Need a little help with this code...
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  1. #1
    Join Date
    Feb 2013
    Posts
    1

    Need a little help with this code...

    So I finally finished compiling my code and it has no errors; however, once I run it it crashes with this error:

    java.lang.NullPointerException
    at CanadianGeography.main(CanadianGeography.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:271)

    This is the code for the program:
    Code:
    import java.awt.*;
    import hsa.Console;
    
    public class CanadianGeography
    {
      static Console c; //output console
      public static void main (String [] args)
      {
        new Console ();
        
        double finalscore, right, wrong;
        int score;
        String prov, cap;
    /*include x as an empty spot so index starts at 1*/
        String [] provanswers = {"x", "Alberta", "Saskatchewan", "Manitoba", "Ontario", "Quebec", "Newfoundland an Labrador", "New Brunswick", "Prince Edward Island", "Nova Scotia"};
        String [] capanswers = {"x", "Edmonton", "Regina", "Winnipeg","Ottawa", "Quebec City","St.John's", "Fredericton", "Charlottetown", "Halifax"}; 
    
        score = 0;
        
    /*prompt user and check answer*/  
        c.println ("What is the province farthest to the west?");
        prov = c.readString ();
        c.println ("What is the name of the capital there?");
        cap = c.readString ();
       
        if (prov == "British Colombia" & cap == "Victoria")
        {
          score = score + 1; //incriment the score if the answer is right
        }
        else
        {
         score = score + 0;//add zero to the score if the question is wrong
        }
            
    /*loop questions for the other provinces*/
        for (int i= 1; i < 11; i++)
        {
        c.println ("What is the next province east to it?");
        prov = c.readString ();
        c.println ("What is the name of the capital there?");
        cap = c.readString ();
        
    /*check answer for province and capital*/
        if (prov == provanswers [i] && cap == capanswers [i])
        {
         score = score + 1;
        }
        else
        {
         score = score + 0;//add zero to the score if the question is wrong
        }  
        
      }//end of loop
        
    /*tabulate score*/
        wrong = 20 - score;
        right = score - wrong;
        finalscore = score/20;
        
        c.println ("You got" + wrong + "wrong, and " + right + "right out of 20.");
        
        if (score <= 0.50)
        {
          c.println ("Not enough to pass. Try again.");
        }
        else if (score >= 0.90)
        {
          c.println ("You are the most brilliant person in the world");
        }
        else if (score >50 && score <90)
        {
          c.println ("Good");
        }
        
        
      }//end of main method
    }//end of class
    I looked at line 23, but I don't understand what the error is or how to fix it. Can someone help?

  2. #2
    Join Date
    Aug 2011
    Location
    West Yorkshire, U.K.
    Posts
    54

    Re: Need a little help with this code...

    Outside main, you define static Console c ... inside main you create a new Console(), then call methods like readString() on c which has not been initialized.
    Better to define console c = new Console() inside main and remove the static definition from the class - since c is not referenced outside main.
    Your string comparisons won't work because you can't use == with Strings ... and you need a double ampersand for the logical "and" ... i.e.
    Code:
        if (prov.equals("British Colombia") && cap.equals("Victoria"))
        {
          score = score + 1; //incriment the score if the answer is right
    Your loop will throw an exception too because you are looping from 1 to 10 - but the arrays are zero based so your index should be 0 to 9.
    I see you have placed a dummy entry (x) in each array to try to make them 1 based - it's probably better to remove the x and start from zero (i.e. try to think like the computer rather than like a human!). Also use the array ".length" to control the loop - then you are less likely to go beyond the bounds of the array ... i.e.
    Code:
        String [] provanswers = {"Alberta", "Saskatchewan", "Manitoba", "Ontario", 
                                 "Quebec", "Newfoundland an Labrador", "New Brunswick",
                                 "Prince Edward Island", "Nova Scotia"};
        String [] capanswers = {"Edmonton", "Regina", "Winnipeg","Ottawa", 
                                "Quebec City","St.John's", "Fredericton", "Charlottetown",
                                "Halifax"};
    
        ... other stuff omitted ...
    
    /*loop questions for the other provinces*/
        for (int i = 0; 0 < provanswers.length; i++)
        {

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