Write a Java Program to Find missing Number in an Array

Write a Java Program to Find missing Number in an Array thumbnail
By Dhiraj Ray 15 February, 2020

This program is about finding the missing number from an array of length N-1 containing elements from 1 to N in Java. The trick to find the missing number is using the mathematical formula of Sum of Series to find the sum of N numbers and then subtract it from the sum of all the numbers of any given array.

Program Description

Assume, you have a number N(e.g. 9) and an array containing numbers from 1 to N-1 (e.g 1 - 8), meaning there is any random number missing in tha array between 1 to 9. Now, you need to write a Java program to find that missing number from the array.

Below is the sample program:

package com.devglan;

import java.util.Arrays;

public class FindMissingNumber {

    public static int calculateSumOfNNumbers(int n){
        return (n * (n + 1))/2;

    public static int calculateSum(int [] array){
        return Arrays.stream(array).sum();

    public static void main(String [] args){
        int n = 9;
        int[] numbers = {1, 2, 4, 9, 7, 8, 5, 6};

        int nNumberSum = FindMissingNumber.calculateSumOfNNumbers(n);
        int sumOfArray = calculateSum(numbers);

        int missingNumber = nNumberSum - sumOfArray;
        System.out.println(String.format("The missing number is: %s", missingNumber));



The method calculateSumOfNNumbers() find the sum of any natural number. The method calculateSum() calculates the sum of any given array. It uses Java 8 Stream operation to perform the sum.

In the main method, first we calculated the sum of N natural number and then calculated the sum of all the numbers in the given array. The difference between these two summation will give the missing number.


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