为什么我的 do-while 循环在使用 Java 扫描仪输入无效输入后变成无限循环？

Question

When you type in anything besides a number it will just print my "Invalid response:" for forever. But when you type in a number it will go on to the next one (working correctly).

I don't understand why it's continuously printing my error message when it fails the check. Instead, I want it to ask the user for input again.

If you know of a better more efficient way please help me out.

public class Test {

    public static void main(String[] args) {
        //Scanner getting input
        Scanner nin = new Scanner(System.in); //input for numbers
        Scanner sin = new Scanner(System.in); //input for yes or no

        //initialize variables
        String input = "";
        boolean valid = false;
        boolean playAgain = false;

        //Primary Do While
        do {
            System.out.println("This program will ask for three numbers and see which one is the largest\n");
            System.out.println("\n\n");

            //define the range
            int num1 = 0;
            int num2 = 0;
            int num3 = 0;
            int largestNum = 0;

            //numeric input validation who while loop for the first number
            do {
                System.out.println("Please enter the frist number: ");

                //if block to check if input is valid
                if (nin.hasNextInt()) {
                    num1 = nin.nextInt();
                    valid = true;
                } else {
                    System.out.println("Invalid response: Please enter a whole number.\n\n");
                    valid = false;
                    nin.hasNext();
                }


            } while (!valid); //if false run the loop
            valid = false; //this resets the validity for the next number

            //numeric input validation who while loop for the second number
            do {
                System.out.println("Please enter the second number: ");

                //if block to check if input is valid
                if (nin.hasNextInt()) {
                    num2 = nin.nextInt();
                    valid = true;
                } else {
                    System.out.println("Invalid response: Please enter a whole number.\n\n");
                    valid = false;
                    nin.hasNext();
                }

            } while (!valid); //if false run the loop
            valid = false; //this resets the validity for the next number

            //numeric input validation who while loop for the third number
            do {
                System.out.println("Please enter the second number: ");

                //if block to check if input is valid
                if (nin.hasNextInt()) {
                    num3 = nin.nextInt();
                    valid = true;
                } else {
                    System.out.println("Invalid response: Please enter a whole number.\n\n");
                    valid = false;
                    nin.hasNext();
                }

            } while (!valid); //if false run the loop


            //passing numbers to method and saving results to variable 
            largestNum = largestNum(num1, num2, num3);

            //printing the results
            System.out.println(largestNum);


        } while (!playAgain);


    }

    private static int largestNum(int num1, int num2, int num3) {
        if (num1 > num2 && num1 > 3) {
            return num1;
        } else if (num2 > num1 && num2 > num3) {
            return num2;
        } else {
            return num3;
        }

    }

}

Answer 1

First, you do not need 2 Scanner objects because you are reading from the same place.

Remember - whatever you need to read, one Scanner per file is enough!

But the real problem is that you do not clear the invalid user input. The method hasNext only checks to see if there is more input ready to be processed, but does nothing with it.

If you find that there is still input but it is not an integer ( hasNext returns true but hasNextInt returns false ) you should call next method, to get the remaining input regardless of type (ignoring the returned value).

Also, since you need 3 loops that are identical, it would be better to just put one loop in to a separate private method that would return the value, and then call that method 3 times.

privat int getNumberFromUser() {
    do {
        //your loop code here
    } while (!valid);

    return num;
}

//in main:
num1 = getNumberFromUser();
num2 = getNumberFromUser();
num3 = getNumberFromUser();

Answer 2

Your code never actually reads the bad input, as it reads the valid input using nextInt . ( hasNext just tells whether or not there is input to be read.)