I am trying to write a program in Java (this is a school assignment that tells you what day of the week a certain date is. (The date should be written on the form yyyy-mm-dd.) I thought I had come up with a solution with the code below, but then I found an error.
When you run the code, and type in 1999-12-31 in the dialog, the program tells you that the entered date (1999-12-31) is a Friday. But when you type in the date 2000-01-01 (which is one day after 1999-12-31), the program tells you that the day is a Sunday! What happened with Saturday? A similar problem happens when you type in 2000-02-29 and 2000-03-01, they both give Wednesday as an answer!
What I have yet noticed, this error appears only when you enter a date between 2000-01-01 and 2000-02-29. I would be very grateful if someone could please help me to find the cause of the error and to solve the problem!
import static javax.swing.JOptionPane.*;
import static java.lang.Math.*;
public class DateCalc {
// Here, between the DateCalc class declaration and the main method, several methods used in the program are
// constructed.
// The method isLeapYear tests whether the entered year is a leap year or not.
private static boolean isALeapYear(int year) {
// If the year is a multiple of 4 and not a multiple of 100, or a multiple of 400, then it is a leap year.
if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
return true;
}
else {
return false;
}
}
// A method that tests whether a given string is written as a valid date.
private static boolean isAValidDate(int year, int month, int day) {
int maxValidYear = 9999;
int minValidYear = 1754;
if (year > maxValidYear || year < minValidYear) {
return false;
}
if (month < 1 || month > 12) {
return false;
}
if (day < 1 || day > 31) {
return false;
}
// Handle the February Month
if (month == 2) {
if (isALeapYear(year)) {
return (day <= 29); // This statement is true if the value of the day is less than or equal to 29 if the month is February within a leap year.
// Otherwise the statement is false and the method will return the boolean value false.
}
else {
return (day <= 28); // This statement is true if the value of the day is less than or equal to 28 if the month is February within a non-leap year.
// Otherwise the statement is false and the method will return the boolean value false.
}
}
// Month of April, June, September and November must have number of days less than or equal to 30.
if (month == 4 || month == 6 || month == 9 || month == 11) {
return (day <= 30);
}
return true;
}
// A method that calculates the day number within the year.
private static int dayNumberWithinYear(int year, int month, int day) {
// An array which stores the number of days in the different months (when the year is not a leap year).
// (Index 0 is the number of days in January, index 1 is the number of days in February, etc.)
int[] monthStructure = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
// If the entered year is a leap year, then the monthStructure array will be initialized with an extra day in February, i.e the leap day.
if (isALeapYear(year)) {
monthStructure[1] = 29;
}
int sumDaysInPreviousMonths = 0;
int daysInTheCurrentMonth = day;
int dayNumber = 0;
// Loops through all the months (index 0 is January, index 1 is February, etc.).
for (int i = 0; i < month - 1; i++) {
sumDaysInPreviousMonths += monthStructure[i];
}
dayNumber = sumDaysInPreviousMonths + daysInTheCurrentMonth;
return dayNumber;
}
// A method that decides the day of the week of an entered date.
private static void weekDay(int year, int month, int day) {
// The number of days that have passed since January 1, 1754, excluding the days of the entered year and
// excluding the leap days.
int sumDaysInOrdinaryYears = (year - 1754) * 365;
int sumLeapDaysInLeapYears = 0;
// Suppose the entered year is n. The for-loop goes through all the years from year n-1 to year 1754, and
// checks if the current year in the loop is a leap year. The number of leap years between year 1754 and n-1
// is equal to the number of days that will get added (beside from the days in ordinary years) to the total
// days from January 1, 1754 to the entered date.
for (; year > 1754; year -= 1) {
if (isALeapYear(year)) {
sumLeapDaysInLeapYears += 1;
}
}
// The sum of all days from year 1754 to year n-1 (if the entered year is n), is equal to the sum of days in
// the ordinary years and the leap days in the years.
int sumDaysInEveryYearExcludingTheEntered = sumDaysInOrdinaryYears + sumLeapDaysInLeapYears;
int sumDaysInTotalYears = sumDaysInEveryYearExcludingTheEntered + dayNumberWithinYear(year, month, day);
int weekDay = sumDaysInTotalYears % 7;
if (weekDay == 0) {
showMessageDialog(null, "The date is a monday.");
}
else if (weekDay == 1) {
showMessageDialog(null, "The date is a tuesday.");
}
else if (weekDay == 2) {
showMessageDialog(null, "The date is a wednesday.");
}
else if (weekDay == 3) {
showMessageDialog(null, "The date is a thursday.");
}
else if (weekDay == 4) {
showMessageDialog(null, "The date is a friday.");
}
else if (weekDay == 5) {
showMessageDialog(null, "The date is a saturday.");
}
// If weekDay == 6
else {
showMessageDialog(null, "The date is a sunday.");
}
}
public static void main(String[] args) {
// This is step 3 in the laboratory instruction.
while (true) {
String date = showInputDialog("Please, enter a date on the form yyyy-mm-dd");
// If the user clicks 'Cancel' or clicks 'OK' when the dialog box is empty, the program will exit.
if (date == null || date.length() == 0) {
break;
}
int y = Integer.parseInt(date.substring(0,4));
int m = Integer.parseInt(date.substring(5,7));
int d = Integer.parseInt(date.substring(8));
if (!isAValidDate(y, m, d)) {
showMessageDialog(null, "Error! The entered date is invalid. " +
" Please enter a valid date on the form yyyy-mm-dd");
}
else {
weekDay(y, m, d);
}
}
}
}
