Problems with date validation code

[Philip Hager]

Any help would be greatly appreciated, and if you find anything wrong that the compiler missed, feel free to point it out.
Here is the code I have so far that I can't get to compile because of one error:

LegalDate.java:30: error: variable loop is already defined in method main(String[])
char loop = aString.charAt(0);
^
1 error

import java.util.Scanner;

public class LegalDate {

    static final int maxYear = 2050;

    public static void main (String [] args) {
        int month, day, year;
        boolean leapYear, validDate, notValidDate = false;
        char loop = 0;

        Scanner kb = new Scanner (System.in);
        
        while (loop != 'N'){
        System.out.println("Please enter the month, day, and year in interger form.");

        System.out.print("Month: ");
        month = kb.nextInt();
        System.out.print("Day: ");
        day = kb.nextInt();
        System.out.print("Year: ");
        year = kb.nextInt();

        leapYear = isLeapYear(year);
        validDate = isValidDate(month, day, year);

        System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");
        System.out.println("Would you like to validate another date? (Y/N) ");
        String aString = kb.next();
        char loop = aString.charAt(0);
        if (loop == 'N')
        {
        break;
        }
    }
   } 
    public static int numDaysInMonth(int month, boolean isLeapYear) {
        if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)
        {
        return 31;
        }
        if (month == 4 || month ==6 || month == 9 || month == 11)
        {
        return 30;
        }
        if (month == 2)
        {
        return 28;
        } 
        else if (isLeapYear)
        {
        return 29;
        }
        else
        {
             return 0;
        }
    }

    public static boolean isLeapYear(int year) {
        return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
    }

    public static boolean isValidDate(int month, int day, int year) {
        return (month >= 1 && month <= 12) && (day >= 1 && day <= numDaysInMonth(month, isLeapYear(year))) && (year >= 1900 && year <= maxYear);
    }
    public static boolean isNotValidDate(int month, int day, int year) {
        return (month < 1 || month > 12) || (day < 1 || day > numDaysInMonth (month, isLeapYear (year))) || (year < 1900 || year > maxYear);
    }
}

[Carey Brown]

Lines 10 and 30 define the same variable. This is not allowed.

[Philip Hager]

Yes, but when I delete the one at the top I get this error:

LegalDate.java:13: error: cannot find symbol
while (loop != 'N'){
^
symbol: variable loop
location: class LegalDate
1 error

[Paul Clapham]

It looks to me like those two variables are supposed to be one variable used for a single purpose. Therefore you only need to declare it once, not twice.

When you removed one of the declarations you had a problem. But that's because you removed the wrong one.

[Carey Brown]

On line 30 - change

char loop = aString.charAt(0);

to

loop = aString.charAt(0);

[Philip Hager]

Ok, so that problem is fixed, thanks.

Now I'm having an two issues:

First - The code isn't correctly validating dates
Second - It gets stuck in an infinite loop

This is the relevant code for the first problem (how can I set the char loop variable to the input Y/N from user):

char loop = 0;

        Scanner kb = new Scanner (System.in);
        
        while (loop != 'N'){
        System.out.println("Please enter the month, day, and year in interger form.");

        System.out.print("Month: ");
        month = kb.nextInt();
        System.out.print("Day: ");
        day = kb.nextInt();
        System.out.print("Year: ");
        year = kb.nextInt();

        leapYear = isLeapYear(year);
        validDate = isValidDate(month, day, year);

        System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");
        System.out.println("Would you like to validate another date? (Y/N) ");
        String aString = kb.next();
        if (loop == 'N')
        {
        break;

Here is the code for the second issue:

public static boolean isLeapYear(int year) {
        return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
    }

    public static boolean isValidDate(int month, int day, int year) {
        return (month >= 1 && month <= 12) && (day >= 1 && day <= numDaysInMonth(month, isLeapYear(year))) && (year >= 1900 && year <= maxYear);
    }
    public static boolean isNotValidDate(int month, int day, int year) {
        return (month < 1 || month > 12) || (day < 1 || day > numDaysInMonth (month, isLeapYear (year))) || (year < 1900 || year > maxYear);
    }

[Philip Hager]

First problem has been fixed, only second problem remains.

[Paul Clapham]

Philip Hager wrote:This is the relevant code for the first problem (how can I set the char loop variable to the input Y/N from user):

You had a line of code which did that, but you deleted it. Don't delete the part of the code which does what you want, just delete the part which makes it a redundant declaration.

[Paul Clapham]

Philip Hager wrote:Here is the code for the second issue:

There weren't any loops in that code, so that code couldn't possibly be causing an infinite loop. The cause for your infinite loop must be elsewhere.

[Carey Brown]

        System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");
        System.out.println("Would you like to validate another date? (Y/N) ");
        String aString = kb.next();
        if (loop == 'N')
        {
        break;

You took out the assignment to loop below line 3.

[Philip Hager]

The loop problem has been fixed, and I substituted the switch command for the if command when validating the days in a month, but I still get an output of "The date is valid and is not in range."

The code is supposed to check the year to be in a specific range, if any of them are out of that range that is the only time it should give me the "The date is valid and is not in range", but I get it all the time.

