Java Collection Framework

Master the powerful framework that makes storing and managing groups of objects easy, flexible, and efficient!

What is Collection Framework?

The Java Collection Framework is a unified architecture for storing, manipulating, and managing groups of objects. It provides ready-made data structures and algorithms, saving you from writing complex code from scratch.

Think of it as a Swiss Army knife for data storage - instead of building your own tools, you get powerful, pre-built, tested structures ready to use!

  • Collections: Groups of objects stored together
  • Interfaces: Define standard operations (List, Set, Map, Queue)
  • Classes: Concrete implementations (ArrayList, HashSet, HashMap)
  • Algorithms: Built-in methods for sorting, searching, shuffling

💡 Why Use Collections? Arrays have fixed size, but collections are dynamic - they grow and shrink automatically. Plus they come with powerful built-in methods!

Collection Framework Hierarchy

The Collection Framework is organized in a clear hierarchy:

Collection (Interface)
    │
    ├── List (Interface) - Ordered, allows duplicates
    │   ├── ArrayList
    │   ├── LinkedList
    │   └── Vector
    │
    ├── Set (Interface) - Unordered, no duplicates
    │   ├── HashSet
    │   ├── LinkedHashSet
    │   └── TreeSet
    │
    └── Queue (Interface) - FIFO order
        ├── PriorityQueue
        └── LinkedList

Map (Interface) - Key-Value pairs (separate hierarchy)
    ├── HashMap
    ├── LinkedHashMap
    └── TreeMap
  • List: Maintains insertion order, allows duplicate elements
  • Set: No duplicates allowed, no guaranteed order
  • Queue: FIFO (First In First Out) processing
  • Map: Stores key-value pairs, keys must be unique

Core Collection Interfaces

1. List Interface

An ordered collection that allows duplicate elements. Elements can be accessed by their index position (like arrays but dynamic).

Implementations: ArrayList, LinkedList, Vector

Use when: You need ordered data with index access and duplicates are okay

2. Set Interface

A collection that contains no duplicate elements. Great for storing unique items without caring about order.

Implementations: HashSet (fastest), LinkedHashSet (maintains insertion order), TreeSet (sorted)

Use when: You need unique elements and don't care about order

3. Queue Interface

Designed for holding elements prior to processing. Typically follows FIFO (First In First Out) order.

Implementations: PriorityQueue, LinkedList

Use when: You need to process elements in a specific order (task scheduling, print queue)

4. Map Interface

Stores data in key-value pairs. Each key maps to exactly one value. Keys must be unique but values can be duplicated.

Implementations: HashMap (fastest), LinkedHashMap (insertion order), TreeMap (sorted by keys)

Use when: You need to look up values using unique keys (like a dictionary)

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