The Collectors.maxBy() method is used to find the maximum element from a stream based on a certain comparator. It returns an Optional which contains the maximum element according to the provided comparator, or an empty Optional if there are no elements in the stream.

Here’s a simple example where we have a list of integers, and we want to find the biggest integer:

package org.kodejava.stream;

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

public class MaxByDemo {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

        Optional<Integer> maxNumber = numbers.stream()
                .collect(Collectors.maxBy(Comparator.naturalOrder()));

        maxNumber.ifPresent(System.out::println);
    }
}

In this example:

• We create a Stream from the list of integers.
• We then use Collectors.maxBy(Comparator.naturalOrder()) to get the maximum number. Comparator.naturalOrder() is a shortcut for Comparator.comparing(Function.identity()).
• Collectors.maxBy() returns an Optional because the stream could be empty.
• We print the maximum number if it exists.

When you run this program, it will print “5” because 5 is the biggest number in the list.

Keep in mind that if the stream is empty, maxNumber will be an empty Optional, and nothing will be printed.