Example: Java Program to Implement Merge Sort Algorithm

import java.util.Arrays;

// Merge sort in Java

class Main{

  // Merge two sub arrays L and M into array
  void merge(int array[], int p, int q, int r){

    int n1 = q - p + 1;
    int n2 = r - q;

    int L[] = new int[n1];
    int M[] = new int[n2];

    // fill the left and right array
    for (int i = 0; i < n1; i++)
      L[i] = array[p + i];
    for (int j = 0; j < n2; j++)
      M[j] = array[q + 1 + j];

    // Maintain current index of sub-arrays and main array
    int i, j, k;
    i = 0;
    j = 0;
    k = p;

    // Until we reach either end of either L or M, pick larger among
    // elements L and M and place them in the correct position at A[p..r]
    // for sorting in descending
    // use if(L[i] >= <[j])
    while (i < n1 && j < n2) {
      if (L[i] <= M[j]) {
        array[k] = L[i];
        i++;
      } else {
        array[k] = M[j];
        j++;
      }
      k++;
    }

    // When we run out of elements in either L or M,
    // pick up the remaining elements and put in A[p..r]
    while (i < n1) {
      array[k] = L[i];
      i++;
      k++;
    }

    while (j < n2) {
      array[k] = M[j];
      j++;
      k++;
    }
  }

  // Divide the array into two sub arrays, sort them and merge them
  void mergeSort(int array[], int left, int right){
    if (left < right) {

      // m is the point where the array is divided into two sub arrays
      int mid = (left + right) / 2;

      // recursive call to each sub arrays
      mergeSort(array, left, mid);
      mergeSort(array, mid + 1, right);

      // Merge the sorted sub arrays
      merge(array, left, mid, right);
    }
  }

  public static void main(String args[]){

    // created an unsorted array
    int[] array = { 6, 5, 12, 10, 9, 1 };

    Main ob = new Main();

    // call the method mergeSort()
    // pass argument: array, first index and last index
    ob.mergeSort(array, 0, array.length - 1);

    System.out.println("Sorted Array:");
    System.out.println(Arrays.toString(array));
  }
}

Output 1

Unsorted Array:
[6, 5, 12, 10, 9, 1]
Sorted Array:
[1, 5, 6, 9, 10, 12]

Here, the elements of the array are sorted in ascending order. If we want to sort the elements in descending order, then inside the first while loop of the merge() method, we can change the code as:

// the less than sign is changed to greater than
if (L[i] >= M[j]) {