Java 8 Streams - Sum of all the items of the List

Example 1: Using Collectors.summingInt()

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

public class Main {

    public static void main(String[] args) {
        List<Integer> integers = Arrays.asList(2, 9, 10, 8, 4, 7);
        Integer sum = integers.stream()
                .collect(Collectors.summingInt(Integer::intValue));
        System.out.println("Sum= " + sum);
    }
}

Here, stream() method returns a sequential Stream with List as its source.

summingInt() method returns a Collector that produces the sum of a integer. 

Similarly, the Collectors class provides summingDouble() and summingLong(). 

Example 2: Using Stream.reduce() 

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

public class Main {

    public static void main(String[] args) {
        List<Integer> integers = Arrays.asList(2, 9, 10, 8, 4, 7);
        Integer sum = integers.stream()
                .reduce(0, Integer::sum);
        System.out.println("Sum="+sum);
    }
}

reduce() is terminal operations, here it helps us to sum all elements of list.

Example 3: Using IntStream.sum()

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

public class Main {

    public static void main(String[] args) {
        List<Integer> integers = Arrays.asList(2, 9, 10, 8, 4, 7);
        Integer sum = integers.stream()
                .mapToInt(Integer::intValue)
                .sum();
        System.out.println("Sum= " + sum);
    }
}

Here mapToInt() method convert Stream of Integer to IntStream. 

sum() is terminal operation which returns sum of elements in a stream.

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