What does the reduce method do?

The reduce method in Java is used for sequentially processing stream elements with accumulation of the result. It takes two parameters:

• An accumulator (bi-function) that combines the current accumulated result and the next element;
• (optionally) an initial value for the accumulator.

Ultimately, reduce reduces a stream of elements to a single value, such as a sum, product, or concatenation.

Example of summing numbers using reduce:

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

public class ReduceExample {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        int sum = numbers.stream()
                         .reduce(0, (acc, x) -> acc + x);
        System.out.println("Sum: " + sum); // Outputs Sum: 15
    }
}

If the initial value is not specified, reduce returns an Optional<T>, since the stream may be empty.