java.util.Scanner strange behaviour when used with System.in

Question

This simple piece of code:

import java.util.Scanner;

public class TestScanner {

    public static void main(String[] args){

        Scanner sc1 = new Scanner(System.in);
        int number1 = sc1.nextInt();
        sc1.close();

        Scanner sc2 = new Scanner(System.in);
        int number2 = sc2.nextInt();
        sc2.close();
    }

}

always gives me this error:

Exception in thread "main" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Scanner.java:838)
    at java.util.Scanner.next(Scanner.java:1461)
    at java.util.Scanner.nextInt(Scanner.java:2091)
    at java.util.Scanner.nextInt(Scanner.java:2050)
    at mainpkg.TestScanner.main(TestScanner.java:14)

This one doesn't work either. This time it falls into a never-ending loop!

import java.util.Scanner;

public class TestScanner {

    public static void main(String[] args){

        Scanner sc1 = new Scanner(System.in);
        int number1 = sc1.nextInt();
        sc1.close();

        Scanner sc2 = new Scanner(System.in);
        while(!sc2.hasNextInt())
            ;
        int number2 = sc2.nextInt();
        sc2.close();
    }

}

Why?

I hope there was some good reason for giving down votes to this question! — RGO, Jan 19 '13 at 02:02

score 2 · Accepted Answer · edited Jan 19 '13 at 02:07

2

The problem is that sc1.close(); closes the underlying stream. The second time, you're trying to read from a closed stream, so it obviously fails.

edited Jan 19 '13 at 02:07

tckmn

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answered Jan 19 '13 at 01:55

aditsu quit because SE is EVIL

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Does it mean after the `Scanner` is closed, it couldn't be used again or is there any solution on this? – RGO Jan 19 '13 at 01:57
@Reza err... don't close the scanner? – tckmn Jan 19 '13 at 02:00
2

I think once System.in gets closed, you can't use it anymore. The solution is to use only one Scanner. – aditsu quit because SE is EVIL Jan 19 '13 at 02:00
@aditsu I agree with you. – RGO Jan 19 '13 at 02:04
@Doorknob yeah. I think the Scanner mustn't be closed. – RGO Jan 19 '13 at 02:06
1

@Reza: Indeed. But you also need to handle the case where there is no more input, so you will still need to use `hasNextInt`. – Oliver Charlesworth Jan 19 '13 at 02:10
@OliCharlesworth Agreed. – RGO Jan 19 '13 at 02:11
@OliCharlesworth in the case of System.in, hasNextInt actually waits for more input, then checks whether the input looks like an int. "No more input" can happen only if the user sends an EOF character (ctrl-d in linux, possibly ctrl-z in windows) or if the input is redirected from a file. – aditsu quit because SE is EVIL Jan 19 '13 at 02:17

score 0 · Answer 2 · answered Jan 19 '13 at 01:50

0

You should be checking each time with hasNextInt, rather than just assuming that there's more to read.

answered Jan 19 '13 at 01:50

Oliver Charlesworth

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It doesn't work for me. Could you put a working code here, please? – RGO Jan 19 '13 at 01:55
2

@Reza: I'm not going to write your code for you. I suggest referring to a book on Java, or perhaps a tutorial, e.g. http://docs.oracle.com/javase/tutorial/essential/io/scanning.html. – Oliver Charlesworth Jan 19 '13 at 01:56
There is no mentioning of such a problem there. This is why I asked it here. `hasNextInt` cannot solve the issue (I couldn't. Please see the update) – RGO Jan 19 '13 at 02:00
@Reza: There's a working example of how to use `hasNext` and `next`. You should use that pattern. – Oliver Charlesworth Jan 19 '13 at 02:02

java.util.Scanner strange behaviour when used with System.in

2 Answers2