Java program to print day name for given date

Write a  java program to accept a date in the string format dd/mm/yyyy and accept the name of the day on the the 1st January of the corresponding year. Find the day for the given date.

Example: Input:

Date: 5/7/2001

Day on 1st January: MONDAY

Output:

Day on 5/7/2001

THURSDAY

Run the program on the Following input:

import java.util.*;

public class DayName
{
    public static boolean isLeapYear(int y) 
    {
        boolean ret = false;

        if (y % 400 == 0) {
            ret = true;
        }
        else if (y % 100 == 0) {
            ret = false;
        }
        else if (y % 4 == 0) {
            ret = true;
        }
        else {
            ret = false;
        }

        return ret;
    }
    public static void main(String args[]) 
    {
        int monthDays[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        String dayNames[] = {"MONDAY", "TUESDAY", "WEDNESDAY", "THURSDAY",
                            "FRIDAY", "SATURDAY", "SUNDAY"};
        Scanner in = new Scanner(System.in);
        System.out.print("Enter Date(dd/mm/yyyy): ");
        String dateStr = in.nextLine();
        StringTokenizer st = new StringTokenizer(dateStr, "/");
        int tokenCount = st.countTokens();
        if (tokenCount <= 0 || tokenCount > 3) {
            System.out.println("Invalid Date");
            return;
        }

        int day = Integer.parseInt(st.nextToken());
        int month = Integer.parseInt(st.nextToken());
        int year = Integer.parseInt(st.nextToken());
        
        boolean leapYear = isLeapYear(year);
        if (leapYear) {
            monthDays[1] = 29;
        }
        
        if (month < 1 || month > 12) {
            System.out.println("Invalid Month");
            return;
        }
        
        if (day < 1 || day > monthDays[month - 1]) {
            System.out.println("Invalid Day");
            return;
        }

        System.out.print("Day on 1st January: ");
        String startDayName = in.nextLine();
        int startDayIdx = -1;
        for (int i = 0; i < dayNames.length; i++) {
            if (dayNames[i].equalsIgnoreCase(startDayName)) {
                startDayIdx = i;
                break;
            }
        }
        
        if (startDayIdx == -1) 
        {
            System.out.println("Invalid Day Name");
            return;
        }
        //Calculate total days
        int tDays = 0;
        for (int i = 0; i < month - 1; i++) {
            tDays += monthDays[i];
        }
        
        tDays += day;
        
        int currDayIdx = tDays % 7 + startDayIdx - 1;
        if (currDayIdx >= 7) {
            currDayIdx -= 7;
        }
        
        System.out.println("Day on " + dateStr + " : " + dayNames[currDayIdx]);
    }   
}

Enter Date(dd/mm/yyyy): 04/9/1998

Day on 1st January: THURSDAY

Day on 04/9/1998 : FRIDAY