Why different type of exceptions thrown in below scenario?

Question

I am trying to get input from user by using Scanner and DataInputStream. Here is my code that I'm using:

Scenario 1:

Scanner scanner = new Scanner(System.in);
System.out.print("Enter a number: ");
double d1 = scanner.nextDouble();

Scenario 2:

DataInputStream din = new DataInputStream(System.in);
System.out.print("Enter a number: ");
double d2 = Double.parseDouble(in.readLine());

When providing input as some characters like abc:

In scenario 1, I'm getting InputMismatchException. In scenario 2, I'm getting NumberFormatException.

Why Scanner throws different exception? Can someone please clarify.

Answer 1

The JavaDoc of Scanner.nextDouble() says:

Scans the next token of the input as a double. This method will throw InputMismatchException if the next token cannot be translated into a valid double value. If the translation is successful, the scanner advances past the input that matched.

Returns:

The double scanned from the input

Throws:

InputMismatchException - if the next token does not match the Float regular expression, or is out of range
NoSuchElementException - if the input is exhausted
IllegalStateException - if this scanner is closed

Check your Scanner.class source:

public double nextDouble() {
        // Check cached result
        if ((typeCache != null) && (typeCache instanceof Double)) {
            double val = ((Double)typeCache).doubleValue();
            useTypeCache();
            return val;
        }
        setRadix(10);
        clearCaches();
        // Search for next float
        try {
            return Double.parseDouble(processFloatToken(next(floatPattern())));
        } catch (NumberFormatException nfe) {
            position = matcher.start(); // don't skip bad token
            throw new InputMismatchException(nfe.getMessage());
    }
}

When trying to parse, if Double.parseDouble() throws NumberFormatException (as per your Scenario 2), then Scanner.nextDouble() throw InputMismatchException (as per your Scenario 1).