You now know how to store and retrieve data by custom keys using HashMap, and how to maintain collections of unique elements using HashSet. You practiced inserting with .put() and .add(), retrieving with .get() and .getOrDefault(), checking existence with .containsKey() and .contains(), and iterating with .keySet() and .entrySet().
You also learned the frequency counter pattern for counting occurrences and saw specialized variants: TreeMap and TreeSet for sorted order, and LinkedHashMap for insertion order. Choosing the right collection type for the job is now part of your toolkit.