Quick sort problem

the professor wrote a quick sort algorithm on the board and just told us to copy and read it at home. How ever it didn't run and i tried my best to learn it online and fix the code but it still doesn't seem to run, or im misunderstanding quick sort.

Code:

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public class QuickSort{
        public QuickSort(int[] anArray){
                a=anArray;
        }
public void sort(int from,int to){
        if(from>=to) return;
        int p=partition(from,to);
        sort(from,to);
        sort(p+1,to);
        }
private int partition(int from,int to){
        int pivot=a[from];
        int j=to+1;
        int i=from;
        while(i<j){
                i++;
                while(a[i]<pivot)i++;
                        j--;
                        while(a[j]>pivot)j--;
                        if(i<j)
                                swap(i,j);
                }
                return j;
        }
        private void swap(int i,int j){
                int temp=a[i];
                a[i]=a[j];
                a[j]=temp;
        }
private int[] a;
}

public class QuickSortTester {
        public static void main(String args[]){
                ArrayUtil au=new ArrayUtil();
                int[] a=au.randomIntArray(20, 100);
                QuickSort qs=new QuickSort(a);
                qs.sort(0,19); //2 random numbers i put, cuz i don't understand it.
                au.print(a);
                }
        }
}

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If anyone can point out where its wrong, that will be great. Thanks in advance.