Scanner does not read user input once the thread throws an exception in Java

Question

I have a separate thread class and below is the run method,

    @Override
    public void run() {

        Scanner scanner = new Scanner(System.in);
        int iRequestType = 0;
        int iRequestId = 0;

        while (!exit){
            try {
                System.out.print("\nRealTime - 1\nHttpUrl - 2\nSelect one : ");
                iRequestType = scanner.nextInt(); //it will wait here after the exception occurs

                System.out.print("\nEnter Request ID : ");
                iRequestId = scanner.nextInt();

                if(iRequestType == 1 && !map_RealTimeRequests.isEmpty()){
                    sendRealTimeRequest(iRequestId);
                } else if (iRequestType == 2 && !map_HttpUrlRequests.isEmpty()){
                    sendHttpUrlRequest(iRequestId);
                } else if(iRequestType > 2) {
                    System.out.println("Invalid List Number");
                }else {
                    System.out.println("Empty List");
                }
            } catch (InputMismatchException e){
                System.out.println("Invalid request type or ID : " + e);
                scanner.close();
                scanner = new Scanner(System.in);
            }

        }
    }

If I enter numbers, the program works fine. When I enter a character, it goes to the catch block and and execute the lines within it as expected. Once the execution of catch block completes, it will iterate again and ask for the keyboard inputs. But when I try to enter a value (a number or a character), it does not read anything and keep waiting without executing the next lines. What I want to do is, ask the user to enter a number and if the user enter a character, print an error message in console and iterate the while loop again. How can I solve this issue? Thanks in advance!

Check this : https://stackoverflow.com/questions/2496239/how-do-i-keep-a-scanner-from-throwing-exceptions-when-the-wrong-type-is-entered — Som, May 26 '20 at 04:59
Check this : https://stackoverflow.com/questions/2912817/how-to-use-scanner-to-accept-only-valid-int-as-input — Som, May 26 '20 at 05:00
Does this answer your question? [How to use Scanner to accept only valid int as input](https://stackoverflow.com/questions/2912817/how-to-use-scanner-to-accept-only-valid-int-as-input) — Som, May 26 '20 at 05:01
Please clarify your question. ``But when I try to enter a value (even a number)...`` what's the value that triggers the problem? ``...keep waiting without executing the next lines`` so on which line it get's stuck. And in which way do you think it's related to an exception? Try to reduce your code as in [How to create a Minimal, Reproducible Example](https://stackoverflow.com/help/minimal-reproducible-example). — Ivo Mori, May 26 '20 at 05:04
Exception should trigger if the user enter an character instead of a number. The problems occurs after an exception triggered. If I enter a character (let's say "q"), it triggers the exception and execute the catch block. Then it will iterate again and waits for user inputs (I have added a comment). Then it should execute the rest of the lines if I enter the correct value(a number). But without executing the rest of the code, it waits @IvoMori — Manujaya Premathilaka, May 26 '20 at 05:31
I have updated the question. Can you please re-check it? @IvoMori — Manujaya Premathilaka, May 26 '20 at 05:32
otherwise it will keep the last inserted value. @AlwaysLearning — Manujaya Premathilaka, May 26 '20 at 06:04
Curious I don't get the same behaviour... I run into ``Exception in thread "Thread-3" java.util.NoSuchElementException`` trying to reproduce your steps. I also can provoke that error by running your code once (very first while-loop execution) and just entering q... (triggered by your 2nd scanner.nextInt()) My suggestions: 1) Do your user input validation without relying on exception(s): see other Q/A as mentioned by @Som. 2) ___Don't replace/close the scanner___. 3) Provide a complete and easy reproducible example when asking for debugging help. Thx. — Ivo Mori, May 26 '20 at 06:37

Always Learning · Accepted Answer · 2020-05-26T06:59:48.943

0

You should not call scanner.close() because that closes System.in. You don't want to close System.in you just want to clear out the buffer. You can do that by replacing the scanner as you did (without the close() call), or you can call nextLine() if it's single-line input like this:

} catch (InputMismatchException e){
  System.out.println("Invalid request type or ID : " + e);
  scanner.nextLine();
}

edited May 26 '20 at 06:59

answered May 26 '20 at 06:10

Always Learning

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Doing this change to the OP's code fixes the ``Exception in thread "Thread-3" java.util.NoSuchElementException`` (uncaught exception) as described in my comment. – Ivo Mori May 26 '20 at 06:17
I changed the catch to accept any Exception object – Always Learning May 26 '20 at 06:24
I found that the ``Exception in thread "Thread-3" java.util.NoSuchElementException`` is triggered by the OP's ``scanner.close();`` statement in the catch block. Given the OP's original code and just changing the catch ``InputMismatchException`` to ``Exception`` results in an infinite-loop. ___The crucial point is as you stated "not to call ``scanner.close()``"___. So your original answer, ``catch (InputMismatchException e`` is actually "fine", i.e it doesn't make a difference in this case. – Ivo Mori May 26 '20 at 06:55
makes sense - I put the example back and avoiding .close should solve things – Always Learning May 26 '20 at 07:00

Scanner does not read user input once the thread throws an exception in Java

1 Answers1