java.lang.NullPointerException form user input

Question

Hey guys I'm trying to create a loop until a correct character choice is entered by the user. When I enter a wrong choice I get the error java.lang.NullPointerException. It might be with the way I'm inputing but I don't want to change that if I don't have to. choice is a private member of the class.

char wf() { 
    Scanner input = new Scanner(System.in); 
    System.out.println("What is your choice? (x/o)"); 
    choice = input.findInLine(".").charAt(0);

    while (choice != 'x' && choice != 'o') { 
        System.out.println("You must enter x or o!");
        choice = input.findInLine(".").charAt(0);
    }

    return choice; 
}//end wf

Is findLineAt returning null? If so, that's the problem, cause you call charAt right after. If findLineAt was null, you will not be able to call methods since there was no object to call methods from (instead there was null), hence the NPE. — Vince
– Vince, Commented Oct 11, 2015 at 17:14
Since it is clear that findLineAt returns null you only have to understand why. You can do that by reading the JavaDoc and by debugging this method, to see what happens there. — Tom
– Tom, Commented Oct 11, 2015 at 17:20

Tsung-Ting Kuo · Accepted Answer · 2015-10-11 17:23:30Z

1

Change the function as below (I have tested this code):

char wf() { 
    Scanner input = new Scanner(System.in); 
    System.out.println("What is your choice? (x/o)"); 
    char choice = input.findInLine(".").charAt(0);

    while (choice != 'x' && choice != 'o') { 
        System.out.println("You must enter x or o!");
        choice = input.next().charAt(0);
    }

    return choice; 
}//end wf

answered Oct 11, 2015 at 17:23

Tsung-Ting Kuo

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Comments

ergonaut · Accepted Answer · 2015-10-11 17:14:49Z

1

Check input.findInLine(".") to see if it null. If you don't have the expected input, it won't return anything..

answered Oct 11, 2015 at 17:14

ergonaut

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Comments

Mohammad tanvirul islam · Accepted Answer · 2015-10-11 17:24:30Z

1

change your code like below

char wf() { 
Scanner input = new Scanner(System.in); 
System.out.println("What is your choice? (x/o)"); 
if(input.findInLine(".") !=null){
choice = input.findInLine(".").charAt(0);
while (choice != 'x' && choice != 'o') { 
    System.out.println("You must enter x or o!");
    choice = input.findInLine(".").charAt(0);
 }
}
return choice; 
}//end wf

answered Oct 11, 2015 at 17:24

Mohammad tanvirul islam

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1 Comment

Tom Over a year ago

Can you explain why it should work, even though it doesn't?

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