Question

I’m learning java and I’m trying to make a very simple application that does conversion of currency. You enter a rate, a direction (e.g : from euro to dollars or reverse) and an amount. the numbers valid non negative numbers.

So far I managed to make it so that the number can’t be negative; now I need to throw an error if it’s not a number.
I have following code:

public void setKoers(double koers) 
            throws NegativeValueException, NumberFormatException{
    if (koers > 0 ) {
            this.koers=koers;
        } else {
            throw new NegativeValueException("negative number");
        }
}

and my main looks like

    try   {
            cal.setKoers( Double.parseDouble(args[0])); 

          } catch(NegativeValueException e) {
             System.out.println(e.getMessage());
          } catch (NumberFormatException e) {
             System.out.println( e.getMessage());
          }

So how can I check if koers is a number or not.
I know I could put try and catch the error in my code, but I think this would go against the logic of where and how to deal with errors: in my main function I should catch any NumberFormatException

Answer 1

You do not need the NumberFormatException since Double.parseDouble() takes care of it for you. If it is not a proper number (in this case a Double) than the parseDouble() method will throw a NumberFormatException for you.

Here’s how I would write it: (just take out the NumberFormatException)

public void setKoers(double koers) throws NegativeValueException {
    if (koers > 0 ) {
            this.koers=koers;
        } else {
            throw new NegativeValueException("negative number");
        }
}

try{
    cal.setKoers( Double.parseDouble(args[0]));

    } catch(NegativeValueException e) {
        System.out.println(e.getMessage());
    } catch (NumberFormatException e) {
        System.out.println( e.getMessage());
    }