Date Validation

[Kara Wilimas]

Below is my current program for my java class. The assignment is to validate a date when entered by user.

//author: KW 

   import java.util.*;
public class Date{
   public static void main (String arg[]){ 
   
   int month = 0;
   int day = 0;
   int year = 0;
   
   Scanner scan = new Scanner(System.in); 
   
   try{ 
      System.out.println("Enter a month: "); //user 
      month = scan.nextInt();
   }  
  
   catch(InputMismatchException ime){
      System.out.println("You did not enter a number");
   }  
   
  //month validation 
  
  if ((month < 1) || (month > 12)) 
      System.out.println(" The month entered is not valid");
   
  else
        
    
   try{
    System.out.println("Enter a year: "); //user 
    year = scan.nextInt();
    } 
    
   catch(InputMismatchException ime){
      System.out.println("You did not enter a number");
   } 

   // year Validation
 
   if ((year < 1900) || (year > 2050)) 
      System.out.println( year + " is not in range");
  
         
  //Leap Year 
  
  
      boolean leapYear = ((year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0));

        if (leapYear)
        {
            System.out.println(year + " is a leap year.");
        } 
        else
            System.out.println(year + " is not a leap year.");  
  
  
      try{
      System.out.println("Enter a day: "); //user 
      day = scan.nextInt();
   }  
   
      catch(InputMismatchException ime){
      System.out.println("You did not enter a number");
   }  
   
  
 //dayInMonth validation 
        
     boolean dayinMonth31 = (month == 1 || month == 3 ||month == 5 ||month == 7 ||month == 8 ||month == 10 ||month == 12 && day <= 31);
        if (dayinMonth31)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
      
     boolean dayinMonth30 = (month == 4 || month == 6 ||month == 9 ||month == 11 && day <= 30);
        if (dayinMonth30)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
            
     boolean dayinMonth29 = (month == 2  && leapYear && day <= 29);
        if (dayinMonth29)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
       
     boolean dayinMonth28 = (month == 2  && day <= 28);
        if (dayinMonth28)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid"); 
     
   }  
}

My issue with my program is the output for the day.
If I enter 30 for the date I get this: Date is invalid
Date is invalid
Date is invalid
Date is invalid
Date is invalid

I wanted to know how what is wrong and why it is doing this.

[Junilu Lacar]

Hello, and welcome to the Ranch!

First, if you're going to post code, please UseCodeTags ←click. They make it easier to read your post. I've added them for you this time.

Now to your question. You should review this page: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html because your boolean expressions are not working the way you probably think they're working. In particular, && takes precedence over || in a boolean expression, much like how * takes precedence over + in an arithmetic expression. So, an expression like (a || b && c) is evaluated as (a || (b && c)), not ((a || b) && c).

[Kara Wilimas]

Hello,

Thank you for that.

I added the parenthesis but when an int variable for day is entered it appears to run through all four booleans and not just the one that applies. So I get four outputs of either "Date is valid" or "Date is invalid"

How do I fix this problem?

boolean dayinMonth31 = ((month == 1 || month == 3 ||month == 5 ||month == 7 ||month == 8 ||month == 10 ||month == 12) && day <= 31);
        if (dayinMonth31)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
      
     boolean dayinMonth30 = ((month == 4 || month == 6 ||month == 9 ||month == 11) && day <= 30);
        if (dayinMonth30)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
            
     boolean dayinMonth29 = (month == 2  && leapYear && day <= 29);
        if (dayinMonth29)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid");
       
     boolean dayinMonth28 = (month == 2  && day <= 28);
        if (dayinMonth28)
        {
            System.out.println("Date is valid");
        } 
        else
            System.out.println("Date is invalid"); 

[Junilu Lacar]

Also, if you were wondering why your program prints out "Date is invalid" multiple times, it's because your if-else statements are all executed independently of each other. That is, each one of them will execute regardless of what already happened with any previously executed if-else statements. You could chain your if-else statements to make them dependent on the results of previous ones.

This code will always result in two actions being executed: (action1 or action2) and (action3 or action4)

   // execute action1 or action2
   if (condition1)
      action1
   else
      action2

   // execute action3 or action4
   if (condition3)
      action 3
   else 
      action 4

In order for action4 to execute, only condition3 has to be false; it doesn't matter what condition1 is.

