Core Libraries & Data Structures

This is Tier 3. With the object model behind you, these six chapters give you the standard library's workhorses — the growable, hashed, and sorted collections you'll reach for every day — and the contracts that make your own types behave correctly inside them. The thread running through the tier: a collection is only as good as the equals/hashCode of what you put in it, and the modern type tools (generics, records, sealed) let you model data so the compiler catches mistakes for you.

Six chapters, in order:

1. Strings in Depth — the cost of immutability, StringBuilder, interning, formatting, and text blocks.
2. The Collections Framework — List and program-to-the-interface, the Iterator, and choosing an implementation.
3. Sets & Maps — uniqueness and key→value lookup, hash buckets vs ordering, the counting idiom.
4. equals & hashCode — the contract that makes objects usable as set elements and map keys.
5. Generics — type parameters, bounds, wildcards (PECS), and type erasure.
6. Enums & Records — fixed constant sets, immutable data carriers, and a first look at sealed.

Every code block with a ▶ Run button is live. The highest-value habit at this tier is to respect the contracts: when you put your own type into a HashSet or use it as a HashMap key, ask whether its equals/hashCode are correct — and prefer a record, which gets them right for free.