---
title: "Collections and data structures"
chapter: "06"
---

# Collections and data structures

Choose a collection from required ordering, uniqueness, lookup, mutation,
concurrency, and memory behavior.

## Core interfaces

| Type | Main property |
|---|---|
| `List` | Ordered sequence, duplicates allowed |
| `Set` | Unique elements |
| `Map` | Key-to-value association |
| `Queue` / `Deque` | Processing order and double-ended operations |

`ArrayList` is the usual list default. `HashSet` and `HashMap` use hash codes
and equality. `TreeSet` and `TreeMap` maintain sorted order. `LinkedHashMap`
maintains encounter order. Java 21 introduced sequenced collection interfaces
for first/last and reversed views.

## Equality contract

If two objects are equal, they must have the same hash code. Fields used in
`equals`/`hashCode` should not change while the object is a hash key. Records
generate value-based equality from their components.

## Mutability

`List.of` creates an unmodifiable list and rejects null elements.
`Collections.unmodifiableList` creates a read-only view of a backing list; the
backing list can still change. `List.copyOf` creates an unmodifiable snapshot
subject to element mutability.

## Iteration

Do not structurally modify most collections while using a for-each loop except
through the iterator's supported removal. Concurrent modification detection is
best-effort, not a thread-safety mechanism.

## Concurrent choices

Use `ConcurrentHashMap` for highly concurrent map access, `CopyOnWriteArrayList`
for read-heavy/small write workloads, and blocking queues for producer-consumer
coordination. Measure instead of synchronizing every collection blindly.

## Feynman check

A list is a numbered shelf, a set is a guest list with no duplicates, a map is
a dictionary, and a queue is a line of work.