Scanner input validation in while loop

Question

I've got to show Scanner inputs in a while loop: the user has to insert inputs until he writes "quit". So, I've got to validate each input to check if he writes "quit". How can I do that?

while (!scanner.nextLine().equals("quit")) {
    System.out.println("Insert question code:");
    String question = scanner.nextLine();
    System.out.println("Insert answer code:");
    String answer = scanner.nextLine();

    service.storeResults(question, answer); // This stores given inputs on db
}

This doesn't work. How can I validate each user input?

Not sure for how many its the same case, but this actually worked properly for me. Not sure what exactly didn't work for you. Remember to give "quit" only and not any other case versions of it. — Rahul
– Rahul, Commented Nov 13, 2013 at 10:10
When asking about something that "doesn't work", specify in what way it doesn't work. How did its behavior differ from what you expected? — LarsH
– LarsH, Commented Jun 1, 2018 at 12:53

LarsH · Accepted Answer · 2018-06-01 13:02:30Z

11

The problem is that nextLine() "Advances this scanner past the current line". So when you call nextLine() in the while condition, and don't save the return value, you've lost that line of the user's input. The call to nextLine() on line 3 returns a different line.

You can try something like this

    Scanner scanner=new Scanner(System.in);
    while (true) {
        System.out.println("Insert question code:");
        String question = scanner.nextLine();
        if(question.equals("quit")){
            break;
        }
        System.out.println("Insert answer code:");
        String answer = scanner.nextLine();
        if(answer.equals("quit")){
            break;
        }
        service.storeResults(question, answer);
    }

edited Jun 1, 2018 at 13:02

LarsH

28.1k9 gold badges99 silver badges162 bronze badges

answered Nov 13, 2013 at 10:04

Ruchira Gayan Ranaweera

35.7k17 gold badges78 silver badges117 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

drewteriyaki Over a year ago

Why is it that the Scanner scanner = new Scanner(System.in) is outside the while loop?

LarsH Over a year ago

@drewteriyaki: You don't want a new Scanner created for every user input. A single Scanner on the single system input stream keeps a consistent state on that stream.

user2986555 · Accepted Answer · 2013-11-13 10:09:09Z

4

Try:

while (scanner.hasNextLine()) {
    System.out.println("Insert question code:");
    String question = scanner.nextLine();
    if(question.equals("quit")){
     break;
    }

    System.out.println("Insert answer code:");
    String answer = scanner.nextLine();

    service.storeResults(question, answer); // This stores given inputs on db
}

answered Nov 13, 2013 at 10:09

user2986555

592 bronze badges

3 Comments

Kurt Bourbaki Over a year ago

What's the difference between while (scanner.hasNextLine()) and while (true) (like in Ruchira's answer) in this case?

user2986555 Over a year ago

while(true) will terminate only in break and while(scanner.hasNextLine()) terminate at EOF.

Kurt Bourbaki Over a year ago

But how can it reach the end of file if there will always be a scanner.nextLine() (even if empty)?

mosaad · Accepted Answer · 2013-11-13 10:06:43Z

0

always check if scanner.nextLine is not "quit"

while (!scanner.nextLine().equals("quit")) {
    System.out.println("Insert question code:");
    String question = scanner.nextLine();
    if(question.equals("quit"))
     break;

    System.out.println("Insert answer code:");
    String answer = scanner.nextLine();
    if(answer.equals("quit"))
      break;

    service.storeResults(question, answer); // This stores given inputs on db

}

answered Nov 13, 2013 at 10:06

mosaad

2,3415 gold badges27 silver badges49 bronze badges

Collectives™ on Stack Overflow

Scanner input validation in while loop

3 Answers 3

2 Comments

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related