Approach 1

import java.util.*;
import java.util.stream.*;

public class MaxExample {
    public static void main(String[] args) {
        List<Integer> nums = Arrays.asList(3, 7, 2, 9, 5);

        int max = nums.stream()
                      .mapToInt(Integer::intValue)
                      .max()
                      .orElseThrow();

        System.out.println(max);
    }
}

Approach 2

Optional<Integer> maxNumber = numbers.stream()
                                     .reduce(Integer::max);