Collections Overview

A unified architecture for storing, managing, and manipulating groups of objects.
Includes interfaces, implementations (classes), and utility methods.

Core Interfaces

Interface	Description
Collection	Root interface (for lists, sets, queues)
List	Ordered collection with duplicates allowed
Set	Unordered, no duplicates
Queue	FIFO structure (some support LIFO)
Map	Key-value pairs (not part of Collection)

Map is a part of the framework, but not a subtype of Collection

Collection Hierarchy (Simplified)

          Iterable
              │
         Collection
        /    |      \
     List   Set    Queue
 
    Map (separate branch)

Key Features

• Built-in data structures.
• Ready-to-use implementations.
• Generic support (e.g. List).
• Algorithms in Collections class.
• Consistent API (add, remove, size, contains…).

Example

List<String> list = new ArrayList<>();
list.add("Hello");
 
Set<Integer> set = new HashSet<>();
set.add(10);
 
Map<String, Integer> map = new HashMap<>();
map.put("A", 1);

Common Implementations

Type	Classes
List	ArrayList, LinkedList, Vector
Set	HashSet, LinkedHashSet, TreeSet
Queue	PriorityQueue, ArrayQueue
Map	HashMap, LinkedHashMap, TreeMap, Hashtable

Tip

Be ready to explain:

• why use List vs Set vs Map,
• the core difference between Collection and Collections,
• why Map is not a subtype of Collection.

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Parent: _Collections