java.lang.NumberFormatException: empty String?

[a22asin]

Hey, ok so i keep getting this error for my program when asking the user for the value of x:

Exception in thread "main" java.lang.NumberFormatException: empty String
at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
at java.lang.Double.parseDouble(Unknown Source)
at question2.PolynomialTest.main(PolynomialTest.java:25)



coming from this part of my code:

String x;
do{
System.out.print ("Enter a value of x for which to evaluate the polynomial: ");
x = key.nextLine();
if (!x.equals("quit")){
double in = Double.parseDouble(x);
double result = p.evaluate(in);
System.out.println ("For x = " + x +", polynomial =" + result);
}
}while(!x.equals("quit"));


The Program will run right till it asks for an input for x, it wont let me input a value, it just gives the error. I tried initializing x as a random string at the start since the error says "empty string", i tried making "quit" a variable and comparing it to x instead. But nothing seems to work and i cant figure out why im getting this. Any ideas? oh and line 25 would be when i parse the double.

[DemonWasp]

"Empty string" is different from "null string". Empty means "no content", null means "no value".

So after x = key.nextLine(); , x isn't null, it's just empty. Without seeing how 'key' is set up, I can't tell you anything more about why that would be the case (don't forget: the 'nextLine()' method will return an empty string if it encounters an extra line-break in its input).

[a22asin]

DemonWasp @ Wed Jan 22, 2014 10:26 pm wrote:

"Empty string" is different from "null string". Empty means "no content", null means "no value".

So after x = key.nextLine(); , x isn't null, it's just empty. Without seeing how 'key' is set up, I can't tell you anything more about why that would be the case (don't forget: the 'nextLine()' method will return an empty string if it encounters an extra line-break in its input).

Well the error occurs before i have a chance to enter a value for x. The key is the scanner object and works ealier in the code, so thats why i dont get whats wrong. Heres my entire code as maybe something may have been wrong in between.

code:

package question2; import java.util.*; public class PolynomialTest {         public static void main(String[] args){                 Scanner key = new Scanner (System.in);                                          System.out.print ("Enter polynomial degree: ");                 int index = key.nextInt();                 Polynomial p = new Polynomial(index);                                 for (int i = 0; i <= index; i++){                         System.out.print ("Enter polynomial degree " + i + ": ");                         p.setCoefficient(i, key.nextDouble());                  }                                 System.out.println(p);                 String x;                 do{                         System.out.print ("Enter a value of x for which to evaluate the polynomial: ");                         x = key.nextLine();                         if (!x.equals("quit")){                                 double in = Double.parseDouble(x);                                 double result = p.evaluate(in);                                 System.out.println ("For x = " + x +", polynomial =" + result);                         }                 }while(!x.equals("quit"));         } }

[Tony]

Think about the difference between

code:

int index = key.nextInt();

and

code:

x = key.nextLine();

hint: read the documentation.
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html#nextInt()
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html#nextLine()