Develop Reference

How to handle invalid input

Can anyone suggest an example on how to handle invalid inputs, I have got the logic wrong for input from the keyboard. So, I need !myScanner.hasNextInt() or to put that loop in a method which I can reuse for ever.
package dayoftheyear;
import java.util.Scanner;
/**
*
* #author abdal
*/
public class DayOfTheYear {
/**
* Ask the user to enter a month, day, and year as integers
* separated by spaces then display the number of the days since the
* beginning of the year.
* Don’t forget about leap year.
* Sample: user inputs ‘3 1 2000’, output is ‘61’.
* #param args Unused.
*/
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
String enter = "A-z";
int a=31;
int b=1;
boolean dateCheck;
int month;
int day;
int year;
do {
System.out.print("Enter a valid month day year separated by spaces");
if (s.hasNextInt()) {
month= s.nextInt();
day=s.nextInt();
year=s.nextInt();
if (month >= b && month <= a || day>=b && day<=a || year>=b) {
int numberOfDay = countDays(month, day, year);
System.out.println(+ month + "/" + day + "/" + year + " is a day number "
+ numberOfDay + " of that year");
dateCheck = true;
} else {
System.out.println("Enter a valid month day year separated by spaces");
dateCheck = false;
}
} else {
System.out.println("Not a date");
month = 0;
day=0;
year=0;
s.next();
dateCheck = false;
}
} while (!dateCheck);
/**
* Get the number of days since the start of the year.
* Declare a 12 element array and initialize it with the number of
* days in each month.
* #param month Month to count from.
* #param day Day of the month to count to.
* #param year The year to count from.
* #return Days since the start of the given year.
*/
} public static int countDays(int month, int day, int year) {
int monthLength[] = {
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};
int days = 0;
if (isLeapYear(year) && month > 2)
days += 1;
for (int i = 0; i < month - 1; i++)
days += monthLength[i];
return days += day;
}
/**
* Check if a year is a leap year.
* #param year Year to check.
* #return True if the year is a leap year.
*/
public static boolean isLeapYear(int year) {
if (year % 4 != 0) return false;
else if (year % 100 != 0) return true;
else return year % 400 == 0;
}
}

Replace
if (month >= b && month <= a || day>=b && day<=a || year>=b)
with
if (month >= 1 && month <= 12 && day >= 1 && day <= 31 && year >= 1)
A sample run after this change:
Enter a valid month day year separated by spaces: 13 1 2000
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 3 32 2000
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 3 1 2
3/1/2 is a day number 60 of that year
Update1: the following condition will hold good also for February
if ((month == 2 && day >= 1 && day <= 28 && year >= 1 && !isLeapYear(year))
|| (month == 2 && day >= 1 && day <= 29 && year >= 1 && isLeapYear(year))
|| (month != 2 && month >= 1 && month <= 12 && day >= 1 && day <= 31 && year >= 1))
A sample run:
Enter a valid month day year separated by spaces: 2 29 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 13 2 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 1 32 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 1 2 3
1/2/3 is a day number 2 of that year
Update2: the following code addresses all the issues raised in the comments
int monthLength[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
if ((month == 2 && day >= 1 && day <= 28 && year >= 1 && !isLeapYear(year))
|| (month == 2 && day >= 1 && day <= 29 && year >= 1 && isLeapYear(year))
|| (month != 2 && month >= 1 && month <= 12 && day >= 1 && day <= monthLength[month-1] && year >= 1))
A sample run:
Enter a valid month day year separated by spaces: 2 29 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 13 2 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 1 32 2001
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 6 31 2020
Enter a valid month day year separated by spaces
Enter a valid month day year separated by spaces: 7 31 2020
7/31/2020 is a day number 213 of that year

