由于nextDouble（）而出现异常，如何避免这种情况？

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");
        }
    }
}

Answer 1

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

Answer 2

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");
    }
}