Phase 6: Java Collections Framework & Generics

The Collections Framework (`java.util`) provides reusable data structures. Master the core interfaces and performance trade-offs.

1. Collections Framework Architecture

                          Iterable
                              │
                         Collection
                   ┌──────────┼──────────┐
                 List    Set     Queue
                   │          │
              ArrayList    HashSet     (Map is a separate root!)
              LinkedList   TreeSet        HashMap / TreeMap
        

Interface	Duplicates Allowed?	Ordered / Indexed?	Key Implementation Classes
`List<E>`	✅ Yes	✅ Ordered by insertion index	`ArrayList`, `LinkedList`, `Vector`
`Set<E>`	❌ No (Unique elements only)	❌ Unordered (`HashSet`) / Sorted (`TreeSet`)	`HashSet`, `TreeSet`, `LinkedHashSet`
`Map<K,V>`	Keys: ❌ No, Values: ✅ Yes	Key-Value pair lookup	`HashMap`, `TreeMap`, `Hashtable`

2. ArrayList vs LinkedList (Frequent Exam Comparison)

Feature	ArrayList	LinkedList
Underlying Data Structure	Resizable Dynamic Array.	Doubly Linked List (Nodes with pointers).
Random Access (`get(index)`)	Fast $O(1)$ constant time.	Slow $O(n)$ traversal.
Insertion / Deletion	Slow $O(n)$ (requires shifting elements).	Fast $O(1)$ (just updates node pointers).
Memory Overhead	Low (only stores elements).	Higher (stores element + next & prev pointers).

3. Generics (<T>) & Type Safety

Generics allow classes, interfaces, and methods to operate on specified data types while providing compile-time type safety.

Without Generics: Requires explicit type casting and causes runtime ClassCastException risk.
With Generics: List<String> list = new ArrayList<>(); Compiler catches type mismatches at compile-time!

⚡ Quick Self-Test

Q: Is Map a child interface of Collection in Java?

Answer: No!.
Map<K,V> is a standalone interface in the Java Collections Framework because it deals with Key-Value pairs rather than single element collections.