How to sum a List of integers using Java 8 Stream

February 03, 2023

In this section, we will show you how to sum a List of integers using Java 8 Stream.

Following ways can be used to sum a List of integers:

1. By using mapToInt() method

2. By using summarizingInt() method

3. By using reduce() method

Example 1: By using mapToInt () method

This mapToInt () method is an intermediate operation which returns an IntStream consisting of the results of applying the given function to the elements of this stream.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {

    // Driver code
    public static void main(String[] args)
    {
        //Creating List of integers
        List<Integer> numberList =
                new ArrayList<Integer>(Arrays.asList(5, 2, 3, 4, 9));

        //Using mapToInt
        int sum = numberList.stream().mapToInt(Integer::intValue).sum();
        System.out.println("Sum of integers => "+sum);
    }

}

Console Output:

Sum of integers => 23

Example 2: By using summarizingInt() method

The summarizingInt () is a static method of the Collectors class that is used to return the summary statistics of the results obtained from applying the passed ToIntFunction implementation to a set of input elements.

ToIntFunction Interface is a part of the java.util.function package which has been introduced since Java 8, to implement functional programming in Java. It represents a function which takes in an argument of type T and produces an int-valued result.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Main {

    // Driver code
    public static void main(String[] args)
    {
        //Creating List of integers
        List<Integer> numberList =
                new ArrayList<Integer>(Arrays.asList(5, 2, 3, 4, 9));

        //Using summarizingInt
        long sum = numberList.stream().
                collect(Collectors.
                        summarizingInt(Integer::intValue)).getSum();
        System.out.println("Sum of integers => "+sum);
    }
}

Console Output:

Sum of integers => 23

Example 3: By using reduce() method

In Java 8, the Stream.reduce() combine elements of a stream and produces a single value.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {

    // Driver code
    public static void main(String[] args)
    {
        //Creating List of integers
        List<Integer> numberList =
                new ArrayList<Integer>(Arrays.asList(5, 2, 3, 4, 9));

        //Use reduce
        int sum = numberList.stream().
                reduce(Integer::sum).get().intValue();
        System.out.println("Sum of integers => "+sum);
    }
}

Console Output:

Sum of integers => 23

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