ein ziehmlich nices programm

package programming.set6.date;

/**
 * A class that provides basic functions to handle dates.
 *
 * @author Bjarne
 */
public class Date {

    /**
     * the year of an instance of the class
     */
    private int year;

    /**
     * the month of an instance of the class
     */
    private int month;

    /**
     * the day of an instance of the class
     */
    private int day;

    /**
     * An array that contains the number of days that each month has, where the
     * 0-th element represents January and the 11-th element represents
     * December.
     */
    private static final int[] DaysInMonth = { 31, 28, 31, 30, 31, 30, 31, 31,
            30, 31, 30, 31 };

    /**
     * A second array that contains the number of days for each month within a
     * leap year.
     */
    private static final int[] DaysInMonth_LeapYear = { 31, 29, 31, 30, 31, 30,
            31, 31, 30, 31, 30, 31 };

    /**
     * An array that contains the names of the months.
     */
    private static final String[] MonthNames = { "January", "February",
            "March", "April", "May", "June", "July", "August", "September",
            "October", "November", "December" };

    /**
     * Returns the number of days in a given month.
     *
     * @param year
     *            the year is needed to check whether it is a leap year
     * @param month
     *            the given month
     * @return days in the given month
     */
    public static int getDaysInMonth(int year, int month) {

        // return 0 for an invalid month

        if (month < 1 || month > 12) {

            return 0;
        }

        // check for leap year

        if (year % 4 == 0 && year % 100 != 0 || year % 4 == 0
                && year % 100 == 0 && year % 400 == 0) {


            // If the year is a leap year, return the value from the leap year array.

            // As January is the first month, but the 0-th element in the array,
            // we need to subtract 1 from month to get the correct value.


            return DaysInMonth_LeapYear[month - 1];
        }

        // If the year isn't a leap year, use the default array.

        else {

            return DaysInMonth[month - 1];
        }
    }

    /**
     * Returns true for a valid date and false for an invalid one.
     *
     * @param year
     *            the year of the given date
     * @param month
     *            the month of the given date
     * @param day
     *            the day of the given date
     * @return boolean
     */
    public static boolean validate(int year, int month, int day) {

        // check for valid month in the range from 1 - 12
        // and for a day greater 0.

        if (month > 0 && month < 13 && day > 0) {

            // If we have a leap year, compare the value of the given
            // day to the corresponding value in the leap year array.

            if (year % 4 == 0 && year % 100 != 0 || year % 4 == 0
                    && year % 100 == 0 && year % 400 == 0) {

                if (day <= DaysInMonth_LeapYear[month - 1]) {

                    return true;
                }

            // otherwise, use the default array

            } else if (day <= DaysInMonth[month - 1]) {

                return true;
            }
        }

        // if the month is outside of the range 1 - 12, or the day is smaller than one,
        // or the value of the given day is greater than the number of days in the respective month,
        // return false.

        return false;
    }

    /**
     * To be able to declare further constructors, we need to declare this 0 -
     * constructor. (Would have been nice to know before, though)
     */
    public Date() {
        year = 0;
        month = 0;
        day = 0;
    }

    /**
     * A constructor that initializes Date with the parameters year, month and
     * day. If the given values don't represent a valid date, an
     * IllegalArgumentException is thrown.
     *
     * @param year
     *            the year of the date
     * @param month
     *            the month of the date
     * @param day
     *            the day of the date
     */
    public Date(int year, int month, int day) {

        if (validate(year, month, day) == false) {
            throw new IllegalArgumentException("This is not a valid date.");
        }

        this.year = year;
        this.month = month;
        this.day = day;

    }

    /**
     * Getter method to obtain the year of a date.
     *
     * @return year
     */
    public int getYear() {
        return this.year;
    }

    /**
     * Getter method to obtain the month of a date.
     *
     * @return month
     */
    public int getMonth() {
        return this.month;
    }

    /**
     * Getter method to obtain the day of a date.
     *
     * @return day
     */
    public int getDay() {
        return this.day;
    }

    /**
     * Calculates the position of a given day within the given year
     *
     * @return position of the day
     */
    public int dayOfYear() {

        // The variable n is used to sum up all days of the previous months.

        int n = 0;

        // as usual, if the year is a leap year, the leap year array is used.

        if (year % 4 == 0 && year % 100 != 0 || year % 4 == 0
                && year % 100 == 0 && year % 400 == 0) {

            for (int i = 0; i < this.month - 1; i++) {
                n += DaysInMonth_LeapYear[i];
            }

        } else {
            for (int i = 0; i < this.month - 1; i++) {
                n += DaysInMonth[i];
            }
        }

        // We add the days of our given date to n and thus get the position of the day within the year.

        int DayOfYear = this.day + n;
        return DayOfYear;
    }

    /**
     * Calculates the difference (in days) between two dates within the same
     * year
     *
     * @param other
     *            the other date
     * @return difference of the positions
     */
    public int sameYearDiff(Date other) {

        // The method will only work for dates with the same year
        // and otherwise return 0.

        if (this.year - other.year != 0) {
            return 0;
        }

        int diff = other.dayOfYear() - this.dayOfYear();
        return diff;
    }

    /**
     * Creates a string that displays a given date. The array that contains the names
     * of the months is used here.
     */
    public String toString() {

        String month = MonthNames[this.month - 1];
        String date = month + " " + this.day + ", " + this.year;
        return date;

    }
}