arraylist - Merge sort with array lists in java -

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so homework have write program mergesorts array lists code works regular arrays, wondering if me figure out went wrong, because code throws ton of null pointer exceptions, , have tried fix them, when fix 1 goes another...and on....

thanks! code:

private static arraylist<integer> numbers= new arraylist<integer>(); private static arraylist<integer> helper; private static int number; public static void sort(arraylist<integer> mynumbers){     for(int i=0; i<mynumbers.size();i++){         numbers.add(mynumbers.get(i));     }     //numbers=mynumbers;     number = mynumbers.size()-1;      mergesort(0, number -1); } private static void mergesort(int low, int high){     //check if low smaller high, if not array sorted     if(low<high){         //get index of element in middle         int middle=low+(high-low)/2;         //sort left side of array         mergesort(low, middle);         //sort right side of array         mergesort(middle +1, high);         //combine them both         merge(low, middle, high);     } } private static void merge(int low, int middle, int high){     //copy both parts helper array     for(int i=high;i>low;i++){         helper.add((numbers.get(i)));     }      int i=low;     int j=middle+1;     int k=low;     //copy smallest mynumbers either left or right side original array     while(i<middle  && j<high){         if(helper.get(i)< helper.get(j)){             numbers.set(k,(helper.get(i)));             i++;         }         else{             numbers.set(k,(helper.get(j)));             j++;         }         k++;     }     //copy rest of left side of array target array     while(i<middle){         numbers.set(k,helper.get(i));         k++;         i++;     } }

returns:

exception in thread "main" java.lang.nullpointerexception @ binarysearch.merge(binarysearch.java:61) @ binarysearch.mergesort(binarysearch.java:55) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.mergesort(binarysearch.java:51) @ binarysearch.sort(binarysearch.java:43) @ binarysearch.main(binarysearch.java:25)

here's culprit:

for(int i=high;i>low;i++){     helper.add((numbers.get(i))); }

use for(int i=high; i>=low; i--) { instead.


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