Since date format is not subject to change in real world, it should be appropriate to use some hard coding. Dividing the problem into several methods is always recommended.
You could add following methods to your class:
private static boolean validDate(int month, int day, int year) {
if (year < 1) {
return false; // no B.C.
}
if (month > 1 && month < 13) {
if (month == 2) { // handle February
return validFeb(day, year);
} else if (month % 2 == 1 && month < 8
|| month % 2 == 0 && month >= 8) { // 31-day months
return valid31(day);
} else { // 30 day months
return valid30(day);
}
}
return false;
}
Validate 30-day months:
private static boolean valid30(int day) {
if (day > 1 && day < 31) {
return true;
}
return false;
}
Validate 31-day months
private static boolean valid31(int day) {
if (day > 1 && day < 32) {
return true;
}
return false;
}
Validate February
private static boolean validFeb(int day, int year) {
if (isLeapYear(year)) {
if (day > 1 && day < 30) {
return true;
}
} else {
if (day > 1 && day < 29) {
return true;
}
}
return false;
}
Then your do-while loop should look something like so:
do {
System.out.print("Enter a valid month day year separated by spaces\n");
if (s.hasNextInt()) month = s.nextInt();
else s.next();
if (s.hasNextInt()) day = s.nextInt();
else s.next();
if (s.hasNextInt()) year = s.nextInt();
else s.next();
int numberOfDaysSinceStart = 0;
if (validDate(month, day, year)) {
dateCheck = true;
numberOfDaysSinceStart = countDays(month, day, year);
System.out.println(month + "/" + day + "/" + year + " is a day number "
+ numberOfDaysSinceStart + " of that year");
} else {
dateCheck = false;
}
} while (!dateCheck);

You can add edge cases forever. There is a reason why time-related calculations are nightmare outsourced to libraries written by some poor souls that are paid to try to cover them all. Java has such built in, take a look at java.util.Calendar (Gregorian implementation).
You set it to year/month/day, and it will puke out an exception if anything is wrong while trying to get the result.
Calendar c = Calendar.getInstance();
c.set(year, month, day);
try {
c.getTime();
} catch (Exception e) {
// wrong date format
}

I think the code below can help you, comment if you have any questions.
while(true) {
try{
System.out.print("Enter a valid month day year separated by spaces");
month= s.nextInt();
day=s.nextInt();
year=s.nextInt();
if (month >= 1 && month <= 12 || day>=1 && day<=31 || year>=1) {
System.out.println(+ month + "/" + day + "/" + year + " is a day number "+ " of that year");
break;
} else {
System.out.println("Enter a valid month day year separated by spaces");
}
} catch(InputMismatchException e) {
System.out.println("Enter a valid month day year separated by spaces");
}
s.next();
}

thanks I updated my program and it is working;
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int year = 0;
int month = 0;
int day = 0;
boolean dateCheck;
do {
System.out.print("Enter a valid month day year separated by spaces\n");
if (s.hasNextInt()) month = s.nextInt();
else s.next();
if (s.hasNextInt()) day = s.nextInt();
else s.next();
if (s.hasNextInt()) year = s.nextInt();
else s.next();
int numberOfDaysSinceStart = 0;
if (month >= 1 && month <= 12 && day >= 1 && day <= 31 && year >= 1) {
dateCheck = true;
numberOfDaysSinceStart = countDays(month, day, year);
System.out.println(month + "/" + day + "/" + year + " is a day number "
+ numberOfDaysSinceStart + " of that year");
} else {
dateCheck = false;
}
} while (!dateCheck);
/**
* Get the number of days since the start of the year.
* Declare a 12 element array and initialize it with the number of
* days in each month.
* #param month Month to count from.
* #param day Day of the month to count to.
* #param year The year to count from.
* #return Days since the start of the given year.
*/
} public static int countDays(int month, int day, int year) {
int monthLength[] = {
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};
int days = 0;
if (isLeapYear(year) && month > 2)
days += 1;
for (int i = 0; i < month - 1; i++)
days += monthLength[i];
return days += day;
}
/**
* Check if a year is a leap year.
* #param year Year to check.
* #return True if the year is a leap year.
*/
public static boolean isLeapYear(int year) {
if (year % 4 != 0) return false;
else if (year % 100 != 0) return true;
else return year % 400 == 0;
}
}

