How to use .nextInt() and hasNextInt() in a while loop

Question

So I want my program to read an input, which has some integers in one line, for example:

1 1 2

Then it should read every integer separately and print it in a new line. The number of integers the program has to read is not given in advance, so what I am trying to do is use a while loop, which ends after there are no more integers to read. This is the code I wrote:

while (scan.hasNextInt()) {
    int x = scan.nextInt();
    System.out.println(x);
}

but it's not working correctly, because the loop never ends, it just wants the user to input more integers. What am I missing here?

Answer 1

The hasNextInt call blocks until it has enough information to make the decision of "yes/no".

Press Ctrl+Z on Windows (or Ctrl+D on "unix") to close the standard input stream and trigger an EOF. Alternatively, type in a non-integer and press enter.

Console input is normally line-buffered: enter must be pressed (or EOF triggered) and the entire line will be processed at once.

Examples, where ^Z means Ctrl+Z (or Ctrl+D):

1 2 3<enter>4 5 6^Z   -- read in 6 integers and end because stream closed
                      -- (two lines are processed: after <enter>, after ^Z)
1 2 3 foo 4<enter>    -- read in 3 integers and end because non-integer found
                      -- (one line is processed: after <enter>)

---

See also:

• How do I get Java's hasNextInt() to stop waiting for ints without inputting a character?
• hasNext() - when does it block and why?
• Java Scanner hasNext() doesn't return false

Answer 2

If you like to stop your loop after the line, create your Scanner like this:

public static void main(final String[] args) {
    Scanner scan = new Scanner(System.in).useDelimiter(" *");
    while (scan.hasNextInt() && scan.hasNext()) {
        int x = scan.nextInt();
        System.out.println(x);
    }

}

The trick is to define a delimiter that contains whitespace, the empty expression, but not the next line character. This way the Scanner sees the \n followed by a delimiter (nothing) and the input stops after pressing return.

Example: 1 2 3\n will give the following tokens: Integer(1), Integer(2), Integer(3), Noninteger(\n) Thus the hasNextInt returns false.