1/**
 2 * Assignment 2, COMP268 Class: WeekDay.java
 3 *
 4 * @description A class used to determine the week day for any given date.
 5 * @author: mo khan Student ID: 3431709
 6 * @date Jul 13, 2019
 7 * @version 1.0
 8 */
 9package Q6;
10
11import java.util.*;
12
13public class WeekDay {
14  private static final String[] DAYS =
15      new String[] {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
16  private static final int[] MONTHS = new int[] {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
17
18  /**
19   * Returns the day of the week that a certain date falls on.
20   *
21   * @param day the day of the month
22   * @param month the month of the year
23   * @param year the year
24   * @return the name of the day of the week. (Sunday to Saturday)
25   */
26  public String getWeekDay(int day, int month, int year) {
27    this.ensureValidDate(year, month, day);
28    return DAYS[daysSinceEpoch(day, month, year) % 7];
29  }
30
31  private int daysSinceEpoch(int day, int month, int year) {
32    int days = ((year - 1900) * 365) + ((year - 1900) / 4);
33    if (isLeapYear(year) && (month == 1 || month == 2)) days -= 1;
34    days += daysThisYearUpTo(day, month);
35    return days;
36  }
37
38  private boolean isLeapYear(int year) {
39    return year % 4 == 0;
40  }
41
42  private int daysThisYearUpTo(int day, int month) {
43    int days = 0;
44    for (int i = 1; i < month; i++) days += daysInMonth(i);
45    return days + day;
46  }
47
48  private int daysInMonth(int month) {
49    return MONTHS[month - 1];
50  }
51
52  private void ensureValidDate(int year, int month, int day) {
53    if (month < 1 || month > 12 || day < 1 || day > MONTHS[month - 1])
54      throw new IllegalArgumentException();
55  }
56
57  /**
58   * The entry point to the console application.
59   *
60   * @param args the argument vector given to the program.
61   */
62  public static void main(String[] args) {
63    Scanner in = new Scanner(System.in);
64
65    System.out.println("--- Question 6 ---");
66    System.out.println("Enter year:");
67    int year = in.nextInt();
68    if (year < 1900 || year > 2100) {
69      System.out.println("Invalid year.");
70      System.exit(-1);
71    }
72
73    System.out.println("Enter month:");
74    int month = in.nextInt();
75    if (month < 1 || month > 12 || (year == 1900 && month < 3) || (year == 2100 && month > 2)) {
76      System.out.println("Invalid month.");
77      System.exit(-1);
78    }
79
80    System.out.println("Enter day:");
81    int day = in.nextInt();
82
83    System.out.println();
84    System.out.println("Today is:");
85
86    try {
87      System.out.println(new WeekDay().getWeekDay(day, month, year));
88    } catch (IllegalArgumentException error) {
89      System.out.println("Invalid date.");
90      System.exit(-1);
91    }
92    System.out.println();
93  }
94}