Date.java

package HWs;
/* Date.java */

import java.io.*;

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

	/* Put your private data fields here. */

	/**
	 * Constructs a date with the given month, day and year. If the date is not
	 * valid, the entire program will halt with an error message.
	 * 
	 * @param month is a month, numbered in the range 1...12.
	 * @param day   is between 1 and the number of days in the given month.
	 * @param year  is the year in question, with no digits omitted.
	 */
	public Date(int month, int day, int year) {
		if (isValidDate(month, day, year)) {
			this.month = month;
			this.day = day;
			this.year = year;
		} else {
			System.out.println("Error: Date entered is invalid");
			System.exit(0);
		}
	}

	/**
	 * Constructs a Date object corresponding to the given string.
	 * 
	 * @param s should be a string of the form "month/day/year" where month must be
	 *          one or two digits, day must be one or two digits, and year must be
	 *          between 1 and 4 digits. If s does not match these requirements or is
	 *          not a valid date, the program halts with an error message.
	 */
	public Date(String s) {
		String monthTxt = s.substring(0, s.indexOf('/'));
		String dayTxt = s.substring(s.indexOf('/') + 1, s.indexOf('/', s.indexOf('/') + 1));
		String yearTxt = s.substring(s.indexOf('/', s.indexOf('/', s.indexOf('/') + 1)) + 1, s.length());

		if ((dayTxt.length() >= 1 && dayTxt.length() <= 2) && (monthTxt.length() >= 1 && monthTxt.length() <= 2)
				&& (yearTxt.length() >= 1 && yearTxt.length() <= 4)
				&& (isValidDate(Integer.parseInt(monthTxt), Integer.parseInt(dayTxt), Integer.parseInt(yearTxt)))) {
			month = Integer.parseInt(monthTxt);
			day = Integer.parseInt(dayTxt);
			year = Integer.parseInt(yearTxt);
		} else {
			System.out.println("Error: Date entered is invalid");
			System.exit(0);
		}

	}

	/**
	 * Checks whether the given year is a leap year.
	 * 
	 * @return true if and only if the input year is a leap year.
	 */
	public static boolean isLeapYear(int year) {
		return ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0);
	}

	/**
	 * Returns the number of days in a given month.
	 * 
	 * @param month is a month, numbered in the range 1...12.
	 * @param year  is the year in question, with no digits omitted.
	 * @return the number of days in the given month.
	 */
	public static int daysInMonth(int month, int year) {
		if (month == 9 || month == 4 || month == 6 || month == 11) {
			return 30;
		} else if (month == 2) {
			if (isLeapYear(year)) {
				return 29;
			} else {
				return 28;
			}
		} else {
			return 31;
		}
	}

	/**
	 * Checks whether the given date is valid.
	 * 
	 * @return true if and only if month/day/year constitute a valid date.
	 *
	 *         Years prior to A.D. 1 are NOT valid.
	 */
	public static boolean isValidDate(int month, int day, int year) {
		return (month >= 1 && month <= 12 && year >= 1 && day >= 1 && day <= daysInMonth(month, year));
	}

	/**
	 * Returns a string representation of this date in the form month/day/year. The
	 * month, day, and year are expressed in full as integers; for example,
	 * 12/7/2006 or 3/21/407.
	 * 
	 * @return a String representation of this date.
	 */
	public String toString() {
		return month + "/" + day + "/" + year;
	}

	/**
	 * Determines whether this Date is before the Date d.
	 * 
	 * @return true if and only if this Date is before d.
	 */
	public boolean isBefore(Date d) {
		if (year < d.year) {
			return true;
		} else if (year == d.year) {
			if (month < d.month) {
				return true;
			} else if (month == d.month) {
				return day < d.day;
			}
		}
		return false;
	}

	/**
	 * Determines whether this Date is after the Date d.
	 * 
	 * @return true if and only if this Date is after d.
	 */
	public boolean isAfter(Date d) {
		if (year > d.year) {
			return true;
		} else if (year == d.year) {
			if (month > d.month) {
				return true;
			} else if (month == d.month) {
				return day > d.day;
			}
		}
		return false;
	}

