Java Collections Framework

- July 18, 2025

Chapter 11: Java Collections Framework

🔰 Introduction

The Java Collections Framework (JCF) is a set of classes and interfaces that implement commonly reusable collection data structures. It provides algorithms and utilities to manage groups of objects effectively.

🌲 Core Interfaces in Collections

Collection: Root interface of the collection hierarchy.
List: An ordered collection that allows duplicate elements (e.g., ArrayList, LinkedList).
Set: A collection that does not allow duplicate elements (e.g., HashSet, TreeSet).
Map: Stores key-value pairs (e.g., HashMap, TreeMap).
Queue: Designed for holding elements prior to processing (e.g., LinkedList, PriorityQueue).

🔄 List Interface

Ordered, allows duplicate elements. Access by index.

List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");

🧱 Set Interface

Unordered, no duplicate elements.

Set<String> cities = new HashSet<>();
cities.add("Delhi");
cities.add("Delhi"); // ignored

🔐 Map Interface

Stores key-value pairs. Keys are unique.

Map<Integer, String> map = new HashMap<>();
map.put(1, "Java");
map.put(2, "Python");

🔁 Queue Interface

Used in scenarios like message processing.

Queue<String> queue = new LinkedList<>();
queue.offer("A");
queue.poll();

🌐 Important Implementations

ArrayList: Dynamic array
LinkedList: Doubly-linked list
HashSet: Uses hashing for uniqueness
TreeSet: Sorted set
HashMap: Key-value with hash table
TreeMap: Sorted map
PriorityQueue: Elements processed based on priority

🧰 Utility Class: Collections

Collections.sort(list);
Collections.reverse(list);
Collections.max(list);

📌 Benefits of Collections Framework

Code reusability
High performance and scalability
Standard architecture
Reduced programming effort

📘 Chapter Summary

List: Ordered and allows duplicates
Set: Unordered and unique
Map: Key-value pairs
Queue: FIFO operations
Collections: Utility class for algorithms

❓ FAQs with Answers

Q1: What is the Java Collections Framework?

A: It is a unified architecture for representing and manipulating collections, enabling efficient data processing.

Q2: What is the difference between List and Set?

A: List allows duplicate elements and maintains order. Set does not allow duplicates and is unordered (unless using TreeSet).

Q3: What are the key implementations of Map?

A: HashMap, TreeMap, and LinkedHashMap.

Q4: When should I use a Queue in Java?

A: When you need FIFO operations like task scheduling or request handling.

Q5: How does HashSet ensure uniqueness?

A: It uses the hashCode() and equals() methods to determine uniqueness.

Q6: What is the Collections utility class?

A: A class in java.util that provides static methods like sort, max, min, reverse for collection objects.

Q7: Difference between HashMap and TreeMap?

A: HashMap is unordered, TreeMap maintains sorted keys.

Q8: Can you store null in Set or Map?

A: HashSet allows one null element, HashMap allows one null key and multiple null values.

Q9: What is the advantage of using generics with collections?

A: Type safety, code clarity, and eliminating runtime ClassCastException.

Q10: Which collection is best for frequent insertion and deletion?

A: LinkedList or ArrayDeque for Queue operations.

Written by Op Verma

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