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Program: Implement merge sort in java.

Merge sort is a divide and conquer algorithm.

Steps to implement Merge Sort:

1) Divide the unsorted array into n partitions, each partition contains 1 element. Here the one element is considered as sorted.
2) Repeatedly merge partitioned units to produce new sublists until there is only 1 sublist remaining. This will be the sorted list at the end.

Merge Sort

Merge sort is a fast, stable sorting routine with guaranteed O(n*log(n)) efficiency. When sorting arrays, merge sort requires additional scratch space proportional to the size of the input array. Merge sort is relatively simple to code and offers performance typically only slightly below that of quicksort.

package com.java2novice.sorting;

public class MyMergeSort {
        private int[] array;
        private int[] tempMergArr;
        private int length;

        public static void main(String a[]){
                int[] inputArr = {45,23,11,89,77,98,4,28,65,43};
                MyMergeSort mms = new MyMergeSort();
                for(int i:inputArr){
                System.out.print(" ");
        public void sort(int inputArr[]) {
                this.array = inputArr;
                this.length = inputArr.length;
                this.tempMergArr = new int[length];
                doMergeSort(0, length - 1);

        private void doMergeSort(int lowerIndex, int higherIndex) {
                if (lowerIndex < higherIndex) {
                        int middle = lowerIndex + (higherIndex - lowerIndex) / 2;
                        // Below step sorts the left side of the array
                        doMergeSort(lowerIndex, middle);
                        // Below step sorts the right side of the array
                        doMergeSort(middle + 1, higherIndex);
                        // Now merge both sides
                        mergeParts(lowerIndex, middle, higherIndex);

        private void mergeParts(int lowerIndex, int middle, int higherIndex) {

                for (int i = lowerIndex; i <= higherIndex; i++) {
                        tempMergArr[i] = array[i];
                int i = lowerIndex;
                int j = middle + 1;
                int k = lowerIndex;
                while (i <= middle && j <= higherIndex) {
                        if (tempMergArr[i] <= tempMergArr[j]) {
                                array[k] = tempMergArr[i];
                        } else {
                                array[k] = tempMergArr[j];
                while (i <= middle) {
                        array[k] = tempMergArr[i];


4 11 23 28 43 45 65 77 89 98 
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Java Sorting Algorithms Examples

  1. Implement bubble sort in java.
  2. Implement selection sort in java.
  3. Implement insertion sort in java.
  4. Implement quick sort in java.
  5. Implement merge sort in java.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java™ Platform Standard Ed. 8 - API Specification | Java is registered trademark of Oracle.
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