# Java Program to Implement Merge Sort Algorithm

#### In this example, we will learn to execute merge sort algorithm in Java.

## 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]) {
```

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