Determining valid dates in Java

Question

I have a homework assignment which asks to have the user input a date in Java in the (mm/dd/yyyy) format, then to determine if the entered date is valid. I have been able to successfully do this for every month, save February, because you must take leap years into account.

I have this code:

import java.util.Scanner;

/**
 * 
 * @author Andrew De Forest 
 * @version v1.0
 * 
 */
public class exc6
{
    public static void main (String[] args)
    {  
        //Initialize a string
        String getInput;
        //Initialize some integers
        int month, day, year;
        //Make a boolean
        boolean validDate;
        //Set the date to false
        validDate = false;
        //Ask for input
        System.out.println("Enter a date (mm/dd/yyyy)");
        //Initialize the scanner
        Scanner keyboard = new Scanner (System.in);
        //Get input & use a delimiter
        keyboard.useDelimiter("[/\n]");
        month = keyboard.nextInt();
        day = keyboard.nextInt();
        year = keyboard.nextInt();

        if((month >= 1 && month <= 12) && (day >= 1 && day <= 31))
        {
            //For months with 30 days
            if((month == 4 || month == 6 || month == 9 || month == 11) && (day <= 30))
            {
                validDate = true;
            }

            //For months with 31 days
            if((month == 1 || month == 2 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && (day <= 31))
            {
                validDate = true;
            }

            //For February
            if((month == 2) && (day < 30))
            {
                //Boolean for valid leap year
                boolean validLeapYear = false;

                //A leap year is any year that is divisible by 4 but not divisible by 100 unless it is also divisible by 400
                if((year % 400 == 0) || ((year % 4 == 0) && (year %100 !=0)))
                {
                    validLeapYear = true;
                }

                if (validLeapYear == true && day <= 29)
                {
                    validDate = true;
                }

                else if (validLeapYear == false && day <= 28)
                {
                    validDate = true;
                }
            }
        }

        //If the date is valid
        if(validDate == true)
        {
            System.out.println(month + "/" + day + "/" + year + " is a valid date.");
        }

        else
        {
            System.out.println("Invalid date!");
        }

    }
}

The part I'm most concerned with is this:

    //For February
    if((month == 2) && (day < 30))
    {
        //Boolean for valid leap year
        boolean validLeapYear = false;

        //A leap year is any year that is divisible by 4 but not divisible by 100 unless it is also divisible by 400
        if((year % 400 == 0) || ((year % 4 == 0) && (year %100 !=0)))
        {
            validLeapYear = true;
        }

        if (validLeapYear == true && day <= 29)
        {
            validDate = true;
        }

        else if (validLeapYear == false && day <= 28)
        {
            validDate = true;
        }
    }
}

As far as I can tell, it looks correct. However, when I input something like 2/29/2011, it returns as a valid date (which it should not, as 2011 was not a leap year). Why is this? What am I missing, or passing over, that causes bad dates to return valid?

Try to make comments meaningful, and not just a duplicate of the code. Also, when posting code, try to remove things not relevant, for example, the opening javadoc adds nothing of value and takes up noticeable vertical space. — Dave Newton
– Dave Newton, Commented Feb 13, 2012 at 0:56
Can't you use any java library? java.util.Calendar, org.joda.time.DateTime?? — Marsellus Wallace
– Marsellus Wallace, Commented Feb 13, 2012 at 2:23
Normally, yes...but this particular assignment required us to follow specific directions ;) — Andrew De Forest
– Andrew De Forest, Commented Feb 13, 2012 at 3:09

Matti Virkkunen · Accepted Answer · 2012-02-13 00:51:48Z

4

if((month == 1 || month == 2 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && (day <= 31))

This line already catches February.

answered Feb 13, 2012 at 0:51

Matti Virkkunen

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1 Comment

linuxunil Over a year ago

For java 8, you have java.time.Month with static values for each month. To get as int just do : Month.July.getValue()

duffymo · Accepted Answer · 2012-02-13 00:58:54Z

2

DateFormat dateFormat = new SimpleDateFormat("yyyy-MMM-dd");
dateFormat.setLenient(false);
String dateAsString = "2011-Feb-29";
Date date = dateFormat.parse(dateAsString);  // throws an exception; invalid date

answered Feb 13, 2012 at 0:58

duffymo

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3 Comments

BalusC Over a year ago

While indeed the right way, I'm not sure if the tutor allows using DateFormat for this homework assignment.

Andrew De Forest Over a year ago

@duffymo I'm sure this would be the correct way to do this in the future, but for now the assignment is to pick apart the user's input ;)

duffymo Over a year ago

Oh, well. Learning comes in all forms.

MRAB · Accepted Answer · 2012-02-13 01:06:46Z

0

First you're setting validDate to true because month is 2.

Next you're setting validLeapYear to false because it's not a leap year.

(validLeapYear == true && day <= 29) isn't true.

(validLeapYear == false && day <= 28) also isn't true.

Therefore, validDate is still true.

answered Feb 13, 2012 at 1:06

MRAB

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