Find Missing Number In An ArrayList

import java.util.ArrayList;

public class MissingNumberInArrayList {

// We find out sum of ‘n’ numbers by using formula n*(n+1)/2.
static int sumOfNnumbers(int n) {
int sum = (n * (n + 1)) / 2;
return sum;
}

//we will find sum of all elements of array list
static int sumOfElements(ArrayList<Integer> list) {
int sum = 0;

for (int i = 0; i < list.size(); i++) {
sum = sum + list.get(i);
}
return sum;
}

// Another Approach, It will find multiple missing elements

static ArrayList<Integer> findMissingElements(ArrayList<Integer> oldList,
int numbers) {
ArrayList<Integer> newList = new ArrayList<Integer>(numbers);

for (int i = 1; i <= numbers; i++) {
newList.add(i);
}

newList.removeAll(oldList);
return newList;
}

public static void main(String[] args) {
int numbers = 10;

ArrayList<Integer> list = new ArrayList<Integer>();
for (int i = 1; i <= numbers; i++) {
if (i != 5)     // Skip one number
list.add(i);
}

System.out.println(list);

int sumOfNnumbers = sumOfNnumbers(numbers);

int sumOfElements = sumOfElements(list);

//Missing_Number = (Sum of 1 to ‘n’ numbers) – (Sum of elements of array list )

int missingNumber = sumOfNnumbers - sumOfElements;

System.out.println("Missing Number is = " + missingNumber);

// Another Approach

System.out.println("Missing Numbers are = "
+ findMissingElements(list, numbers));
}
}