Java implementation of in-place Quick Sort. See http://en.wikipedia.org/wiki/Quick_sort#In-place_version

QuickSort.java

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

/**
 * See http://en.wikipedia.org/wiki/Quick_sort#In-place_version
 */
public class QuickSort {

    private static final Random random = new Random();
    private static final int RANDOM_INT_RANGE = 100;

    public static void main(String[] args) {
        int[] arr = newTestArray(25);
        int[] copy = Arrays.copyOf(arr, arr.length);

        System.out.println("Starting array: " + Arrays.toString(arr));

        sort(arr);

        // check the result
        Arrays.sort(copy);
        if(Arrays.equals(arr, copy)) {
            System.out.println("Sorted array: " + Arrays.toString(arr));
        } else {
            System.err.println("ERROR: expected " + Arrays.toString(copy) + " but was " + Arrays.toString(arr));
        }


    }

    private static int[] newTestArray(int size) {
        final int[] arr = new int[size];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = random.nextInt(RANDOM_INT_RANGE);
        }
        return arr;
    }

    private static void sort(int[] arr) {
        if(arr.length > 0)
            sortInPlace(arr, 0, arr.length - 1);
    }

    private static void sortInPlace(int[] arr, int left, int right) {

        if(left >= right) return; // sorted

        final int range = right - left + 1;
        int pivot = random.nextInt(range) + left;

        int newPivot = partition(arr, left, right, pivot);

        sortInPlace(arr, left, newPivot - 1);
        sortInPlace(arr, newPivot + 1, right);


    }

    private static int partition(int[] arr, int left, int right, int pivot) {

        // we're going to move around the values in the array.
        // > than this pivot value goes to the right,
        // < than this pivot value goes to the left
        int pivotVal = arr[pivot];

        // temporarily move the pivot value out of the way...
        swapArrayVals(arr, pivot, right);

        int storeIndex = left;
        for(int i = left; i <= (right - 1); i++) {
            if(arr[i] < pivotVal) {
                swapArrayVals(arr, i, storeIndex);
                storeIndex++;
            }
        }

        swapArrayVals(arr, storeIndex, right);

        System.out.println("Array sorted so far: " + Arrays.toString(arr));

        return storeIndex;
    }

    private static void swapArrayVals(int[] arr, int from, int to) {
        int fromVal = arr[from];
        int toVal = arr[to];
        arr[from] = toVal;
        arr[to] = fromVal;
    }

}