Date calculator: Tell what day of the week a certain date is

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);
}
}
}
}

Instead of asking us to debug through your entire code, perhaps consider LocalDate to get the desired result:
LocalDate ldt = LocalDate.parse("1999-12-31");
System.out.println(ldt.getDayOfWeek());
LocalDate ldt2 = LocalDate.parse("2000-01-01");
System.out.println(ldt2.getDayOfWeek());
Output:
FRIDAY
SATURDAY

The Problem is with finding the number of leap year. Your logic is counting the year 2000 also. The number of leap years should be same for 1999-12-31 and 2000-01-01. You need to consider year 2000 only if the month is greater than February. Increment the sumLeapDaysInLeapYears only if the input date is greater than Feb 28th

Swap Value of an Array in a Loop

I was trying to build a beginner calendar exercise project and all I want is if last date of January ends on Sunday then start next month from Monday. Here is my code;
public static void main(String[] args)
{
String[] months= {"January","Febuary","March","April","May","June"
,"July","August","september","October","November"
,"December"
};
String[] weekdays= {"Sun","Mon","Tue","Wed","Thu","Fri","Sat"};
int x;
for(int k=0;k<12;k++)
{
if(months[k]=="April" ||months[k]=="june" ||months[k]=="September" ||months[k]=="November")
{x=30;}
else if(months[k]=="Febuary")
{x=28;}
else {x=31;}
System.out.print(months[k]+"\n");
for(int i=0;i<weekdays.length;i++)
{
System.out.print("\t"+weekdays[i]);
}System.out.println();
for(int m=1;m<=x;m++) {
if(((m-1) %7) == 0) //line break after 7 characters
{
System.out.println();
}
System.out.print("\t"+m);
}
System.out.println();
}
}

I think you use not correct approach. Working with dated relates to working with Locale, because all these names like name for the month, weekdays in different format are already in the JVM. You should use it:
public static void printCalendar(LocalDate date, Locale locale) {
DateFormatSymbols symbols = new DateFormatSymbols(locale);
WeekFields weekFields = WeekFields.of(locale);
printMonthName(symbols, date);
printWeekdayNames(symbols, weekFields);
printWeekdays(weekFields, date);
}
private static void printMonthName(DateFormatSymbols symbols, LocalDate date) {
System.out.println(symbols.getMonths()[date.getMonthValue() - 1]);
}
private static void printWeekdayNames(DateFormatSymbols symbols, WeekFields weekFields) {
String[] weekdays = symbols.getShortWeekdays();
DayOfWeek firstDayOfWeek = weekFields.getFirstDayOfWeek();
int offs = firstDayOfWeek == DayOfWeek.SUNDAY ? 1 : firstDayOfWeek.ordinal() + 2;
for (int i = 0; i < 7; i++)
System.out.print('\t' + (offs + i >= weekdays.length ? weekdays[(offs + i) % 7] : weekdays[offs + i]));
System.out.println();
}
private static void printWeekdays(WeekFields weekFields, LocalDate date) {
LocalDate cur = date.withDayOfMonth(1).with(weekFields.dayOfWeek(), 1);
boolean stop = false;
do {
if (cur.getMonthValue() == date.getMonthValue())
System.out.format("\t%2d", cur.getDayOfMonth());
else
System.out.format("\t ");
cur = cur.plusDays(1);
if (cur.getDayOfWeek() == weekFields.getFirstDayOfWeek()) {
System.out.println();
stop = cur.getMonthValue() != date.getMonthValue();
}
} while (!stop);
}
Test1:
printCalendar(LocalDate.of(2018, Month.DECEMBER, 17), Locale.US);
December
Sun Mon Tue Wed Thu Fri Sat
1
2 3 4 5 6 7 8
9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30 31
Test2:
printCalendar(LocalDate.of(2018, Month.DECEMBER, 17), Locale.ITALIAN);
dicembre
lun mar mer gio ven sab dom
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31

Your program still needs some adjustments:
1. The first day of the year can be sunday or other day of the week.
2. If the last day of the of the month is sunday, you should continue the order and not starting with sunday from the beginning.
I will propose you to adjust you current code, or you can use an existing library https://www.javatpoint.com/java-util-calendar.

