Challenge 1: Iterating a HashMap and a Missing-Key Lookup — Possible Solution
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MapDemo.java:

import java.util.HashMap;
import java.util.Map;

public class MapDemo {
    public static void main(String[] args) {
        Map<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 30);
        ages.put("Bob", 25);
        ages.put("Carol", 41);

        for (Map.Entry<String, Integer> entry : ages.entrySet()) {
            System.out.println(entry.getKey() + " -> " + entry.getValue());
        }

        System.out.println("Missing key result: " + ages.get("Dave"));
    }
}

Output (HashMap iteration order is not guaranteed, so entry order may vary):

    Alice -> 30
    Bob -> 25
    Carol -> 41
    Missing key result: null

Explanation:
entrySet() returns a Set of Map.Entry objects, each supplying
getKey()/getValue(), giving a genuinely iterable view of the map's
pairs. ages.get("Dave") returns null rather than throwing, since
"Dave" was never put into the map -- get() on a missing key simply
reports "no value found" via null, not an exception.

WHY THIS WORKS AS AN ANSWER
------------------------------
This uses entrySet() exactly as the chapter demonstrates for
iteration, and shows get()'s real behavior on a missing key (null,
not an exception), a detail worth confirming explicitly rather than
assuming.