Challenge 1: Filter, Square, and Collect — Possible Solution
====================================================================

StreamDemo.java:

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

public class StreamDemo {
    public static void main(String[] args) {
        List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8);

        List<Integer> result = numbers.stream()
            .filter(n -> n % 2 == 0)
            .map(n -> n * n)
            .collect(Collectors.toList());

        System.out.println(result);
    }
}

Output:

    [4, 16, 36, 64]

Explanation:
filter(n -> n % 2 == 0) keeps only even numbers (2, 4, 6, 8). map(n ->
n * n) squares each surviving element. collect(Collectors.toList())
is the terminal operation -- nothing in the pipeline actually runs
until this call, at which point every element flows through filter
then map in sequence, and the final results are gathered into a new
List. A method reference wasn't used here since squaring isn't an
existing single method call the way String::toUpperCase was in the
chapter's own example -- n -> n * n has no equivalent method to
reference.

WHY THIS WORKS AS AN ANSWER
------------------------------
This builds the exact filter-then-map-then-collect pipeline shape the
chapter introduces, and correctly recognizes that squaring has no
existing method to reference, so a lambda is the right and only choice
there, while still noting where a method reference would apply if the
operation were different.