The code below will result in exactly one of action1, action2, action3, or action4 being executed:

   if (condition1)
      action1
   else if (condition2)
      action2
   else if (condition3)
      action3
   else 
      action4

In order for action4 to execute, (condition1 && condition2 && condition3) has to be false. Do you see the difference?

[Kara Wilimas]

Hello,

Yeh I understand. That was actually something I was thinking of doing.


Okay so I think I fixed that problem but now it is telling me
Date.java:91: error: illegal start of statement
at the end of my code and I checked all my brackets and they are paired so I don't what is wrong.

 import java.util.*;
  
public class Date{
   public static void main (String arg[]){
   
   int month = 0;
   int day = 0;
   int year = 0;
   
   Scanner scan = new Scanner(System.in); 
   
    try{ 
      System.out.println("Enter a month: "); //user 
      month = scan.nextInt();
    }  
  
    catch(InputMismatchException ime){
      System.out.println("You did not enter a number");
    }  
   
  //month validation 
  
    if((month < 1) || (month > 12)) 
      System.out.println(" The month entered is not valid");
        
    
    try{
    System.out.println("Enter a year: "); //user 
    year = scan.nextInt();
    } 
    
    catch(InputMismatchException ime){
      System.out.println("You did not enter a number");
    } 

   // year Validation
 
    if((year < 1900) || (year > 2050)) 
      System.out.println( year + " is not in range");
  
         
  //Leap Year 
  
  
      boolean leapYear = ((year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0));

        if (leapYear)
        {
            System.out.println(year + " is a leap year.");
        } 
        else
            System.out.println(year + " is not a leap year.");  
  
  
       try{
       System.out.println("Enter a day: "); //user 
       day = scan.nextInt();
       }  
   
       catch(InputMismatchException ime){
       System.out.println("You did not enter a number");
       }  
   
  
 //dayInMonth validation 
        
      if((month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && day <= 31)        
      {     
            System.out.println("Day is valid");
      }
      else
      if((month == 4 || month == 6 ||month == 9 ||month == 11) && day <= 30)
      {     
            System.out.println("Day is valid");
      }
      else
      if(month == 2  && leapYear && day <= 29)
      {
            System.out.println("Date is valid");
      } 
      else
      if(month == 2  && day <= 28)
      {
            System.out.println("Date is valid");
      } 
      else 
            
            System.out.println(" Day entered is not valid");
     
}
   }  

[Kara Wilimas]

Uhhh....okay never mind. It is now working o.o

[Campbell Ritchie]

Welcome again

You need a little thought about repeated if‑elses. You can convert something like this to the second version, which is much neater and easier to read:-

if (mark > 100 || mark < 0)
   // ...
else if (mark >= 90)
   // ...
else if (mark <= 19)
   // ...
else if (mark >= 80)
   // ...
else if (mark < 80 && mark >= 70)
   // ...
else if (mark >= 60)
   // ...
else if (mark <= 29)
   // ...
else if (mark >= 50)
   // ...
else if (mark < 50 && mark >= 40)
   // ...
else
   // ...

if (mark > 100 || mark < 0)
   // ...
else if (mark < 20)
   // ...
else if (mark < 30)
   // ...
else if (mark < 40)
   // ...
else if (mark < 50)
   // ...
else if (mark < 60)
   // ...
else if (mark < 70)
   // ...
else if (mark < 80)
   // ...
else if (mark < 90)
   // ...
else
   // ...

[Junilu Lacar]

Another thing you can do to clean up your code is to remove repetitions. For example, System.out.println("Day is valid") is repeated multiple times in your code. You should separate logic to check validity from your display statements. Your code will be cleaner and clearer if you do something like this:

   boolean isFebruary = (month == 2);
   boolean isLeapYear = ... 
   boolean isLongMonth = ... //does month have 31 days?

   boolean isValid;

   // run the gauntlet of checks for invalid conditions
   if (isFebruary && ...)  
      isValid = false;
   else if (isLeapYear && ...)  
      isValid = false;
   else if ( /* whatever other conditions */) 
      isValid = false;
   else // passed all checks so it must be valid
      isValid = true;

   // now display results
   if (isValid) 
      System.out.println("Valid");
   else 
      System.out.println("Invalid");

I suggest you try to restructure your code this way and compare it to your original version. I think you'll find that this way is a lot easier to understand and it looks a lot cleaner. There are other ways you can clean it up but this is a good start. Baby steps.