Challenge 1: A List<Integer> Declared via the Interface — Possible Solution
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ListDemo.java:

import java.util.ArrayList;
import java.util.List;

public class ListDemo {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(40);
        numbers.add(50);

        for (int n : numbers) {
            // Each call to numbers.add(10) autoboxes the primitive int
            // literal 10 into an Integer object before it's stored --
            // List can only hold objects, never a raw int. Then, in
            // this for-each loop, each Integer is auto-UNBOXED back
            // into a primitive int to be assigned to the loop variable
            // n, since n is declared as a primitive int here, not
            // Integer.
            System.out.println(n);
        }
    }
}

Output:

    10
    20
    30
    40
    50

WHY THIS WORKS AS AN ANSWER
------------------------------
The variable is declared as List<Integer>, the interface, while
ArrayList<> supplies the actual implementation, matching the chapter's
own "declare with the interface" guidance, and the comment correctly
identifies both the autoboxing (on add) and auto-unboxing (in the
for-each) points, tying back to java1-2's boxing material.