Java While Loop

Question

Whenever I try to run this loop, I receive:

Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:840)
    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 test.readNumber(Lab2.java:28)
    at test.go(Lab2.java:15)
    at test.main(Lab2.java:7)

I'm trying to repeatedly prompt the user until the keyboard input is positive. Can anyone tell me how I can go about doing that without running into error?

    public int readNumber() {
        int x = keyboard.nextInt();
        while (x < 0) {
            System.out.println("Please enter a positive number.");
            x = keyboard.nextInt();
        }   
        return x;
    }

"InputMismatchException - if the next token does not match the Integer regular expression, or is out of range" docs.oracle.com/javase/7/docs/api/java/util/… — Héctor Valls
– Héctor Valls, Commented Feb 3, 2015 at 15:29

udalmik · Accepted Answer · 2015-02-03 15:42:03Z

3

It is always a good idea to check the docs. The exception is thrown if input data doesn't match the "Integer" pattern. Wrap input with try/catch:

public int readNumber() {
    int x = -1;
    while (x < 0) {
        System.out.println("Please enter a positive number.");
        try {
            x = keyboard.nextInt();
        } catch (java.util.InputMismatchException e) {
            // oops, wrong input
        }
    }   
    return x;
}

edited Feb 3, 2015 at 15:42

answered Feb 3, 2015 at 15:32

udalmik

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1 Comment

ControlAltDel Over a year ago

breaks if initial input is not an int

ControlAltDel · Accepted Answer · 2015-02-03 15:32:12Z

1

public int readNumber() {
        int x = -1;
        do {
            try {
               System.out.println("Please enter a positive number.");
               x = keyboard.nextInt();
            } catch (InputMismatchException e) {
              // normally you always want to handle the exception but in this case it is okay because mismatched input is the same as a non-positive integer input
            }
        } while (x < 0);
        return x;
    }

answered Feb 3, 2015 at 15:32

ControlAltDel

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Comments

user3248346 · Accepted Answer · 2015-02-03 15:31:19Z

0

You are either not supplying an integer or giving a value for an integer which is out of range.

answered Feb 3, 2015 at 15:31

user3248346

1 Comment

Grant Park Over a year ago

You were right, turns out I was giving a value that was out of range. Thanks a lot!

Jens · Accepted Answer · 2015-02-03 15:31:25Z

0

You have to read the EOL after reading te int value.

public int readNumber() {
        int x = keyboard.nextInt();
        keyboard.readNextLine();
        while (x < 0) {
            System.out.println("Please enter a positive number.");
            x = keyboard.nextInt();
            keyboard.readNextLine();  
        }   
        return x;
    }

answered Feb 3, 2015 at 15:31

Jens

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