我正在尝试使用 Java 做一个日历程序，但是当它打印时，月初是错误的，它弄乱了输出

Question

I'm a beginner in programming and I am making a Java program for calendar. I don't know how to make the start of the month on a certain day. As an example, I want the start of November 2022 to be on Tuesday. My current code messed up the output, please help. Here's my code:

import java.util.Scanner;
public class printCalendar {
    //calculating the month's days
    public static int dayOfMonth(int mon, int day, int yr) {
        int y = yr - (14 - mon) / 12;
        int x = y + y/4 - y/100 + y/400;
        int m = mon + 12 * ((14 - mon) / 12) - 2;
        int d = (day + x + (31*m)/12) % 7;
        return d;
    }

    //function for leap year detection
    public static boolean leapYear(int yr) {
        if ((yr % 4 == 0) && (yr % 100 != 0))
            return true;
        if (yr % 400 == 0)
            return true;

        return false;
    }

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        System.out.print("Enter full year (e.g., 2022): ");
        int yr = scan.nextInt();
        System.out.print("Enter month as a number between 1 and 12: ");
        int mon = scan.nextInt();
        

        String[] month = {"",
            "January", "February", "March",
            "April", "May", "June",
            "July", "August", "September",
            "October", "November", "December"
        };
        // days of a month ( maximum 31 )
        int[] days = {
            0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
        };

        // Leap year checking
        if (mon == 2 && leapYear(yr)) days[mon] = 29;

        // month name and days name
        System.out.println(" " + month[mon] + " " + yr);
        System.out.println("----------------------------");
        System.out.println("Sun Mon Tue Wed Thu Fri Sat");

        // first day
        int d = dayOfMonth(mon, 1, yr);

        // printing the calendar
        for (int i = 0; i < d; i++)
            System.out.print(" ");
        for (int i = 1; i <= days[mon]; i++) {
            System.out.printf("%2d ", i);
            if (((i + d) % 7 == 0) || (i == days[mon])) System.out.println();
        }           
    }
}

Here's the example output if I want the program to display November 2022 using the code I attached. I want the start of the month to be Tuesday (like a real calendar) instead of Sunday. enter image description here

Answer 1

I suggested you can try the way to format a string with printf or stirng.format to correct messy output. I've verified the code you posted and used the printf method to gave a space of 4 character for every words.

If you would like to see the effect, the below is what I'm trying.

import java.util.Scanner;
public class StackOverFlowTest {
    //calculating the month's days
    public static int dayOfMonth(int mon, int day, int yr) {
        int y = yr - (14 - mon) / 12;
        int x = y + y/4 - y/100 + y/400;
        int m = mon + 12 * ((14 - mon) / 12) - 2;
        int d = (day + x + (31*m)/12) % 7;
        return d;
    }

    //function for leap year detection
    public static boolean leapYear(int yr) {
        if ((yr % 4 == 0) && (yr % 100 != 0))
            return true;
        return yr % 400 == 0;
    }

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        System.out.print("Enter full year (e.g., 2022): ");
        int yr = scan.nextInt();
        System.out.print("Enter month as a number between 1 and 12: ");
        int mon = scan.nextInt();

        String[] month = {"",
                "January", "February", "March",
                "April", "May", "June",
                "July", "August", "September",
                "October", "November", "December"
        };
        // days of a month ( maximum 31 )
        int[] days = {
                0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
        };

        // Leap year checking
        if ( mon== 2 && leapYear(yr)) days[mon] = 29;

        // month name and days name
        System.out.println(" " + month[mon] + " " + yr);
        System.out.println("----------------------------");
        String[] dayOfWeek = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};

        // first day
        int d = dayOfMonth(mon, 1, yr);

        // printing the dayOfWeek
        for (String s : dayOfWeek) {
            System.out.printf("%-4s", s);
        }
        System.out.printf("%n");

        // printing the calendar
        for (int i = 0; i < d; i++)
            System.out.printf("%4s", "");
        for (int i = 1; i <= days[mon]; i++) {
            System.out.printf("%-4d", i);
            if (((i + d) % 7 == 0) || (i == days[mon])) System.out.println();
        }
    }
}

OUTPUT:

Enter full year (e.g., 2022): 2022
Enter month as a number between 1 and 12: 11
 November 2022
----------------------------
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