reduce

Definition

reduce combines all items of a stream into a single value by applying an operation again and again, such as adding numbers. It needs a starting value and a rule for combining two values.

Example code

Adding numbers, and finding the longest word

import java.util.List;

class Demo {
    public static void main(String[] args) {
        List<Integer> numbers = List.of(4, 8, 15, 16, 23, 42);

        int sum = numbers.stream().reduce(0, (a, b) -> a + b);       // start at 0, add each number
        System.out.println(sum);

        // for sums, the built-in is simpler and avoids boxing
        System.out.println(numbers.stream().mapToInt(Integer::intValue).sum());

        String longest = List.of("java", "kotlin", "go").stream()
                .reduce("", (a, b) -> b.length() > a.length() ? b : a);
        System.out.println(longest);
    }
}

// Output:
// 108
// 108
// kotlin