While loop skips user input

Question

I'm trying to keep a scanner command in a while loop until the user gives good input. If I run the following, it keeps looping through without waiting for input if I get the input wrong first (but waits for the first input). Why is that?

while (true){ 
    System.out.println("Enter temperature in Fahrenheit: ");
    try {
        x = in.nextFloat();
        break;
    } catch (InputMismatchException e) {
        System.out.println("Please enter a valid temperature!");
        e.printStackTrace();
        continue;
    }
}

We need to see more code. Maybe a complete running solution as we cannot tell what `in` does. Also, the continue isn't needed there. The behavior is the same without it. — just_a_programmer, Mar 06 '20 at 17:27
@just_a_programmer safe to assume that `in` is a `Scanner`... because of the `.nextFloat()` method. Make sure you're calling `.nextLine()` after `.nextFloat()` to **consume the newline character**. — sleepToken, Mar 06 '20 at 17:30

score 2 · Accepted Answer · answered Mar 06 '20 at 17:32

Try this way:

    while (true){ 
        System.out.println("Enter temperature in Fahrenheit: ");
        try {
            x = Float.parseFloat(in.nextLine());
            break;
        } catch (Exception e) {
            System.out.println("Please enter a valid temperature!");
            e.printStackTrace();
            continue;
        }
    }

Dave Costa · Answer 2 · 2020-03-06T17:33:21.920

1

When a scanner throws an InputMismatchException, the scanner will not pass the token that caused the exception, so that it may be retrieved or skipped via some other method.

per https://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html

It's rereading the same token over and over, continually trying to convert it to a float and failing.

You need to consume that token inside the exception handler, so that the next iteration of the loop will block waiting for more input.

edited Mar 06 '20 at 17:33

answered Mar 06 '20 at 17:30

Dave Costa

44,298
8
52
70

Perhaps also suggest how he could avoid this problem – sleepToken Mar 06 '20 at 17:31

While loop skips user input

2 Answers2