inputMismatchException Java reading doubles from plain text file

Question

Using

 double variable = inputFile.nextDouble();

Gives the mismatch error and I can't figure out why... Anyone know what's up?

The input file is just a bunch of doubles like 5.0...

Okay here is the code snippet

String fileName;
Scanner scanner = new Scanner(System.in);
System.out.println("\nEnter file name that contains the matrix and vector: ");
fileName = scanner.nextLine();
Scanner inputFile = new Scanner(fileName);


double a1 = inputFile.nextDouble();

the input file is a plain text document .txt in this format

5.0 4.0 -3.0
4.0 2.0  5.0
6.0 5.0 -2.0
-13.0 4.0 12.0

I don't understand why it wouldn't take those as doubles...

As far as what its expecting the format of the file to be... I suppose binary? isn't that the default? I didn't specify in the code...

Answer 1

Add a check beforehand

if (inputFile.hasNextDouble()) { 
 double variable = inputFile.nextDouble();
} else if (inputFile.hasNext()) {
 System.out.println("Not double at token " + inputFile.next());
}

in order to identify why and where exactly it fails.

It could be that your delimiter isn't " " and that you haven't specified it manually. To set the delimiter, call one of useDelimiter(...) functions.

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Answer 2

InputMismatchException is the result of a scanner attempting to parse a string into a format to which it cannot to parsed. For example, calling Double.parseDouble on a string such as "3.3 meters" will throw a NumberFormatException. As iccthedral added, even a string as nontrivial as "3.0 " (notice the whitespace) will result in an NFE.

When a NumberFormatException occurs in Scanner.nextDouble() , the NFE is wrapped and rethrown in an InputMismatchException, which is what is occuring here.

To ensure that your Scanner can read a double, call Scanner#hasNextDouble() and only proceed to acquire the double if the scanner has that next double.