JavaAlgorithms/Sorts/MergeSort.java

package Sorts;

import static Sorts.SortUtils.print;

/**
 * This method implements the Generic Merge Sort
 *
 * @author Varun Upadhyay (https://github.com/varunu28)
 * @author Podshivalov Nikita (https://github.com/nikitap492)
 * @see SortAlgorithm
 */

class MergeSort implements SortAlgorithm {

    /**
     * This method implements the Generic Merge Sort
     *
     * @param unsorted the array which should be sorted
     * @param <T>      Comparable class
     * @return sorted array
     */
    @Override
    @SuppressWarnings("unchecked")
    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
     *              Recursively sorts the array in increasing order
     **/
    private static <T extends Comparable<T>> void doSort(T[] arr, int left, int right) {
        if (left < right) {
            int mid = left + (right - left) / 2;
            doSort(arr, left, mid);
            doSort(arr, mid + 1, right);
            merge(arr, left, mid, right);
        }

    }

    /**
     * This method implements the merge step of the merge sort
     *
     * @param arr   The array to be sorted
     * @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;
        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 program
    public static void main(String[] args) {

        // Integer Input
        Integer[] arr = {4, 23, 6, 78, 1, 54, 231, 9, 12};
        MergeSort mergeSort = new MergeSort();
        mergeSort.sort(arr);

        // Output => 1	   4  	 6	9	12	23	54	78	231
        print(arr);

        // String Inpu
        String[] stringArray = {"c", "a", "e", "b", "d"};
        mergeSort.sort(stringArray);
        //Output => a	b	c	d	e
        print(stringArray);
    }
}