Scanner input validation in while loop

问题

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?

回答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);
    }

回答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
}

回答3:

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 

}

来源：https://stackoverflow.com/questions/19950713/scanner-input-validation-in-while-loop