I am getting an exception due to nextDouble(), how can I avoid this?

Question

I'm an AP computer science and I am coding java. There is only a limited number of methods and classes that I can use. For instance I am not allowed to use hasNextLine() . This is the error it gives me once I enter the value for "how many euros is one dollar." It allows me to enter that value and then asks to enter the dollar value. However, before I can enter it, this error shows up:

Exception in thread "main" java.lang.NumberFormatException: empty String
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)   
at java.lang.Double.parseDouble(Double.java:538)
at CurrencyConverter.main(CurrencyConverter.java:20)

public class Currency
{
    private double rate;

    public Currency()
    {
        rate = 0.0;
    }

    public Currency(double newRate)
    {
        rate = newRate;
    }

    public double convert(double dollar)
    {
        double euro = dollar * rate;
        return euro;
    }
}

import java.util.Scanner;
import java.util.Scanner;
public class CurrencyConverter
{
    public static void main(String [] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("How many euros is one dollar?");
        double exchangerate = input.nextDouble();
        System.out.println("Dollar value (Q to quit):");
        String dollarvalue = input.nextLine();
        double dv = 0.0;
        String bvalue = "";
        String bvaluetwo = "Q";
        if (dollarvalue.equals(bvalue))
        {
            dollarvalue = "test";
        }
        else if (!dollarvalue.equals(bvaluetwo))
        {   
            dv = Double.parseDouble(dollarvalue);
        } 

        Currency exchange = new Currency(exchangerate);

        while (dollarvalue != "Q")
        {
            double eurovalue = exchange.convert(dv);
            System.out.println(dv + " dollar = " + eurovalue + " euro");
        }
    }
}

^ thats a problem you have but also... try putting in a `nextLine()` in there to clear it out. — 3kings, Nov 06 '15 at 04:42
And put `Double.parseDouble()` in `try catch` block, it may throw `NumberFormatException` — Apurva, Nov 06 '15 at 04:45
From the error it sounds like you are not entering a number for the exchange rate but instead pressing the enter key... — cjds, Nov 06 '15 at 04:46
no i am not entering anything and the error shows up and so should i use the equals method? — kay, Nov 06 '15 at 04:46
@KeyanKazemian error shows up because " i am not entering anything" ,`double exchangerate = input.nextDouble();` expects you to enter a double value. — Ramanlfc, Nov 06 '15 at 05:02
no you dont understand it doesn't give the opportunity to enter anything @Ramanlfc — kay, Nov 06 '15 at 05:06
yes but i'm already using it. where else should i use it?? @Sekula1991 — kay, Nov 06 '15 at 05:15
Have in mind, that even if you get into while method, you will have endless loop. Do you want to have your program check for dollar value until Q is pressed? — Sekula1991, Nov 06 '15 at 05:15

Ramanlfc · Answer 1 · 2015-11-06T05:23:02.197

double exchangerate = input.nextDouble(); this will give you exception if there is no input , always use scanner's hasNext() method before calling any of the next methods

Here is your main culprit :

   System.out.println("Dollar value (Q to quit):");
            String dollarvalue = input.nextLine();
            double dv = 0.0;
            String bvalue = "";
            String bvaluetwo = "Q";
            if (dollarvalue == bvalue) {
                dollarvalue = "test";
            } else if ((dollarvalue != bvaluetwo)) {
                dv = Double.parseDouble(dollarvalue);
 }

You are storing a String of "Q" or other string values entered by user , then in dv = Double.parseDouble(dollarvalue); ,you are parsing(converting) it into a double which is throwing java.lang.NumberFormatException exception

im not allowed to use hasNext() and also what do you mean parse your input? — kay, Nov 06 '15 at 04:47

score 0 · Accepted Answer · answered Nov 06 '15 at 05:22

Try puting this into your main method instead:

        public static void main(String [] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("How many euros is one dollar?");
        double exchangerate = input.nextDouble();
        //You have to put this in order to continue
        input.nextLine();
        System.out.println("Dollar value (Q to quit):");
        String dollarvalue = input.nextLine();
        double dv = 0.0;
        String bvalue = "";
        String bvaluetwo = "Q";
        if (dollarvalue == bvalue)
        {
            dollarvalue = "test";
        }
        else if ((dollarvalue != bvaluetwo))
        {   
            dv = Double.parseDouble(dollarvalue);
        } 

        Currency exchange = new Currency(exchangerate);

        while (!dollarvalue.equals("Q"))
        {
            double eurovalue = exchange.convert(dv);
            System.out.println(dv + " dollar = " + eurovalue + " euro");
            System.out.println("Dollar value (Q to quit):");
            dollarvalue = input.nextLine();
             if (!dollarvalue.equals("Q")) {     
             dv = Double.parseDouble(dollarvalue);
             eurovalue = exchange.convert(dv);
                System.out.println(dv + " dollar = " + eurovalue + " euro");
            } 

        }
        System.out.println("You pressed Q, have a nice day");
    }
}

First you had to put input.nextLine(); below inputNextDouble(); , then i fixed your while loop because your previous loop was endless, in the while loop you get another input, and if its "Q" it just passes through, otherwise it parses your double value, and so on until you press "Q"- which was i guess your intention with that code of yours. :) — Sekula1991, Nov 06 '15 at 05:41
If you have questions with nextLine(), read this: http://stackoverflow.com/questions/13102045/skipping-nextline-after-using-next-nextint-or-other-nextfoo-methods — Sekula1991, Nov 06 '15 at 05:43
Well, i dont see a point of `bvalue` and `bvaluetwo` but thats up to you. — Sekula1991, Nov 06 '15 at 05:45

I am getting an exception due to nextDouble(), how can I avoid this?

2 Answers2