Java Collections Framework

How the framework is organized — Iterable, Collection, Map, the optional-operations contract, and why "Collection" and "Collections" are different things.

How ArrayList grows, when (almost never) to pick LinkedList, and the tarpit of Arrays.asList vs List.of vs Collections.unmodifiableList.

The three production-grade Set implementations: hash-based, insertion-ordered, and sorted.

The single most-asked Java topic. Inside hash(), resize(), treeification, load factor, and the contract for null.

How LinkedHashMap weaves a doubly-linked list through HashMap, and how that gives you an LRU cache in 5 lines.

Red-black tree under the hood, log-n everything, and the navigation API senior engineers use to solve interval problems.

The contract every Java developer must internalize: five properties of equals, the hashCode invariant, and the inheritance trap.

Natural ordering vs external ordering, and the "consistent with equals" trap most senior engineers can name but not explain.

The Queue throw-vs-return method matrix, why ArrayDeque replaces Stack, and the PriorityQueue iterator trap.

The iteration protocol, modCount and ConcurrentModificationException, and how Spliterator powers parallel streams.

How CHM redesigned itself in Java 8, why size() is approximate, and the right way to use computeIfAbsent.

The producer-consumer toolbox. Which queue each Executors factory uses, why newFixedThreadPool can take down production.

The lesser-known concurrent toolbox: copy-on-write for read-mostly state, skip list for sorted concurrent access, lock-free queues.

Static helpers and wrappers: unmodifiable vs immutable vs synchronized, and the algorithms most people forget.

Factory methods, Stream.toList, Sequenced Collections (Java 21), Records as map keys, virtual-thread implications.