Challenge 2: Predicate and Function Applied to a List — Possible Solution
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FunctionalDemo.java:

import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;

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

        Predicate<Integer> isEven = n -> n % 2 == 0;
        Function<Integer, Integer> square = n -> n * n;

        for (int n : numbers) {
            if (isEven.test(n)) {
                System.out.println(n + " is even, squared: " + square.apply(n));
            }
        }
    }
}

Output:

    2 is even, squared: 4
    4 is even, squared: 16
    6 is even, squared: 36

Explanation:
isEven.test(n) calls Predicate's single abstract method, returning a
boolean. square.apply(n) calls Function's single abstract method,
returning the transformed value. Both are ready-made functional
interfaces from java.util.function -- neither required a custom
interface to be declared, matching the chapter's own tip-box guidance.

WHY THIS WORKS AS AN ANSWER
------------------------------
This uses Predicate<Integer> and Function<Integer, Integer> exactly
as the chapter introduces them -- test() and apply() as each
interface's single abstract method -- applied together over a real
list rather than in isolation.