Capstone: A Music Library

We've covered the whole map: the four collection families, generic types, equality and hashing, comparators, iteration, immutability, and architecture. This chapter is one substantial challenge that exercises all of it.

You'll build a small MusicLibrary that ingests tracks and answers four queries:

1. byArtist(name) — all tracks for an artist, in insertion order
2. genres() — the set of all unique genres
3. topRated(n) — the top n tracks by rating (ties broken by title)
4. albumsByArtist(name) — the unique album names for an artist, in insertion order

Each query corresponds directly to a collection shape we've studied:

What "good" looks like

A clean implementation will:

• Store Tracks in the shape that matches each query (not a flat list scanned every time).
• Use computeIfAbsent to build per-artist lists and sets lazily.
• Use a LinkedHashSet where insertion-ordered uniqueness matters.
• Build topRated with a Comparator.comparingInt(...).reversed().thenComparing(...) chain.
• Return read-only / snapshot collections from every getter.

The challenge

Debussy tracks:
Clair de Lune
Reverie
La Mer
Genres: [classical, impressionist, orchestral]
Top 3:
Clair de Lune (5)
La Mer (5)
Liebestraum (4)
Debussy albums: [Suite Bergamasque, Reveries, Sea Pieces]

What you just demonstrated

That single class touches every major idea in the course:

• Right shape per query — one map of lists, one map of sets, one set, one list (Data Modeling).
• computeIfAbsent for "map of \*" — concise, race-free against itself (Maps).
• LinkedHashSet — uniqueness and insertion order (Sets).
• Comparator composition — reversed().thenComparing(...) to combine two ordering criteria (Comparable & Comparator).
• Safe getters — every returned collection is an unmodifiable view (Immutable Collections + Architecture).
• record-backed value objects for Track — typed data, free equals/hashCode/toString (Equality and Hashing).

Test your understanding