如何在 while 循环中使用 Scanner hasNextInt()？

Question

I cannot get out of while loop.

I do not why sc.hasNextInt() does not return false after last read number.

Should I use another method or is there a mistake in my code?

public static void main(String[] args) {

        // Creating an array by user keyboard input
        Scanner sc = new Scanner(System.in);

        System.out.println("Length of array: ");
        int[] numbers = new int[sc.nextInt()];

        System.out.printf("Type in integer elements of array ", numbers.length);
        int index = 0;
        **while ( sc.hasNextInt()) {**
            numbers[index++] = sc.nextInt();


        }
        // created method for printing arrays
        printArray(numbers);
        sc.close();


}

Answer 1

When you are receiving your input from the console, the Scanner hasNextInt() method placed inside a while loop condition will continue to read (meaning the loop will continue), until one of the following happens:

• You submit a non-numeric symbol (eg a letter).
• You submit a so-called " end of file " character, which is a special symbol telling the Scanner to stop reading.

Thus, in your case you cannot have the hasNextInt() inside your while loop condition - I am showing a solution below with a counter variable that you can use.

However , the hasNextInt() method inside a while loop has its practical usage for when reading from a different source than the console - eg from a String or a file. Inspired from the examples here , suppose we have:

String s = "Hello World! 3 + 3.0 = 6 ";

We can then pass the string s as an input source to the Scanner (notice that we are not passing System.in to the constructor):

Scanner scanner = new Scanner(s);

Then loop until hasNext() , which checks if there is another token of any type in the input. Inside the loop, perform a check if this token is an int using hasNextInt() and print it, otherwise pass the token to the next one using next() :

while (scanner.hasNext()) {
    if (scanner.hasNextInt()) {
        System.out.println("Found int value: " + scanner.next());
    } else {
        scanner.next();
    }
}

Result:

Found int value: 3
Found int value: 6

In the example above, we cannot use hasNextInt() in the while loop condition itself, because the method returns false on the first non-int character that it finds (so the loop closes immediately, as our String begins with a letter).

However, we could use while (hasNextInt()) to read the list of numbers from a file .

Now, the solution to your problem would be to place the index variable inside the while loop condition:

while (index < numbers.length) {
    numbers[index++] = sc.nextInt();
}

Or for clarity`s sake, make a specific counter variable:

int index = 0;
int counter = 0;
while (counter < numbers.length) {
       numbers[index++] = sc.nextInt();
       counter++;
}

Answer 2

Do the following:

Use the input length as the end of the loop.

        // Creating an array by user keyboard input
        Scanner sc = new Scanner(System.in);

        System.out.println("Length of array: ");

        int len = sc.nextInt();
        int[] numbers = new int[len]; // use len here

        System.out.printf("Type in integer elements of array ", numbers.length);
        int index = 0;
        for (index = 0; index < len; index++) { // and use len here
            numbers[index] = sc.nextInt();
        }
        // created method for printing arrays
        printArray(numbers);
        sc.close();

And don't close the scanner.

Answer 3

I cannot get out of while loop.

I do not why sc.hasNextInt() does not return false after last read number.

Should I use another method or is there a mistake in my code?

public static void main(String[] args) {

        // Creating an array by user keyboard input
        Scanner sc = new Scanner(System.in);

        System.out.println("Length of array: ");
        int[] numbers = new int[sc.nextInt()];

        System.out.printf("Type in integer elements of array ", numbers.length);
        int index = 0;
        **while ( sc.hasNextInt()) {**
            numbers[index++] = sc.nextInt();


        }
        // created method for printing arrays
        printArray(numbers);
        sc.close();


}