Collections Framework – Hierarchy
1. Introduction
The Java Collections Framework (JCF) was introduced in Java 1.2 to provide a unified architecture for storing and manipulating groups of objects.
2. Root Interfaces
2.1 Iterable (Java 1.5)
- Root interface of the collection hierarchy
- Enables use of enhanced for-loop (
for-each)
2.2 Collection (Java 1.2)
- Root interface for most collection types
- Extends
Iterable
3. Collection Hierarchy
Iterable (1.5)
|
└── Collection (1.2)
├── List (1.2)
│ ├── ArrayList (1.2)
│ ├── LinkedList (1.2)
│ ├── Vector (1.0) [Legacy]
│ └── Stack (1.0) [Legacy]
│
├── Set (1.2)
│ ├── HashSet (1.2)
│ │ └── LinkedHashSet (1.4)
│ ├── SortedSet (1.2)
│ │ └── NavigableSet (1.6)
│ │ └── TreeSet (1.2)
│
└── Queue (1.5)
├── PriorityQueue (1.5)
├── Deque (1.6)
│ ├── ArrayDeque (1.6)
│ └── LinkedList (1.2)
4. Map Hierarchy (Separate from Collection)
Map (1.2)
├── HashMap (1.2)
│ └── LinkedHashMap (1.4)
├── Hashtable (1.0) [Legacy]
├── SortedMap (1.2)
│ └── NavigableMap (1.6)
│ └── TreeMap (1.2)
├── WeakHashMap (1.2)
├── IdentityHashMap (1.4)
└── ConcurrentHashMap (1.5)
5. Key Interfaces
| Interface | Version | Description |
| Iterable | 1.5 | Supports for-each loop |
| Collection | 1.2 | Root of collection framework |
| List | 1.2 | Ordered collection |
| Set | 1.2 | No duplicates |
| Queue | 1.5 | FIFO structure |
| Deque | 1.6 | Double-ended queue |
| Map | 1.2 | Key-value pairs |
6. Important Implementations
| Class | Version | Notes |
| ArrayList | 1.2 | Dynamic array |
| LinkedList | 1.2 | Doubly linked list |
| Vector | 1.0 | Synchronized (legacy) |
| Stack | 1.0 | LIFO (legacy) |
| HashSet | 1.2 | Uses HashMap internally |
| LinkedHashSet | 1.4 | Maintains insertion order |
| TreeSet | 1.2 | Sorted set |
| PriorityQueue | 1.5 | Heap-based queue |
| ArrayDeque | 1.6 | Faster than Stack/LinkedList |
| HashMap | 1.2 | Most used map |
| LinkedHashMap | 1.4 | Ordered map |
| TreeMap | 1.2 | Sorted map |
| Hashtable | 1.0 | Legacy synchronized map |
| ConcurrentHashMap | 1.5 | Thread-safe high performance |
7. Legacy Classes (Before Collections Framework)
| Class | Version | Remarks |
| Vector | 1.0 | Replaced by ArrayList |
| Stack | 1.0 | Use Deque instead |
| Hashtable | 1.0 | Replaced by HashMap |
8. Notable Enhancements by Java Version
Java 1.2
- Introduced Collections Framework
- List, Set, Map, ArrayList, HashMap, etc.
Java 1.4
- LinkedHashMap, LinkedHashSet
- IdentityHashMap
Java 1.5 (Generics + Concurrency)
- Iterable
- Queue, PriorityQueue
- ConcurrentHashMap
Java 1.6
- Deque, ArrayDeque
- NavigableSet, NavigableMap
Java 1.8
- Streams API
- Default methods (e.g.,
forEach) compute(), merge() in Map
9. Summary
- Collection → List, Set, Queue
- Map → Separate hierarchy
- Legacy classes exist but should be avoided
- Modern Java favors:
ArrayListHashMapConcurrentHashMapArrayDeque
10. Quick Points
- Map is NOT part of Collection hierarchy
- LinkedList implements both List and Deque
- HashSet internally uses HashMap
- TreeSet & TreeMap are sorted (Red-Black Tree)
- ConcurrentHashMap is preferred over Hashtable