Monthly calendar. Help with simplification

[Tony Fisher]

Hello all. The code below builds monthly calendar. It was my homeclass and it works fine. But it looks pretty long and complicated. Can it be simplified? How?
Thank you

public class Calendar {
    public static void main(String[] args){
        int m = Integer.parseInt (args[0]);
        int y = Integer.parseInt (args[1]);
        
// check if leapyear
        boolean isLeapYear;
        isLeapYear = (y % 4 == 0);
        isLeapYear = isLeapYear && (y % 100 != 0);
        isLeapYear = isLeapYear || (y % 400 == 0);
        
// defining month name amd days qty
           String monthName = "";
        switch (m) {
            case 1:  monthName = "January";
                     break;
            case 2:  monthName = "February";
                     break;
            case 3:  monthName = "March";
                     break;
            case 4:  monthName = "April";
                     break;
            case 5:  monthName = "May";
                     break;
            case 6:  monthName = "June";
                     break;
            case 7:  monthName = "July";
                     break;
            case 8:  monthName = "August";
                     break;
            case 9:  monthName = "September";
                     break;
            case 10: monthName = "October";
                     break;
            case 11: monthName = "November";
                     break;
            case 12: monthName = "December";
                     break;
        }
        double monthDays = 0;
        switch(m) {
            case 1: case 3: case 5: case 7: case 8: case 10: case 12:
                monthDays = 31;
                break;
            case 4: case 6: case 9: case 11:
                monthDays = 30;
                break;
            case 2:
                if(isLeapYear == true) monthDays = 29;
                else monthDays = 28;
                break;
        }
               
// first day of the month
        int d = 1;
        int y0 = y - (14 - m) / 12;
        int x = y0 + y0/4 - y0/100 + y0/400;
        int m0 = m + 12 * ((14 - m) / 12) - 2;
        int d0 = (d + x + (31*m0)/12) % 7;
        
//weeks qty
        double weekRows = Math.ceil((monthDays+d0)/7);
               
//calendar header
        System.out.println("  "+ monthName +" "+y);
        System.out.println("S  M  T  W  T  F  S");
        
//calendar days
        for (int j = 1; j <= weekRows; j++){
            int weekN = j*7;
            for(int i = 0; i <7; i++){
                int numDays = i-d0+1+weekN-7;
                if (i+weekN-7<d0){
                    System.out.print ("   ");
                }
                else {
                    if (numDays < 10){
                    System.out.print(numDays+"  ");
                    }
                    if (numDays > 9 && numDays <= monthDays){
                        System.out.print(numDays+" ");
                    }
                }
            }
            System.out.println();
        }
    }
}        

[Junilu Lacar]

You could use the standard date/time library https://docs.oracle.com/javase/tutorial/datetime/

[Liutauras Vilda]

1. You have got only 1 method in your class, and that method is named "main" which suppose to start your program only.
And it should look similar to:

public class Calendar {

    public static void main(String[] args) {
        new Calendar().runProgram(args);
    }

    private void runProgram(String[] args) {
        // code
    }
}

2. Check if leap year suppose to become a method named isLeapYear() and return "true" if it is, and "false" if it is not.
3. Define month suppose to be a separate method also, which a singular task to define which month it is. Could be named for ex. "defineMonthName()"
4. Define the day of the month should be method also, and take a month name as a parameter.

And likely more methods. So, after you get methods created, your program could be similar to:

public class Calendar {

    public static void main(String[] args) {
        new Calendar().runProgram(args);
    }

    private void runProgram(String[] args) {
        int year = Integer.parseInt(args[1]);
        int monthNumber = Integer.parseInt(args[0]);
        String month = defineMonthName(monthNumber);
        int day;

        if (isLeapYear(year)) {
            day = defineMonthDay(monthName);
        }
        else {
           // do another thing
        }

        printCalendar(year, month, day);
    }

    private boolean isLeapYear(int year) {
        // code
    }

    private String defineMonthName(int monthNumber) {
        // code
    }

    // more methods
}

After all this, you might find that you can simplify some parts. If you write program by placing all code in one method, it is very difficult to improve something without breaking down something.
Also, it is very difficult to follow your program flow. For the simple programs one class maybe is enough, but one method is not enough in pretty much all cases. Unless it is a "Hello, World" program.

[Tony Fisher]

Thank you good sir.