Updated Code:

import java.util.Scanner;

public class LegalDate {

    static final int maxYear = 2050;

    public static void main (String [] args) {
        int month, day, year;
        boolean leapYear, validDate, notValidDate = false;
        char loop = 0;

        Scanner kb = new Scanner (System.in);
        
        while (loop != 'N'){
        System.out.println("Please enter the month, day, and year in interger form.");

        System.out.print("Month: ");
        month = kb.nextInt();
        System.out.print("Day: ");
        day = kb.nextInt();
        System.out.print("Year: ");
        year = kb.nextInt();

        leapYear = isLeapYear(year);
        validDate = isValidDate(month, day, year);

        System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");
        System.out.println("Would you like to validate another date? (Y/N) ");
        String aString = kb.next();
        loop = aString.charAt(0);
        if (loop == 'N')
        {
        break;
        }
    }
   } 
    public static int numDaysInMonth(int month, boolean isLeapYear) {
        switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
             return 31;
        case 4:
        case 6:
        case 9:
        case 11:
             return 30;
        case 2:
             if (isLeapYear) {
                 return 29;
             } else {
                 return 28;
             }
        default:
             return 0;
        }
    }
    public static boolean isLeapYear(int year) {
        return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
    }

    public static boolean isValidDate(int month, int day, int year) {
        return (month >= 1 && month <= 12) && (day >= 1 && day <= numDaysInMonth(month, isLeapYear(year)));
    }
    public static boolean isNotValidDate(int month, int day, int year) {
        return (year < 1900 || year > maxYear);
    }
}

[Paul Clapham]

Sorry, I'm not clear about what is the difference between "valid" and "in range". It looks like your variable notValidDate is somehow supposed to tell you whether the date is "in range", or maybe "not in range", but it would help if you wrote some code which assigned it a value other than false.

I think my problem arises from two variables named "validDate" and "notValidDate" which keep wanting to be opposites of each other in my mind, but apparently they are supposed to be independent of each other?

[Philip Hager]

The boolean for isValidDate works as intended, but the one for isNotValidDate does not when used with this part of the code:

System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");

[Paul Clapham]

Are you talking about "isNotValidDate" or "notValidDate"? I ask because the line of code you posted uses "notValidDate", so if that isn't what you expected you should look at the lines of code which assign values to it.

[Philip Hager]

Alright, I got it to work as expected now, not sure why it works correctly this way, but it does. Thanks for all the help!

Updated code once again:

import java.util.Scanner;

public class LegalDate {

    static final int maxYear = 2050;

    public static void main (String [] args) {
        int month, day, year;
        boolean leapYear, validDate, notValidDate = false;
        char loop = 0;

        Scanner kb = new Scanner (System.in);
        
        while (loop != 'N' || loop != 'n'){
        System.out.println("Please enter the month, day, and year in interger form.");

        System.out.print("Month: ");
        month = kb.nextInt();
        System.out.print("Day: ");
        day = kb.nextInt();
        System.out.print("Year: ");
        year = kb.nextInt();

        leapYear = isLeapYear(year);
        validDate = isValidDate(month, day, year);
        notValidDate = isNotValidDate(year, maxYear);

        System.out.printf("%nThe date is %svalid and is %sin range.%n", validDate ? "" : "not ", notValidDate ? "" : "not ");
        System.out.println("Would you like to validate another date? (Y/N) ");
        String aString = kb.next();
        loop = aString.charAt(0);
        if (loop == 'N' || loop == 'n')
        {
        break;
        }
    }
   } 
    public static int numDaysInMonth(int month, boolean isLeapYear) {
        switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
             return 31;
        case 4:
        case 6:
        case 9:
        case 11:
             return 30;
        case 2:
             if (isLeapYear) {
                 return 29;
             } else {
                 return 28;
             }
        default:
             return 0;
        }
    }
    public static boolean isLeapYear(int year) {
        return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
    }

    public static boolean isValidDate(int month, int day, int year) {
        return (month >= 1 && month <= 12) && (day >= 1 && day <= numDaysInMonth(month, isLeapYear(year)));
    }
    public static boolean isNotValidDate(int year, int maxYear) {
        return (year >= 1900 && year <= maxYear);
    }
}

[Winston Gutkowski]

Philip Hager wrote:Alright, I got it to work as expected now, not sure why it works correctly this way, but it does.

Well done! A few minor things for you to consider:

1. isNotValidDate() should probably be called invalidYear(). Ditto for the variable you assign it to.

2. You could make your numDaysInMonth() method a lot more compact by storing month lengths in an array, viz:

private static final int[] monthLengths = new int[]
   {-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

...

public static int numDaysInMonth(int month, int year) {
   if (month == 2)
      return monthLengths[month] + (isLeapYear(year) ? 1 : 0);
   else 
      return monthLengths[month];
}

3. You don't need to go overboard with brackets (although I understand why you did). Also: Java has a '!=' operator, so:
  return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
will work just fine, and may be a bit clearer.

4. If you took the components in year, month, day order, you could check that each was valid when it was entered.
I realise that probably wasn't the exercise here, but you might find this page useful for some general tips on user input.

HIH

Winston

[Campbell Ritchie]

Philip Hager wrote:. . . // ... case 2: if (isLeapYear) { return 29; } else { return 28; } default: return 0; // ...

That should read // ... case 2: return isLeapYear ? 29 : 28; default: throw new IllegalArgumentException("Non-existent month number."); // ...Don't stand for any nonsense about wrong parameters; if there are only twelve months let them know they have got it wrong rather than quietly returning 0.