I'm writing a calendar program in java, but besides oct 2015 the months dont match, i need help please

So basicaly, everything works like it should. But the main thing that should be correct, which is the date, is wrong, so far for october 2015 it is correct but for some reason i cannot detect, the other months i have tried so far are incorrect when compared to the calendar in my computer.
Here is my program, maybe you have some valid input to help me with these issues.
public static boolean isLeapYear(int year) {
int month = 0;
int s = getDaysIn(month, year);
return year % 4 == 0 && (year % 100 != 0) || (year % 400 == 0);
}
public static int getDaysIn(int month, int year) {
switch (month) {
case 1:
return 31;
case 2:
if (isLeapYear(month)) {
return 29;
} else {
return 28;
}
case 3:
return 31;
case 4:
return 30;
case 5:
return 31;
case 6:
return 30;
case 7:
return 31;
case 8:
return 31;
case 9:
return 30;
case 10:
return 31;
case 11:
return 30;
case 12:
return 31;
default:
return -1;
}
}
public static String getMonthName(int month) {
switch (month) {
case 1:
return "January";
case 2:
return "February";
case 3:
return "March";
case 4:
return "April";
case 5:
return "May";
case 6:
return "June";
case 7:
return "July";
case 8:
return "August";
case 9:
return "September";
case 10:
return "October";
case 11:
return "November";
case 12:
return "December";
default:
return "Invalid month.";
}
}
public static int getStartDay(int month, int year) {
int days = 0;
for (int i = 1900; i < year; i++) {
days = days + 365;
if (isLeapYear(i)) {
days = days + 1;
}
}
for (int i = 1; i < month; i++) {
days = days + getDaysIn(month, year);
}
int startday = (days + 1) % 7 + 2;
return startday;
}
public static void displayCalendar(int month, int year) {
String monthName = getMonthName(month);
int startDay = getStartDay(month, year);
int monthDays = getDaysIn(month, year);
System.out.println(" Sun Mon Tue Wed Thu Fri Sat");
int weekDay = startDay - 1;
for (int i = 1; i < startDay; i = i + 1) {
System.out.print(" ");
}
for (int x = 1; x <= monthDays; x++) {
weekDay = weekDay + 1;
if (weekDay > 7) {
System.out.println();
weekDay = 1;
}
System.out.format(" %3d", x);
}
if (weekDay > 7) {
System.out.println();
}
}

