Can't run jar file?

The way to debug ANY nullpointerexception

1) Go to the line. 2) Look at each method call on that line - is is possible that a method called on an object where the object is null ?

a=null ; a.setX("X"); will result in a null pointer exception. 

In your specific case, the line "if (inputLine.length() == 0)" is throwing a null pointer exception. Thus, you should make sure that "inputLine" is not null ....

Any particular reason why you aren't using the Scanner class?

package game; import java.util.Scanner; public class GameHelper { public String getUserInput(String prompt) { System.out.print(prompt + " "); Scanner scan = new Scanner(System.in); String inputline = scan.nextLine(); return inputLine.toLowerCase(); } } 

And if all you ever want it to do is use the result to ParseInt, you could change

String inputLine = scan.nextLine() 

to

int inputNumber = scan.nextInt() 

and obviously change the return type from String to int

Try this

package game; import java.io.*; public class GameHelper { public String getUserInput(String prompt) { String inputLine = null; System.out.print(prompt + " "); try { BufferedReader is = new BufferedReader(new InputStreamReader( System.in)); inputLine = is.readLine(); if (inputLine == null) return null; } catch (IOException e) { System.out.println("IOException: " + e); } return inputLine.toLowerCase(); } } 

To fix your other error surround

 numProbs=(Integer.parseInt(numProbsReader)) 

in try/catch so like this

try{ numProbs=(Integer.parseInt(numProbsReader)) }catch(Exception e){System.err.println("Invalid Input");} 

and that should help