Java Collections Framework

The Collections Framework in Java is a unified architecture for representing and manipulating collections (groups of objects).

Think of a Toolbox 🧰.

• You have different compartments for different tools.
• List: Like a row of slots where order matters (Screwdrivers sorted by size).
• Set: A bag where you just throw things in, and duplicates don't matter (Loose nails).
• Map: Labeled drawers (Label "A" -> Hammer, Label "B" -> Wrench).

The Hierarchy

graph TD
    A[Collection Interface] --> B[List]
    A --> C[Set]
    A --> D[Queue]

    B --> B1[ArrayList]
    B --> B2[LinkedList]
    B --> B3[Vector]

    C --> C1[HashSet]
    C --> C2[LinkedHashSet]
    C --> C3[TreeSet]

    D --> D1[PriorityQueue]
    D --> D2[ArrayDeque]

    M[Map Interface] --> M1[HashMap]
    M --> M2[LinkedHashMap]
    M --> M3[TreeMap]

    style A fill:#f9f,stroke:#333,stroke-width:2px
    style M fill:#ff9,stroke:#333,stroke-width:2px

Key Interfaces

1. List 📜

• Ordered collection (sequence).
• Allows duplicates.
• Access elements by index (0, 1, 2...).
• Examples: ArrayList, LinkedList.

2. Set 🎒

• Unordered collection.
• No duplicates allowed (Unique items only).
• Examples: HashSet, TreeSet.

3. Map 🗺️

• Key-Value pairs.
• Keys must be unique.
• Not technically a child of Collection interface, but part of the framework.
• Examples: HashMap, TreeMap.

Challenge

Complete this chapter to unlock the next one.

Key Takeaways

• Framework: A standard way to handle groups of objects.
• Interfaces: List (Ordered), Set (Unique), Map (Key-Value).
• Implementations: ArrayList, HashSet, HashMap are the most common.

Common Pitfalls

[!WARNING] Raw Types: Don't use List list = new ArrayList();. Always use Generics: List<String> list = ....

Modification: Don't remove items from a collection while looping through it with a for-each loop. It causes a ConcurrentModificationException.

What's Next?

We mentioned <String> and <Integer>. What are those angle brackets? Learn Generics →