Implementation of Merge Sort in Java.

MergeSort.java

/**
 * @file
 *  Merge sort class.
 */

public class MergeSort {


	/**
	 * Printing vector method.
	 *
	 * Prints on screen the elements of the vector, from first to last element.
	 *
	 * @param int[] a
	 *  Vector of integers.
	 * @return void
	 */
	public static void print_vector(int[] a) {

		if (a == null) {
			System.out.println("Null vector.");
			return;
		}

		System.out.println();

		System.out.print("< ");

		for (int i = 0; i < a.length; i++) {

	        System.out.print(a[i] + ", ");
    	}

    	System.out.print(" >");

    	System.out.println();
	}


	/**
	 * Merge sort.
	 *
	 * Recursive version.
	 *
	 * Sort order: Ascending.
	 *
	 * @param int[] a
	 *  Vector of integers.
	 * @param int begin
	 *  Initial index of vector.
	 * @param int end
	 *  Last index of vector.
	 * @return void
	 */
	public static void merge_sort_ascending(int[] a, int begin, int end) {

		if (a == null) return;

		if (end <= begin) return;

		int middle = (begin + end) / 2;

		merge_sort_ascending(a, begin, middle);
		merge_sort_ascending(a, middle + 1, end);

		int[] A = new int[middle - begin + 1];
		int[] B = new int[end - middle];

		for (int i = 0; i <= middle - begin; i++) {

			A[i] = a[begin + i];
		}

		for (int i = 0; i <= end - middle - 1; i++) {

			B[i] = a[middle + 1 + i];
		}

		int i = 0;
		int j = 0;

		for (int k = begin; k <= end; k++) {

			if (i < A.length && j < B.length) {

				if (A[i] < B[j]) {

					a[k] = A[i++];
				}
				else {

					a[k] = B[j++];
				}
			}
			else if (i < A.length) {

				a[k] = A[i++];
			}
			else if (j < B.length) {

				a[k] = B[j++];
			}
		}
	}


	/**
	 * Overloading of merge_sort_ascending() method.
	 *
	 * Simplifies the call of method.
	 */
	public static void merge_sort_ascending(int[] a) {

		merge_sort_ascending(a, 0, a.length - 1);
	}


	/**
	 * Merge sort.
	 *
	 * Recursive version.
	 *
	 * Sort order: Descending.
	 *
	 * @param int[] a
	 *  Vector of integers.
	 * @param int begin
	 *  Initial index of vector.
	 * @param int end
	 *  Last index of vector.
	 * @return void
	 */
	public static void merge_sort_descending(int[] a, int begin, int end) {

		if (a == null) return;

		if (end <= begin) return;

		int middle = (begin + end) / 2;

		merge_sort_descending(a, begin, middle);
		merge_sort_descending(a, middle + 1, end);

		int[] A = new int[middle - begin + 1];
		int[] B = new int[end - middle];

		for (int i = 0; i <= middle - begin; i++) {

			A[i] = a[begin + i];
		}

		for (int i = 0; i <= end - middle - 1; i++) {

			B[i] = a[middle + 1 + i];
		}

		int i = 0;
		int j = 0;

		for (int k = begin; k <= end; k++) {

			if (i < A.length && j < B.length) {

				if (A[i] > B[j]) {

					a[k] = A[i++];
				}
				else {

					a[k] = B[j++];
				}
			}
			else if (i < A.length) {

				a[k] = A[i++];
			}
			else if (j < B.length) {

				a[k] = B[j++];
			}
		}
	}


	/**
	 * Overloading of merge_sort_descending() method.
	 *
	 * Simplifies the call of method.
	 */
	public static void merge_sort_descending(int[] a) {

		merge_sort_descending(a, 0, a.length - 1);
	}


	/**
	 * Examples of use.
	 */
    public static void main(String args[]) {

    	int[] a = { 24, 4, 67, 4, 3, 5, 6, 6, 7, 34, 12, 12, 24, 35, 2, 24, 45, 56, 78 };

    	merge_sort_descending(a);

    	System.out.println("Initial vector:");

    	print_vector(a);
    }
}