Why is this java input validation loop not working

Question

   import java.util.Scanner;

   //this program test input validation yes or no program

public class Fool
 {
   public static void main(String [] args)
   {
    String input;
    char first;
    Scanner keyboard=new Scanner(System.in);

    System.out.println("Enter yes or no ");
         input=keyboard.nextLine();
        first=input.charAt(0);
        System.out.print(first);
         while( first !='y' || first !='n')
          {
        System.out.println("please enter yes or no");
          }

       }
   }

What is trying to get the program to is that the user has to remain in the while loop if the user does not put in yes or no.

Think about that while condition logically, it will always be true. — August
– August, Commented Dec 20, 2014 at 3:13
The while should contain input=keyboard.nextLine();. Otherwise, first stays the same forever. — tokosh
– tokosh, Commented Dec 20, 2014 at 3:28

Madhawa Priyashantha · Accepted Answer · 2014-12-20 03:23:14Z

2

change this to

while( first !='y' || first !='n') {
        System.out.println("please enter yes or no");
}

this

while( first !='y' && first !='n') {
        System.out.println("please enter yes or no");
}

because (first !='y' || first !='n') is always true.

if first =='y' then first !='n' is true

if first =='n' then first !='y' is true.

so while condition is always true what you need is not || but && [and ]

edited Dec 20, 2014 at 3:23

answered Dec 20, 2014 at 3:11

Madhawa Priyashantha

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3 Comments

james Fog Over a year ago

Huh I though the logic would while first does not equal y or first does not equal n, oh I see the or statement needs only one statement to be true so if someone puts in no, then first will not equal 'y' and thus the loop will start.

RobP Over a year ago

Maybe this helps or maybe confuses you more: while( first !='y' && first !='n') is equivalent to while(!(first =='y' || first =='n')) and I find the latter more readable as it's more like how you describe the condition in English.

james Fog Over a year ago

Yes that is Demorgans logicc law

user3575886 · Accepted Answer · 2014-12-20 03:26:14Z

1

while( first !='y' || first !='n') is always true.

Replace your code with:

while( first !='y' && first !='n')

answered Dec 20, 2014 at 3:26

user3575886

Comments

greatwolf · Accepted Answer · 2014-12-20 03:53:58Z

1

while( first !='y' || first !='n') is always true. As OR operation works as follows

condition 1 condition 2 result

TRUE TRUE TRUE

TRUE FALSE TRUE

FALSE TRUE TRUE

FALSE FALSE FALSE

In your case one condition will be always true, therefore it enters in while loop everytime While AND operation works as follows

condition 1 condition 2 result

TRUE TRUE TRUE

TRUE FALSE FALSE

FALSE TRUE FALSE

FALSE FALSE FALSE

so instead of using OR try using AND e.g. while( first !='y' && first !='n')

edited Dec 20, 2014 at 3:53

greatwolf

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answered Dec 20, 2014 at 3:47

Jinendra

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Comments

JamesENL · Accepted Answer · 2017-01-12 07:31:52Z

1

You should change your code to

boolean b=false;
while(b==false){
    if(first !='y' || first !='n'){
        System.out.println("please enter yes or no");
    } else {
        b=true;
    }
}

edited Jan 12, 2017 at 7:31

JamesENL

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answered Dec 20, 2014 at 4:38

Hello World

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