public class Date2Demo {
	public static void main(String[] args) {
		FastDate fd = new FastDate(1, 30, 2020);
		System.out.println(fd);
		fd.next(7);
		System.out.println(fd);
		System.out.println();

		SmallDate sd = new SmallDate(1, 30, 2020);
		System.out.println(sd);
		sd.next(7);
		System.out.println(sd);
	}
}

abstract class Date {	//a class with a missing piece
	protected static final int length[] = {
		0,
		31,	//January
		28,	//February
		31,	//March
		30,	//April
		31,	//May
		30,	//June
		31,	//July
		31,	//August
		30,	//September
		31,	//October
		30,	//November
		31	//December
	};

	Date(int m, int d, int y) {
		if (y == 0) {
			System.err.println("No year 0.");
			System.exit(1);
		}

		if (m < 1 || m > 12) {
			System.err.println("Bad month " + m + ".");
			System.exit(1);
		}

		if (d < 1 || d > length[m]) {
			System.err.println("Bad day " + d
				+ " in month " + m + ".");
			System.exit(1);
		}
	}

	public void next(int i) {
		//Call the no-argument next method i times.
		for (; i > 0; --i) {
			next();
		}
	}

	//A missing piece

	abstract public void next();
}

class FastDate extends Date {
	private int year;
	private int month;
	private int day;

	public FastDate(int m, int d, int y) {
		super(m, d, y);
		year = y;
		month = m;
		day = d;
	}

	public String toString() {
		return month + "/" + day + "/" + year;
	}

	public void next() {
		if (++day > length[month]) {
			day = 1;
			if (++month > 12) {
				month = 1;
				if (year == Integer.MAX_VALUE) {
					System.err.println("already at maximum FastDate "
						+ this);
					System.exit(1);
				}
				++year;
			}
		}
	}
}

class SmallDate extends Date {
	private int day;	//# days before or after January 1, 1 A.D.

	public SmallDate(int m, int d, int y) {
		super(m, d, y);

		day = 365 * (y - 1) + d - 1;

		for (int i = 1; i < m; ++i) {
			day += length[i];
		}

		/*
		Could write the above loop as
		for (--m; m > 0; --m) {
			day += length[m];
		}
		*/
	}

	public void next() {
		if (day == Integer.MAX_VALUE) {
			System.err.println("already at maximum SmallDate "
				+ this);
			System.exit(1);
		}
		++day;
	}

	public String toString() {
		final int year = 1 + day / 365;
		int d          = 1 + day % 365;

		int m = 1;
		for (; d > length[m]; ++m) {
			d -= length[m];
		}
		return m + "/" + d + "/" + year;
	}
}