	/**
	 * Returns the number of this Date in the year.
	 * 
	 * @return a number n in the range 1...366, inclusive, such that this Date is
	 *         the nth day of its year. (366 is used only for December 31 in a leap
	 *         year.)
	 */
	public int dayInYear() {
		int count = 0;
		for (int i = 1; i < month; i++) {
			count += daysInMonth(i, year);
		}
		count += day;
		return count;
	}

	/**
	 * Determines the difference in days between d and this Date. For example, if
	 * this Date is 12/15/2012 and d is 12/14/2012, the difference is 1. If this
	 * Date occurs before d, the result is negative.
	 * 
	 * @return the difference in days between d and this date.
	 */
	public int difference(Date d) {
		int count1 = 0, count2 = 0;
		if(isAfter(d)) {
			for(int i = (year - d.year); i > 0; i--) {
				if(isLeapYear(year-i)) {
					count1 += 366;
				}else {
					count1 += 365;
				}
			}
		} else if(isBefore(d)) {
			for(int i = (d.year - year); i > 0; i--) {
				if(isLeapYear(d.year-i)) {
					count2 += 366;
				}else {
					count2 += 365;
				}
			}
		}
		count1 += dayInYear();
		count2 += d.dayInYear();
		
		return count1 - count2;
	}

	public static void main(String[] argv) {
		System.out.println("\nTesting constructors.");
		Date d1 = new Date(1, 1, 1);
		System.out.println("Date should be 1/1/1: " + d1);
		d1 = new Date("2/4/2");
		System.out.println("Date should be 2/4/2: " + d1);
		d1 = new Date("2/29/2000");
		System.out.println("Date should be 2/29/2000: " + d1);
		d1 = new Date("2/29/1904");
		System.out.println("Date should be 2/29/1904: " + d1);

		d1 = new Date(12, 31, 1975);
		System.out.println("Date should be 12/31/1975: " + d1);
		Date d2 = new Date("1/1/1976");
		System.out.println("Date should be 1/1/1976: " + d2);
		Date d3 = new Date("1/2/1976");
		System.out.println("Date should be 1/20/1976: " + d3);

		Date d4 = new Date("2/27/1977");
		Date d5 = new Date("8/31/2110");

		/* I recommend you write code to test the isLeapYear function! */

		System.out.println("\nTesting before and after.");
		System.out.println(d2 + " after " + d1 + " should be true: " + d2.isAfter(d1));
		System.out.println(d3 + " after " + d2 + " should be true: " + d3.isAfter(d2));
		System.out.println(d1 + " after " + d1 + " should be false: " + d1.isAfter(d1));
		System.out.println(d1 + " after " + d2 + " should be false: " + d1.isAfter(d2));
		System.out.println(d2 + " after " + d3 + " should be false: " + d2.isAfter(d3));

		System.out.println(d1 + " before " + d2 + " should be true: " + d1.isBefore(d2));
		System.out.println(d2 + " before " + d3 + " should be true: " + d2.isBefore(d3));
		System.out.println(d1 + " before " + d1 + " should be false: " + d1.isBefore(d1));
		System.out.println(d2 + " before " + d1 + " should be false: " + d2.isBefore(d1));
		System.out.println(d3 + " before " + d2 + " should be false: " + d3.isBefore(d2));

		System.out.println("\nTesting difference.");
		System.out.println(d1 + " - " + d1 + " should be 0: " + d1.difference(d1));
		System.out.println(d2 + " - " + d1 + " should be 1: " + d2.difference(d1));
		System.out.println(d3 + " - " + d1 + " should be 2: " + d3.difference(d1));
		System.out.println(d3 + " - " + d4 + " should be -422: " + d3.difference(d4));
		System.out.println(d5 + " - " + d4 + " should be 48762: " + d5.difference(d4));
	}
}