Java Tutorial: Collections
ArrayList, LinkedList, HashMap, HashSet, TreeMap, Queue, Stack, and the Collections utility class.
The Collections Framework
Java's collections framework provides ready-made data structures for almost every use case. Every professional Java program uses these daily.
- `ArrayList` — dynamic array, fast random access
- `HashMap` — key-value pairs, O(1) lookup
- `HashSet` — unique items, O(1) contains
- `TreeMap` / `TreeSet` — sorted collections
What you'll learn in this Java collections tutorial
This interactive Java tutorial has 7 hands-on exercises. Estimated time: 22 minutes.
- ArrayList — dynamic arrays — Unlike arrays, `ArrayList` grows automatically. It implements the `List` interface.
- HashMap — key-value pairs — `HashMap<K, V>` maps keys to values with O(1) average lookup:
- HashSet — unique items only — `HashSet<T>` stores unique items with O(1) average `contains()`.
- TreeMap — sorted keys — `TreeMap` keeps keys sorted (ascending by default). Use it when iteration order matters.
- Queue and Deque — `Queue` = FIFO (first in, first out). `Deque` = double-ended queue.
- Collections utility methods — `java.util.Collections` has powerful utilities:
- Build an inventory manager — Final challenge — no starter code.
Java Collections concepts covered
- The Collections Framework