Introduction to Collections in Java

The Collections Framework in Java provides a powerful architecture to store, process, and manipulate groups of objects.

Instead of using arrays (which are fixed size), Collections offer dynamic, flexible, and feature-rich data structures.

What are Collections?

A Collection in Java is a group of objects stored as a single unit.

Java provides various interfaces and classes to handle data efficiently (lists, sets, queues, maps).

Why Do We Need Collections?

• Arrays have fixed size → collections are dynamic.
• Collections come with built-in methods → searching, sorting, iteration.
• They improve code readability and reduce boilerplate.
• Provide standardized interfaces and high-performance implementations.

Benefits of Collections Framework

• Dynamic Size → grow/shrink automatically.
• Reusable Data Structures → List, Set, Map, Queue.
• Algorithms Support → sorting, searching via Collections class.
• Consistent API → same methods across structures.
• High Performance → optimized implementations like ArrayList, HashMap.
• Thread-Safe Options → via synchronized wrappers or concurrent collections.

Key Interfaces in Collections Framework

1. Collection Interface (Root Interface)

The base interface that represents a group of objects (elements).

Super interface for List, Set, Queue.

               Collection (Interface)
        /            |              \\
     List           Set             Queue
     |              |               |
 ArrayList       HashSet        PriorityQueue
 LinkedList      LinkedHashSet  ArrayDeque
 Vector          TreeSet

And outside this hierarchy:

Map (Interface)
   |
HashMap
LinkedHashMap
TreeMap
Hashtable

Maps are not a part of Collection interface, but they are part of the Collections Framework.

2. List Interface

• Ordered collection
• Allows duplicates
• Index-based access (ArrayList, LinkedList)

3. Set Interface

• No duplicates allowed
• No index-based access
• Implementations: HashSet, LinkedHashSet, TreeSet

4. Queue Interface

• Follows FIFO (First In First Out)
• Used for processing tasks
• Implementations: LinkedList, PriorityQueue

5. Map Interface (Separate Hierarchy)

• Stores key–value pairs
• Keys cannot be duplicated
• Implementations: HashMap, LinkedHashMap, TreeMap

Difference Between Array & Collection

Important Classes in Collections Framework

• List → ArrayList, LinkedList, Vector, Stack
• Set → HashSet, LinkedHashSet, TreeSet
• Map → HashMap, LinkedHashMap, TreeMap
• Queue → PriorityQueue, ArrayDeque

Utility Classes

Collections Class (java.util.Collections)

Provides static methods like:

• sort()
• reverse()
• shuffle()
• min(), max()
• binarySearch()

Arrays Class (java.util.Arrays)

Helps in working with arrays:

• sort()
• toString()
• copyOf()
• binarySearch()

Where Collections Are Used?

• Storing data dynamically
• Implementing databases in memory
• Implementing caching, searching, sorting
• Building stacks, queues, hash tables
• Real-world applications (banking, e-commerce, social media apps)

Points to Remember

• Collections Framework = set of interfaces + classes.
• Provides dynamic data structures (List, Set, Map, Queue).
• Offers algorithms via Collections class.
• Replaces many limitations of arrays.
• Makes Java data handling easier, faster, and more flexible.