Java Date find day

Question

Schoolwork assignment…

I need to know which day and day name is on a specific date. And somehow I made a mistake because for years with 19.. it works but at 2000 I can't get the right day anymore. For example if I use the date 9.1.2001 it doesn't say me the day, instead the error of the switch I made occurs.

and i shouldn't use the calendar methods

here is my code:

public class FindDay {

    public static void main(String[] args) {
        System.out.println("Type a day: ");
        int day = In.readInt();
        System.out.println("Type a month: ");
        int month = In.readInt();
        System.out.println("Type a year: ");
        int year = In.readInt();
        
        if(day <  1 || day > 32) {
            System.out.println("Type valid day");
        }else if (month < 1 || month >12) {
            System.out.println("Type valid month");
        }else if (year < 1900) {
            System.out.println("Type valid year");
        }
        
        int wholeDaysYear; //to calculate how much days a year has
        
        if (year % 4 == 0 && !(year % 100 == 0 && year % 400 != 0) ) {
            wholeDaysYear = 366;
        }else {
            wholeDaysYear = 365;
        }
        int monthDays = 0;  //calculates days to this month
        int february;       //if February has 28 or 2 days
        if(wholeDaysYear ==366) {
            february = 29;
        }else {
            february = 28;
        }
        switch(month) { 
        case 1: monthDays= 0; break;
        case 2: monthDays =31; break;
        case 3: monthDays= 31+february; break;
        case 4: monthDays= 31+february+31; break;
        case 5: monthDays= 31+february+31+30; break;
        case 6: monthDays= 31+february+31+30+31; break;
        case 7: monthDays= 31+february+31+30+31+30; break;
        case 8: monthDays= 31+february+31+30+31+30+31; break;
        case 9: monthDays= 31+february+31+30+31+30+31+31; break;
        case 10: monthDays= 31+february+31+30+31+30+31+31+30; break;
        case 11: monthDays= 31+february+31+30+31+30+31+31+30+31; break;
        case 12: monthDays= 31+february+31+30+31+30+31+31+30+31+30; break;
        default: System.out.println("Enter valid month!");break;
        }
        
        int leapYear = ((year-1) / 4 - 474) - ((year-1) / 100 - 18) + ((year-1) / 400 - 4); //calculates the leap years
        int allDays =(((year - leapYear)-1900)*wholeDaysYear)+monthDays+day-1; //Calculates all days
        
        System.out.println(allDays);
        
        int dayName = allDays % 7;
        String dayNames = null; //gives the days their name
        switch (dayName) {
        case 1: dayNames = "Monday";break;
        case 2: dayNames = "Tuesday";break;
        case 3: dayNames = "Wendesday";break;
        case 4: dayNames = "Thursday";break;
        case 5: dayNames = "Friday";break;
        case 6: dayNames = "Saturday";break;
        case 7: dayNames = "Sunday";break;
        default: System.out.println("Type valid Input");break;
        }
        
        System.out.println(dayNames);
    }
}

Answer 1

You don't have to work so hard... assuming you have the day of the month, month of the year and full-year, you can simply do (example):

    LocalDate ld = LocalDate.of(1999, 12, 12);
    System.out.println(ld.getDayOfWeek()); // prints SUNDAY

LocalDate is available since Java 8.

As for your code, two bugs that are easy to spot are:

1. The cases in the switch go from 1 to 7 while they should go from 0 to 6 since you're calculating the reminder out of 7.
2. int dayName = allDays % 7; here you assume that the first day of the year is Monday (according to your case switch) which is not necessarily true.