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?

Solution 1

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);
    }

Solution 2

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
}

Comments

[Kurt Bourbaki]

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

[user2986555]

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

[Kurt Bourbaki]

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

[drewteriyaki]

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

[LarsH]

@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.