Aid with quick sort algorithm in java (netbeans)

I am quite new to programming and I am just starting using java. My task was to write a program using quick sort, I managed to write it but it always gives me an index out of bounds. Could anyone take a look at my code and help me by identifying what I am doing wrong? Thanks

This is the code for the main class.

package quicksort;

public class Quicksort {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here

    int[] x = {5,3,10,1,9,8,7,4,2,6,0};

    quicksort_class q = new quicksort_class(x);

    q.sort();

    for(int i = 0; i < 11-1; i++)
    {
        System.out.println(x[i]);
    }
   }
 }

This is the code for quicksort_class.

public class quicksort_class {

int[] array1 = new int[11];


public quicksort_class(int[] w)
{
    array1 = w;
}
public void partitionstep(int leftlimit, int rightlimit)
{   
    int LPointer = leftlimit;

    int RPointer = rightlimit;

    Random random = new Random();

    int midpoint = random.nextInt(11);

    int checknumber =  array1[midpoint];

    while(LPointer < RPointer)
    {
        while(array1[LPointer] <= checknumber)
        {
            LPointer = LPointer + 1;
        }

        while(array1[RPointer] >= checknumber)  
        {
            RPointer = RPointer --;
        }

     swap(LPointer, RPointer);

     partitionstep(leftlimit, midpoint - 1);

     partitionstep(midpoint + 1, rightlimit);
    }
}

public void swap(int x, int y)
{
int temp = array1[x];

array1[x] = array1[y];

array1[y] = temp;
}

public void sort()
{
partitionstep(0, array1.length - 1);
}
}