Why Arrays.sort() is not working on my arrayB? Also issues with java.lang.NullPointerException in main method

Question

I am currently working on a project where I need to create an initial array (anArray) with randomly generated integers and make an exact copy of those random values in another array (arrayB). I then want to sort the first array with my selectionSort() method, and the second array with Arrays.sort(). I also need to time how long each sorting method takes to complete.

So far, I can populate the first array with random integers. I'm not sure if I have correctly copied the anArray into arrayB because it's not printing as I have coded so far. anArray is sorting as required, and printing through main with no problems. Please check out my code so far and if you can advise on where to add the way to time the sorting processes that would be awesome too!

import java.util.Arrays;
import java.util.Random;

public class TimedSortOne
{

    public static int[] anArray; // initializes the first array
    public static int[] arrayB; // initializes the second array

    /**
      * This method produces integers of random values.
      * @return int randomNum
      */
    private static int randomFill()
    {       
        Random rand = new Random();
        int randomNum = rand.nextInt();
        return randomNum;        
    }

    private static int[] list()
    {
        anArray = new int[1000];
        for(int i=0;i<anArray.length;i++)
        {
            anArray[i] = randomFill();
        }
        return anArray;
    }

    /**
     * This method sorts the values of anArray into ascending order by 
     * repeatedly finding the largest value and moving
     * it to the last index in the array.
     * @param int[] anArray
     */
    private static void selectionSort(int[] anArray)
    {

        for (int lastPlace = anArray.length-1; lastPlace > 0; lastPlace--)
        {
            int maxLoc = 0;

            for (int j = 1; j <= lastPlace; j++)
            {
                if (anArray[j] > anArray[maxLoc]) 
                {
                    maxLoc = j;
                }
            }

            int temp = anArray[maxLoc];
            anArray[maxLoc] = anArray[lastPlace];
            anArray[lastPlace] = temp;

        }
    }


    /**
     * This method populates arrayB with an exact copy
     * of integer values from anArray using System.arraycopy() function.
     * @param anArray
     * @return arrayB
     */
    private static int[] arrayCopyFull(int[] anArray)
    {
        int[] temp = new int[anArray.length];
        System.arraycopy(anArray, 0, temp, 0, anArray.length);
        return temp;
    }

    public static void main(String[] args)
    {

        list();
        arrayCopyFull(anArray);
        selectionSort(anArray);
        System.out.println("The sorted integers in  anArray are:");
            for (int numbers : anArray) {
             System.out.println(numbers);
            }

        System.out.println("The sorted integers in arrayB are:");
            for (int bNumbers : arrayB) {
               System.out.println(bNumbers);
            }
    }

}

So, I'm expecting to produce a printed list of anArray, sorted, with a record of the time it took to sort anArray.

And then, using arrayB as an exact copy of the random integers in anArray, sort arrayB with Arrays.sort() all with a record of the sorting time.

`arrayCopyFull()` is returning the new array but the returned array is never used or assigned to any variable/field ; `arrayB` is never initialized (just writing it in a comment does not do it - it is just being declared) ; BTW comments like `// initializes the first array` don;t help, no reason to write what a command is doing, better describe WHY, and this comment is just wrong — user85421, Apr 16 '19 at 05:38
Looks like your `arrayB` is null. Not able to see where it is initialize in your code snippet. And you are looping over null object causes `NullPointerException` — Sudhir Ojha, Apr 16 '19 at 05:39
I'd strongly recommend making both your `public static` arrays into local variables, as misuse of them will be better detected by the compiler. — Ken Y-N, Apr 16 '19 at 05:40

score 0 · Answer 1 · answered Apr 16 '19 at 06:45

You NPE was caused by not assigning the arrayB to the output of the array copy.

Try this code:

import java.util.Arrays;
import java.util.Random;
import java.util.Date;

public class TimedSortOne
{

    public static int[] anArray; // initializes the first array
    public static int[] arrayB; // initializes the second array

    /**
      * This method produces integers of random values.
      * @return int randomNum
      */
    private static int randomFill()
    {       
        Random rand = new Random();
        int randomNum = rand.nextInt();
        return randomNum;        
    }

    private static int[] list()
    {
        anArray = new int[1000];
        for(int i=0;i<anArray.length;i++)
        {
            anArray[i] = randomFill();
        }
        return anArray;
    }

    /**
     * This method sorts the values of anArray into ascending order by 
     * repeatedly finding the largest value and moving
     * it to the last index in the array.
     * @param int[] anArray
     */
    private static void selectionSort(int[] anArray)
    {

        for (int lastPlace = anArray.length-1; lastPlace > 0; lastPlace--)
        {
            int maxLoc = 0;

            for (int j = 1; j <= lastPlace; j++)
            {
                if (anArray[j] > anArray[maxLoc]) 
                {
                    maxLoc = j;
                }
            }

            int temp = anArray[maxLoc];
            anArray[maxLoc] = anArray[lastPlace];
            anArray[lastPlace] = temp;

        }
    }


    /**
     * This method populates arrayB with an exact copy
     * of integer values from anArray using System.arraycopy() function.
     * @param anArray
     * @return arrayB
     */
    private static int[] arrayCopyFull(int[] anArray)
    {
        int[] temp = new int[anArray.length];
        System.arraycopy(anArray, 0, temp, 0, anArray.length);
        return temp;
    }

    public static void main(String[] args)
    {

        list();
        arrayB = arrayCopyFull(anArray); //forgot to assigned to arrayB

        long startTime = new Date().getTime();
        selectionSort(anArray);
        long endTime = new Date().getTime();
        System.out.println("Time taken for selectionSort(): " + (endTime - startTime) + " seconds.");


        startTime = new Date().getTime();
        Arrays.sort(arrayB);
        endTime = new Date().getTime();
        System.out.println("Time taken for Arrays.sort(): " + (endTime - startTime) + " seconds.");


        System.out.println("The sorted integers in  anArray are:");
            for (int numbers : anArray) {
             System.out.println(numbers);
            }

        System.out.println("The sorted integers in arrayB are:");
            for (int bNumbers : arrayB) {
               System.out.println(bNumbers);
            } 

    }

}

score 0 · Answer 2 · answered Apr 16 '19 at 06:58

0

In your code arrayB is never initialized. Use "arrayB" in place of "temp" inside "arrayCopyFull()" method. That will solve the NPE error.

For calculation of time follow the link here. It has detailed methods that you can use for time addition.

answered Apr 16 '19 at 06:58

Mayank Maria

146
5

Why Arrays.sort() is not working on my arrayB? Also issues with java.lang.NullPointerException in main method

2 Answers2