Calculate sum of all elements in a List in Java

This post will discuss how to calculate the sum of all elements in a List in Java.

1. IntStream’s `sum()` method

A simple solution to calculate the sum of all elements in a List is to convert it into IntStream and call sum() to get the sum of elements in the stream. There are several ways to get IntStream from Stream<Integer> using mapToInt() method.

1. Using method reference Integer::intValue

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().mapToInt(Integer::intValue).sum();

System.out.println(sum); // 15

}

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2. Using method reference Integer::valueOf

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().mapToInt(Integer::valueOf).sum();

System.out.println(sum); // 15

}

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3. Using Lambda expression i -> i

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().mapToInt(i -> i).sum();

System.out.println(sum); // 15

}

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2. Reduce operation

Another solution is to perform reduce operation to perform the addition.

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().reduce((x, y) -> x + y).get();

System.out.println(sum); // 15

}

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We can simplify the above code using method reference Integer::sum, as shown below:

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().reduce(Integer::sum).get();

System.out.println(sum); // 15

}

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The code can be further simplified by removing Optional:

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = nums.stream().reduce(0, Integer::sum);

System.out.println(sum); // 15

}

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3. Using `IntSummaryStatistics`

Finally, we can get IntSummaryStatistics instance from IntStream, which has getSum() method to get the sum of values in it.

import java.util.Arrays;

import java.util.List;

class Main {

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

long sum = nums.stream().mapToInt(Integer::valueOf).summaryStatistics().getSum();

System.out.println(sum); // 15

}

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4. Naive solution

We can even write a custom routine to calculate the sum of all elements in a List using a for-loop, as shown below:

import java.util.Arrays;

import java.util.List;

class Main {

public static int getSum(List<Integer> nums) {

int sum = 0;

for (int i: nums) {

sum += i;

}

return sum;

}

public static void main(String[] args) {

List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);

int sum = getSum(nums);

System.out.println(sum); // 15

}

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That’s all about calculating the sum of all elements in a List in Java.

Calculate sum of all elements in a List in Java

1. IntStream’s sum() method

2. Reduce operation

3. Using IntSummaryStatistics

4. Naive solution

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1. IntStream’s `sum()` method

3. Using `IntSummaryStatistics`