Java Collections Overview

📘 What is it

The Java Collections Framework (JCF) is a set of classes and interfaces that implement commonly used collection data structures. It provides a standardized way to handle groups of objects.

🏗️ Hierarchy

Collection (Interface): The root of the hierarchy.
- List: Ordered collection (ArrayList, LinkedList, Vector).
- Set: Unordered collection, no duplicates (HashSet, LinkedHashSet, TreeSet).
- Queue: FIFO structure (PriorityQueue, Deque).
Map (Interface): Key-Value pairs, not a subtype of Collection (HashMap, TreeMap, LinkedHashMap).

⚡ When to use

Use the Collections Framework whenever you need to store, retrieve, manipulate, and communicate aggregate data.

🧠 Big-O Summary

Collection	Add	Get	Remove
ArrayList	$O(1)$*	$O(1)$	$O(n)$
LinkedList	$O(1)$	$O(n)$	$O(1)$
HashSet	$O(1)$	$O(1)$	$O(1)$
HashMap	$O(1)$	$O(1)$	$O(1)$
TreeSet	$O(\log n)$	$O(\log n)$	$O(\log n)$
*Amortized

❌ Common mistakes

Using Vector: Using the legacy Vector class (it's synchronized and slow; use ArrayList unless you need thread-safety).
Modification during iteration: Trying to remove items from a collection while iterating over it using a simple for-each loop (throws ConcurrentModificationException).
Ignoring Generics: Not using generics (e.g., List<String>), leading to runtime type errors.