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Thread: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer...)

  1. #1
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    Default Practicing before taking a class(Exception in thread "main" java.lang.NullPointer...)

    public class primes
    {
        // instance variables - replace the example below with your own
        private static int[] primes;
        private static int numPrimes;
        private static int ndx;
        private static int testPrime;
     
        /**
         * Constructor for objects of class primes
         */
        public primes()
        {
            // initialise instance variables
        }
     
        /**
         * An example of a method - replace this comment with your own
         * 
         * @param  y   a sample parameter for a method
         * @return     the sum of x and y 
         */
        public static int getPrime(int whichPrime)
        {
            int[] primes = new int[whichPrime];
     
            while (numPrimes < whichPrime)
            {
                for (ndx = 0; ndx < numPrimes && testPrime % primes[ndx] != 0; ndx++
                {
                    ;
                }
                if(ndx == numPrimes)
                {
                    primes[numPrimes++] = testPrime;
                }
                testPrime++;
            }
     
            return primes[whichPrime];
        }
    }
    -------------------------

    import java.util.Scanner;
     
    public class PrimeFinder extends primes
    {
        static public void main (String[] args)
        {
            Scanner input = new Scanner(System.in);
            int whichPrime;
            int finalprime;
     
            System.out.print("Enter which prime you want: ");
            whichPrime = input.nextInt();
     
            finalprime = getPrime(whichPrime);
     
            System.out.println("The #" + whichPrime + " prime is" + finalprime);
        }
    }

    -----------

    error:
    Exception in thread "main" java.lang.NullPointerException
    at primes.getPrime(primes.java:40)
    at PrimeFinder.main(PrimeFinder.java:21)


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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    String s = null;
    System.out.println(s.length());
    The above code will generate a null pointer exception because the s variable is null. On line 40 of your code you have done something similar. You need to see what variables you are using on line 40, find out which one is null and then why it is null.
    Improving the world one idiot at a time!

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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    Thank you for the answer, I was able to get my code to finish without any interferences, however it does not work correctly. The finalPrime always equals 0 for some reason, do you have any idea why this could be?


    Also, I have done this same program as a single file and it worked correctly, I am doing it now as 2 files to practice some new techniques before entering my java class.
    Last edited by JohnBowers; August 8th, 2011 at 08:44 PM.

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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    The code you posted above doesn't even compile.
    for (ndx = 0; ndx < numPrimes && testPrime % primes[ndx] != 0; ndx++
    {
    ;
    }
    Incomplete for loop statement. You also have a missing statement inside loop (which will compile but I have to wonder why you have it)
    Class names should begin with a capital letter.
    Giving your class and an instance variable the same name will only lead to confusion.
    You have 2 arrays called primes.
    Why does PrimeFinder extend primes? Is it a special type of primes?
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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    Another problem
    int[] primes = new int[whichPrime];
    return primes[whichPrime];
    This will cause an ArrayIndexOutOfBoundsException. If you create an array with a size of 10 then the valid indicies are 0 - 9 and 10 is out of bounds.
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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    Quote Originally Posted by Junky View Post
    Another problem
    int[] primes = new int[whichPrime];
    return primes[whichPrime];
    This will cause an ArrayIndexOutOfBoundsException. If you create an array with a size of 10 then the valid indicies are 0 - 9 and 10 is out of bounds.

    That part actually compiled okay, it's just the ending print out always comes back as 0.

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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    It might compile but when you run the code it will throw the out of bounds exception. If it doesn't then the code you are running and the code you posted are different. Please post the latest version of your code.
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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    Yes, the code will compile. But it shouldn't execute without the error.
    it's just the ending print out always comes back as 0.
    Can you show the code that returns a 0?

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    Default Re: Practicing before taking a class(Exception in thread "main" java.lang.NullPointer

    I have just taken the code you posted above, fixed the for loop and ran it. It throws a divide by zero error. I'm damned if I know how you are capable of running the code you posted without getting errors and having a value (any value) displayed.
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