Scanner inside method does not wait for answer

Question

Have been struggling with this for a day, reading the discussion forum back and forth, no result. Anyone can tell me why the second call of the function aMenu() returns a zero and does not wait for new user input instead? I tried various things, like hasNextInt(), nextLine(), nothing worked. Shouldn't hasNextInt() block until the user writes something? How can I solve this? Thanks.

package FirstJavaPackage; import java.util.Scanner; public class testScanner { public static void main(String[] args) { int choice = aMenu(); System.out.println("You typed: "+choice); choice = aMenu(); System.out.println("You typed: "+choice); } public static int aMenu() { int result = 0; System.out.println("In aMenu... enter an int: "); Scanner keyboard = new Scanner(System.in); if (keyboard.hasNextInt()) result = keyboard.nextInt(); keyboard.close(); return result; } }

The output is:

In aMenu... enter an int: 2 You typed: 2 In aMenu... enter an int: You typed: 0

Community · Accepted Answer · 2017-05-23 10:25:18Z

1

You need to re-use the same Scanner object across the calls to aMenu():

public static void main(String[] args) { Scanner keyboard = new Scanner(System.in); int choice = aMenu(keyboard); System.out.println("You typed: "+choice); choice = aMenu(keyboard); System.out.println("You typed: "+choice); } public static int aMenu(Scanner keyboard) { int result = 0; System.out.println("In aMenu... enter an int: "); result = keyboard.nextInt(); return result; }

For further discussion, see How to use multiple Scanner objects on System.in?

edited May 23, 2017 at 10:25

CommunityBot

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answered Mar 7, 2013 at 9:10

NPE

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3 Comments

x x Over a year ago

Thanks all, now it works fine, but I still don't get the logic completely. Ok, I was closing the input stream by the end of the aMenu() function, BUT wasn't I opening it again at the next call, with the line Scanner keyboard = new Scanner(System.in) ? That's how I ment it to happen, and I had already read the links you recommended, but I guess I'm still missing something. Why doesn't it work like that, closing and re-opening the stream later? Thanks again, you are making a great community here!

NPE Over a year ago

@xx: For one thing, when you call keyboard.close(), that will also close System.in. From that point on, you won't be able to read anything from System.in, irrespective of whether you attempt to create any further scanners.

x x Over a year ago

Thanks, now I get it. So, once a stream is closed, it remains closed for the rest of the program and you cannot scan something that does not exist..

Jean Logeart · Accepted Answer · 2013-03-07 09:19:09Z

After the first call, you actually close the System.in input stream.

From the Scanner.close() documentation:

When a Scanner is closed, it will close its input source if the source implements the Closeable interface.

Try not to close your scanner at the end of aMenu: initialize the scanner outside the aMenu method and make the method use it.

Jugal Shah · Accepted Answer · 2013-03-07 09:22:57Z

Since scanner.close will close the entire input source, you should pass scanner to your aMenu method and do something like this:

import java.util.Scanner; public class TestScanner { public static void main(String[] args) { Scanner keyboard = new Scanner(System.in); int choice = 0; do { choice = aMenu(keyboard); System.out.println("You typed: " + choice); } while (choice > 0); keyboard.close(); } public static int aMenu(Scanner keyboard) { int result = 0; System.out.println("In aMenu... enter an int: "); if (keyboard.hasNextInt()) result = keyboard.nextInt(); return result; } }

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