Java: always receive NoSuchElementException when use Scanner

Question

I have this simple code:

    public class Example {
       public Example() {
          Scanner scanner = new Scanner(System.in);
          int row = scanner.nextInt();   // exception at this line
          scanner.close();
       }

    public static void main(String[] args) {
        Example ex1 = new Example();   // this line successfully operate
        Example ex2 = new Example();   // exception : no such element exception at above line
    }
  }

I don't know why I always receive this Exception, when code run to ex2.

That code won't even compile. Please don't post "almost" code to this site. Instead please only post real code. — Hovercraft Full Of Eels, Oct 06 '12 at 11:53
@HovercraftFullOfEels Sorry, I have edited my code. My full code is long, and I type by hand again to this site, so many error found :) — hqt, Oct 06 '12 at 12:07
thanks for the correction. @Issam has your answer it appears. — Hovercraft Full Of Eels, Oct 06 '12 at 12:10
@hqt.. If you google - `Reading user input using Scanner`.. You will notice millions of results screaming at you within 0.001 seconds.. — Rohit Jain, Oct 06 '12 at 12:20
possible duplicate of [Validating input using java.util.Scanner](http://stackoverflow.com/questions/3059333/validating-input-using-java-util-scanner) — Rohit Jain, Oct 06 '12 at 12:22

score 4 · Answer 1 · answered Oct 06 '12 at 12:00

4

You Should add if(Scanner.hasNext()) before invoking scanner.nextInt(); You have the exception because no int found to be read.

answered Oct 06 '12 at 12:00

James from CppDepend Team

1,365
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3

It should be `hasNextInt()`. The `hasNextXXX()` should match the `nextXXX()`. But 1+ since your concept is correct. – Hovercraft Full Of Eels Oct 06 '12 at 12:03
in case that no such int to be read, what should to try next ? And why when I create another object, this error will appear ? (not appear at first time, i create ex1 but ex2). Thanks :) – hqt Oct 06 '12 at 12:19
1

What happen exactly is when you close the first scanner, you close also the stdin as described in the doc of scanner close method(http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html), for this reason the second will not works – James from CppDepend Team Oct 06 '12 at 12:31
it means : we shouldn't close scanner as my example above ? – hqt Oct 06 '12 at 12:47
1

the better way is to use one scanner instance and close it at the end of your main method.no need to have two scanners. – James from CppDepend Team Oct 06 '12 at 12:50

Dunes · Accepted Answer · 2012-10-07T09:09:14.870

The problem is because you close the Scanner which in turn closes the underlying InputStream (in this case stdin). When you try to use stdin in again the Scanner is unable to retrieve any data because stdin has been closed.

If running directly from the commandlne then the correct way to provide access to stdin is to use the Console class. The console class provides a Reader wrapped around stdin that has a no-op close method. eg.

public class Example {
   public Example() {
      Scanner scanner = new Scanner(System.console().reader()); 
      // note change on above line
      int row = scanner.nextInt();
      scanner.close();
   }
}

Note, if you access stdin other than via the Console class then you'll likely cause problems for yourself. And if you invoke your java program other than directly from the command line then you will not get access to the console. For instance, the following will invokations cause problems.

echo 2 3 | java Example

or

Process p = new ProcessBuilder("java", "Example").start();
// write data to process

@IMustBeSomeone Stdin is a generic name for for an input stream that all programs get. In java, `System.in` is the specific object that represents stdin. — Dunes, Jun 10 '16 at 07:11

Java: always receive NoSuchElementException when use Scanner

2 Answers2