        Geburtstag temp = new Geburtstag(this.datum);
        LocalDate today = LocalDate.now();
        if (temp.datum.isAfter(today) || temp.datum.isEqual(today)) {
            // do nothing, birthday wasn't already or is today
        } else if (temp.datum.isBefore(today)) {
            for (int i = 0; temp.datum.isBefore(today) && !temp.datum.isEqual(today); i++) {
                temp.datum = temp.datum.plusYears(1);
            }
        }
        return temp.datum;
    }

    public int alter() {
        LocalDate today = LocalDate.now();
        int years = 0;
        try {
            years = Period.between(this.datum, today).getYears();
            if (this.datum.isEqual(today)) {
                years++;
            }
        } catch (Exception NullPointerException) {
            // date was invalid, do nothing
        }
        return years;
    }
}