Merge Sort Example in Java

Introduction

Merge Sort is a classic Java console program that demonstrates the concept with complete source code and sample output. Sorting algorithms arrange data in order — bubble, selection, merge, quick and heap sort.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

Merge sort is an O(n log n) comparison-based sorting algorithm. Most implementations produce a stable sort, which means that the implementation preserves the input order of equal elements in the sorted output. Conceptually, a merge sort works as: Divide the unsorted list into n sublists, each containing 1 element and repeatedly merge sublists to produce new sorted sublists until there is only 1 sublist remaining. This will be the sorted list.

Merge Sort Example Program

import java.util.Arrays;
public class MergeSort {
	private int[] arr;
    private int[] tempArr;
    private int length;
    public static void main(String a[]){
        int[] arr = {67,89,32,76,41,99,12,05,40,53};
		System.out.println("Array before sorting is ");
		for(int i=0;i < arr.length;i++){
		System.out.println(arr[i]);
		}		
        MergeSort mers = new MergeSort();
        mers.sort(arr);
		System.out.println("Array after sorting is ");
        for(int i:arr){
            System.out.println(i);
        }
    }
    public void sort(int arr[]) {
        this.arr = arr;
        this.length = arr.length;
        this.tempArr = new int[length];
        MergeMethod1(0, length - 1);
    }
    private void MergeMethod1(int lowIndex, int highIndex) {
        if (lowIndex < highIndex) {
            int middle = lowIndex + (highIndex - lowIndex) / 2;
            MergeMethod1(lowIndex, middle);
            MergeMethod1(middle + 1, highIndex);
            mergeMethod2(lowIndex, middle, highIndex);
        }
    }
    private void mergeMethod2(int lowIndex, int middle, int highIndex) {
        for (int i = lowIndex; i <= highIndex; i++) {
            tempArr[i] = arr[i];
        }
        int i = lowIndex;
        int j = middle + 1;
        int k = lowIndex;
        while (i <= middle && j <= highIndex) {
            if (tempArr[i] <= tempArr[j]) {
                arr[k] = tempArr[i];
                i++;
            } else {
                arr[k] = tempArr[j];
                j++;
            }
            k++;
        }
        while (i <= middle) {
            arr[k] = tempArr[i];
            k++;
            i++;
        }
 
    }
}

Sample Output

Array before sorting is
67
89
32
76
41
99
12
05
40
53
Array after sorting is
05
12
32
40
41
53
67
76
89
99

When to use

Use this merge sort example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.Arrays; imports a class used later in the program.

3. int[] arr = {67,89,32,76,41,99,12,05,40,53}; updates a variable used in the calculation or output.

4. A println / print call writes text to the console — part of the sample output below.

5. A loop repeats the block until its condition becomes false.

6. A println / print call writes text to the console — part of the sample output below.

7. MergeSort mers = new MergeSort(); updates a variable used in the calculation or output.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.