Your getStartDay is actually where your error lays :
for (int i = 1; i < month; i++) {
days = days + getDaysIn(month, year);
}
int startday = (days + 1) % 7 + 2;
return startday;
Should be replaced with :
for (int i = 1; i < month; i++) {
days = days + getDaysIn(i, year);
}
int startday = (days % 7) + 2;
return startday;
There is a minor problem with your loop as you see.
You should have had used your incrementor i in your getDaysIn calls to count the days from the begin of the year until the month you want to display.
Second error is here :
int startday = (days + 1) % 7 + 2;
The logic behind this expression is still a mystery for me.
Let's break this off with a concrete example :
Let's say we want to display FEB 1900. Easy to do because we start at the begin of JAN 1900 so we can count the days easily (31 days in january so days = 31)
Which result will the first expression return?
= (31 + 1) % 7 + 2
= 32 % 7 + 2
= 4 + 2
= 6
Because day 1 in your logic is SUNDAY, day 6 is FRIDAY.
Let's check it out :
Not really the expected result right?
I see one more problem with this method.
Let's imagine days = 33 (this will never happens but we need this example here).
= (33 + 1) % 7 + 2
= 34 % 7 + 2
= 6 + 2
= 8
What would the 8th day of the week be?
The explanation of the correct answer is the following :
(days % 7) + 2
days % 7 gives us the value of the last day of the following month when weeks begin with MONDAY.
To this value, we add 2 :
1 to have the beginning day of the wanted month.
1 because our calendar begins with sunday and we'll have to slide our days on the calendar view.
Here is the full code (that I also modified a bit to make it more compact) :
public class Calendar{
public static void main(String[] args) {
// INSERT YOUR TESTS HERE
}
public static boolean isLeapYear(int year) {
return year % 4 == 0 && (year % 100 != 0) || (year % 400 == 0);
}
public static int getDaysIn(int month, int year) {
switch (month) {
case 1 : case 3 : case 5 : case 7 : case 8 : case 10 : case 12 :
return 31;
case 4 : case 6 : case 9 : case 11 :
return 30;
case 2:
return isLeapYear(year) ? 29 : 28;
default:
return -1;
}
}
public static String getMonthName(int month) {
String[] months = {"January", "February", "March", "April", "May", "June",
"July", "August", "September", "October", "November", "December"};
return month <= 12 && month > 0 ? months[month - 1] : "Invalid month";
}
public static int getStartDay(int month, int year) {
int days = 0;
for (int i = 1900; i < year; i++) {
days += 365;
if (isLeapYear(i)) {
days = days + 1;
}
}
for (int i = 1; i < month; i++) {
days = days + getDaysIn(i, year);
}
int startday = (days % 7) + 2;
return startday;
}
public static void displayCalendar(int month, int year) {
String monthName = getMonthName(month);
int startDay = getStartDay(month, year);
int monthDays = getDaysIn(month, year);
System.out.println(" Sun Mon Tue Wed Thu Fri Sat");
int weekDay = startDay - 1;
for (int i = 1; i < startDay; i = i + 1) {
System.out.print(" ");
}
for (int x = 1; x <= monthDays; x++) {
weekDay = weekDay + 1;
if (weekDay > 7) {
System.out.println();
weekDay = 1;
}
System.out.format(" %3d", x);
}
if (weekDay > 7) {
System.out.println();
}
}
}
EDIT
You should try to write something in your main method if nothing displays.
public static void main(String[] args) {
for (int i = 0 ; i < 5 ; i++){
System.out.println(getMonthName(i+1) + " " + (2010 + i));
displayCalendar(i+1, 2010 + i);
System.out.println();
}
}
Gives the following output :
January 2010
Sun Mon Tue Wed Thu Fri Sat
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
February 2011
Sun Mon Tue Wed Thu Fri Sat
1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28
March 2012
Sun Mon Tue Wed Thu Fri Sat
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31
April 2013
Sun Mon Tue Wed Thu Fri Sat
1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30
May 2014
Sun Mon Tue Wed Thu Fri Sat
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

Mistake in my code and lucky answer

Spoiler alert!! for project euler problem 19.
The problem is how many sundays fall on the first of the month from 1901 to 2000. Given information: Jan 1st 1900 was a monday, Jan & march etc have 31 days, April, June etc 30 days and a leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400. which in this case applies to year 2000.
I got the correct answer, but when I checked my results with a real calendar, it turns out my code is counting saturdays. I don't see why it is doing that, can someone help?!
public class Sunday {
public static void main(String[] args) {
int sundayCount = 0;
int currentday = 0; // monday is 0, sunday will be 6
int dayLimit = 0;
for (int year = 1900; year < 2001; year++) {
for (int month = 1; month < 13; month++) {
// 30 days April, June, September, November
if (month == 4 || month == 6 || month == 9 || month == 11)
dayLimit = 30;
// leap year
else if (month == 2 && year % 4 == 0)
dayLimit = 29;
// February not leap year
else if (month == 2 && year % 4 != 0)
dayLimit = 28;
// Jan, March, May, July, August, October, December
else
dayLimit = 31;
for (int day = 1; day <= dayLimit; day++) {
if (day == 1 && currentday == 6 && year > 1900) {
System.out.println("year: " + year);
System.out.println("month: " + month);
sundayCount++;
}
if (currentday < 6)
currentday++;
else
currentday = 0;
}
}
}
System.out.println(sundayCount);
}
}

You forget to add into the code to check for if febuary 1900 should have 29 or 28 days. As 1900 wasn't a leap year it shouldn't have 29 days in 1900.
else if (month == 2 && year % 4 == 0 && year != 1900)
dayLimit = 29;
// February not leap year
else if (month == 2)
dayLimit = 28;
This should fix it.