Java Collections Framework – Complete Tutorial

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Last Updated on: 25th Nov 2025 17:40:52 PM

In real-world applications, we often deal with large amounts of data—customers, orders, products, students, employees, transactions, etc. Storing such data using simple arrays becomes difficult because:

• Arrays have fixed size

• Arrays cannot grow automatically

• Arrays do not provide built-in methods like sorting, searching, insertion, etc.

• Arrays can store only same type of data

• No direct support for objects comparison or data retrieval logic

 

To solve these problems, Java introduced the Collections Framework, a powerful set of classes & interfaces for storing, processing, and manipulating groups of objects efficiently.

 

What is the Java Collections Framework?

The Java Collections Framework is a unified architecture for storing, manipulating, and retrieving groups of objects efficiently. It provides ready-made data structures (such as ArrayList, HashSet, LinkedList, HashMap, TreeMap, etc.) along with standard algorithms (sorting, searching, iteration) that help developers build scalable, flexible, and high-performance applications.
It replaces traditional arrays with powerful, dynamic, growable, and object-oriented data containers.

 

Need for Java Collections

Collections solve major limitations of arrays:

Problem with Arrays	How Collections Solve
Fixed size	Dynamic & growable
Store only same type	Can store heterogeneous objects
No built-in functions	Many built-in methods (add, remove, sort, search)
No advanced data structures	Provides List, Set, Queue, Map
Complex insertion/deletion	Very easy using Collections
Not optimized for performance	Collections are optimized & flexible

 

Java Collections Framework Architecture

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

            Map (Interface)
                 |
     --------------------------------
     |              |               |
   HashMap       LinkedHashMap    TreeMap

 

Major Interfaces in Java Collections Framework

 

1. List Interface

• Ordered collection (elements have index)

• Duplicates allowed

• Best for accessing elements using index

 

Classes under List:

• ArrayList

• LinkedList

• Vector

• Stack

 

2. Set Interface

• No duplicates allowed

• Mostly used for unique values (emails, usernames, IDs)

 

Classes under Set:

• HashSet → fastest, unordered

• LinkedHashSet → ordered (insertion order)

• TreeSet → sorted

 

3. Queue Interface

• Follows FIFO (First In First Out)

• Used in task scheduling, printing jobs, call center queues

 

Classes:

• PriorityQueue

• ArrayDeque

 

4. Map Interface (Key-Value Pairs)

• Stores data as key-value pairs

• Keys must be unique

 

Classes

• HashMap

• LinkedHashMap

• TreeMap

• Hashtable

 

Detailed Explanation of Each Important Class 

 

1. ArrayList (Most Popular Collection Class)

ArrayList is a dynamic array that grows automatically as elements are added. It allows fast access using indexes and is widely used because of its simplicity and high performance.

 

Properties:

• Ordered

• Allows duplicates

• Index-based retrieval

• Fast read, slow insert/delete in middle

 

Example:

import java.util.*;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        list.add("JavaScript");

        System.out.println(list);
    }
}

 

2. LinkedList

LinkedList is a doubly linked list data structure that provides fast insertion and deletion but slower access compared to ArrayList.

 

Properties:

• Good for adding/removing elements

• Bad for accessing by index

 

Example:

LinkedList<Integer> numbers = new LinkedList<>();
numbers.add(10);
numbers.add(20);
numbers.addFirst(5);

 

3. HashSet

HashSet is a collection of unique elements without order. It is the fastest implementation of the Set interface.

 

Properties:

• No duplicates

• No order

• Very fast (O(1) average time)

 

Example:

HashSet<String> set = new HashSet<>();
set.add("A");
set.add("B");
set.add("A"); // Duplicate ignored

 

4. TreeSet

TreeSet stores unique elements in sorted (ascending) order using a Red-Black Tree.

 

Properties:

• Sorted

• Unique

• Slower than HashSet

 

Example:

TreeSet<Integer> ts = new TreeSet<>();
ts.add(30);
ts.add(10);
ts.add(20);  // Output: [10, 20, 30]

 

5. HashMap (Most Used Map Class)

HashMap stores data in key-value pairs using hashing. It provides very fast access and is widely used in databases, caching, user sessions, etc.

 

Properties:

• Key-value storage

• Keys must be unique

• Allows one null key

 

Example:

HashMap<Integer, String> users = new HashMap<>();
users.put(101, "Ravi");
users.put(102, "Sneha");
users.put(103, "Amit");

System.out.println(users);

 

6. LinkedHashMap

Same as HashMap but maintains insertion order.

 

Example:

LinkedHashMap<String, Integer> map = new LinkedHashMap<>();
map.put("Java", 90);
map.put("Python", 80);

 

7. TreeMap

TreeMap stores key-value data in sorted order based on keys.

 

Example:

TreeMap<Integer, String> tm = new TreeMap<>();
tm.put(3, "C");
tm.put(1, "A");
tm.put(2, "B");

 

8. PriorityQueue

A queue where elements are ordered by priority instead of insertion order.

 

Used in:

• Job scheduling

• Process execution

• Call center queue

 

9. Stack

Stack is a LIFO (Last-In-First-Out) structure used for undo operations, function calls, browser history.

 

Example:

Stack<String> stack = new Stack<>();
stack.push("A");
stack.push("B");
stack.pop();

 

List vs Set vs Map

Feature	List	Set	Map
Duplicates	✓ Yes	✗ No	✓ Values only
Ordering	Ordered	Depends	Keys sorted/unordered
Indexing	Yes	No	No
Best Use	Dynamic arrays	Unique items	Key-value storage

 

Real-Life Examples Using Collections

1. E-commerce App

• List → Products

• Set → Unique categories

• Map → Cart items (productId → quantity)

 

2. Online Banking

• List → Transactions

• Set → Unique customers

• Map → AccountNo → Balance

 

3. College Management System

• List → Students

• Set → Unique subjects

• Map → RollNo → Marks

 

When to Use What?

Use Case	Best Collection
Fast search	HashMap
Unique values	HashSet
Ordered list	ArrayList
Sorted data	TreeSet / TreeMap
Frequently insert/delete	LinkedList
Priority-based tasks	PriorityQueue

 

Interview Questions 

1. Difference between ArrayList and LinkedList

2. HashSet vs TreeSet

3. HashMap vs Hashtable

4. Fail-fast vs Fail-safe

5. How HashMap works internally

6. Why Set does not allow duplicates?

 

Keep practicing — you're doing amazing!

Happy Coding!    

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