Merge Sort in java

/**
 * Generic merge sort algorithm.
 *
 * @see SortAlgorithm
 */
class MergeSort implements SortAlgorithm {

    /**
     * Generic merge sort algorithm implements.
     *
     * @param unsorted the array which should be sorted.
     * @param <T> Comparable class.
     * @return sorted array.
     */
    @Override
    public <T extends Comparable<T>> T[] sort(T[] unsorted) {
        doSort(unsorted, 0, unsorted.length - 1);
        return unsorted;
    }

    /**
     * @param arr the array to be sorted.
     * @param left the first index of the array.
     * @param right the last index of the array.
     */
    private static <T extends Comparable<T>> void doSort(T[] arr, int left, int right) {
        if (left < right) {
            int mid = (left + right) >>> 1;
            doSort(arr, left, mid);
            doSort(arr, mid + 1, right);
            merge(arr, left, mid, right);
        }
    }

    /**
     * Merges two parts of an array.
     *
     * @param arr the array to be merged.
     * @param left the first index of the array.
     * @param mid the middle index of the array.
     * @param right the last index of the array merges two parts of an array in
     * increasing order.
     */
    private static <T extends Comparable<T>> void merge(T[] arr, int left, int mid, int right) {
        int length = right - left + 1;
        @SuppressWarnings("unchecked")
        T[] temp = (T[]) new Comparable[length];
        int i = left;
        int j = mid + 1;
        int k = 0;

        while (i <= mid && j <= right) {
            if (arr[i].compareTo(arr[j]) <= 0) {
                temp[k++] = arr[i++];
            } else {
                temp[k++] = arr[j++];
            }
        }

        while (i <= mid) {
            temp[k++] = arr[i++];
        }

        while (j <= right) {
            temp[k++] = arr[j++];
        }

        System.arraycopy(temp, 0, arr, left, length);
    }

    /**
     * Driver code
     */
    public static void main(String[] args) {
        MergeSort mergeSort = new MergeSort();

        Integer[] arr = {4, 23, 6, 78, 1, 54, 231, 9, 12};
        mergeSort.sort(arr);
        for (int i = 0; i < arr.length - 1; ++i) {
            assert arr[i] <= arr[i + 1];
        }

        String[] stringArray = {"c", "a", "e", "b", "d"};
        mergeSort.sort(stringArray);
        for (int i = 0; i < stringArray.length - 1; ++i) {
            assert arr[i].compareTo(arr[i + 1]) <= 0;
        }
    }
}