这个java while 循环有什么问题？

Question

New to Java and learning how to use While loops and random generator. This prints a multiplication question. Every time the user answers a question wrong, it should print the same question. Instead, it exits the program. What should I do?

while (true) {
    Random multiply = new Random();
    int num1 = multiply.nextInt(15);
    int num2 = multiply.nextInt(15);
    int output = num1 * num2;

    System.out.println("What is the answer to " + num1 + " * " + num2);

    Scanner input = new Scanner(System.in);
    int answer = input.nextInt();
    if (answer == output) {
        if (answer != -1)
            System.out.println("Very good!");
    } else {
        System.out.println("That is incorrect, please try again.");
    }
}

Answer 1

If you want to repeat the same question when the user gets the answer wrong, you should use another while inside your main loop.

This inner loop continues to ask as long as you give a wrong answer.

I also replaced nextInt with nextLine , which reads in a whole line of text. This consumes the "Enter" key and is a safer approach at reading from the console. Since the result is now a String you need to use Integer.parseInt to convert it to an int . This throws an exception if you enter anything but a whole number so I wrapped it into a try-catch block.

If you want, you can add an additional check for validating user input. So in case the user wants to stop playing they only need to input "exit" and the whole outer loop will exit.

boolean running = true; // This flag tracks if the program should be running.
while (running) {
    Random multiply = new Random();
    int num1 = multiply.nextInt(15);
    int num2 = multiply.nextInt(15);
    int output = num1 * num2;
    boolean isCorrect = false; // This flag tracks, if the answer is correct

    while (!isCorrect) {
        System.out.println("What is the answer to " + num1 + " * " + num2);

        Scanner input = new Scanner(System.in);
        try {
            String userInput = input.nextLine(); // Better use nextLine to consume the "Enter" key.
            // If the user wants to stop
            if (userInput.equals("exit")) {
                 running = false; // Don't run program any more
                 break;
            }
            int answer = Integer.parseInt(userInput); 
            if (answer == output) {
                if (answer != -1) {
                    System.out.println("Very good!");
                    isCorrect = true; // Set the flag to true, to break out of the inner loop
                }
            } else {
                System.out.println("That is incorrect, please try again.");
            }
        }
        catch(NumberFormatException e) {
            System.out.println("Please enter only whole numbers");
        }
    }
}

Answer 2

Avoid while true. Declare a variable to true, pass the variable to the condición loop and set it to false when the answer is incorrect. You can use break too, but is easier to read the code when you use a exit condition in the while. Also read more about loops https://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html