📖 THE STORY OF JAVA COLLECTIONS

Before JDK 1.2

Java developers mainly used:

• Arrays (fixed size, rigid)
• Legacy classes like Vector and Hashtable

Problems:

• No standard structure
• Inconsistent design
• Hard to maintain and extend

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JDK 1.2 — COLLECTIONS FRAMEWORK

Java introduced the Collections Framework to:

• Standardize data storage
• Provide reusable data structures
• Improve consistency and flexibility

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🌱 ITERABLE (ROOT FOR TRAVERSAL)

Introduced: JDK 1.5
Package java.lang

Purpose:

• Allows objects to be traversed

Supports:

• for-each loop
• Iterator

Note:

• Iterable does NOT store data
• It only allows traversal

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🧠 COLLECTION INTERFACE

Introduced: JDK 1.2
Package: java.util

Role:

• Root interface of the Collections Framework
• Defines basic behavior for all collections

Common Methods:

• add()
• remove()
• contains()
• size()
• isEmpty()
• clear()

Concept:
"If you store objects, you must support these operations."

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🌳 THREE CHILDREN OF COLLECTION

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📋 LIST → ORDERED COLLECTION

Features:

• Allows duplicate elements
• Maintains insertion order
• Index-based storage

Implementations:

• ArrayList → Fast access
• LinkedList → Fast insert/delete
• Vector → Legacy (synchronized)

Use Case:

• When order matters
• When duplicates are allowed

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🔒 SET → UNIQUE COLLECTION

Features:

• Does NOT allow duplicates
• No index-based access
• Useshashing/sorting

Implementations:

• HashSet → Fast, unordered
• LinkedHashSet → Insertion order
• TreeSet → Sorted order

Use Case:

• When uniqueness is required

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📥 QUEUE → FIFO COLLECTION

Introduced: JDK 1.5

Features:

• Follows FIFO (First In First Out)
• Used in scheduling and processing

Implementation:

• PriorityQueue → Priority-based order

Use Case:

• When processing order matters

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🧰 COLLECTIONS CLASS (UTILITY)

Package java.util

Features:

• Utility class
• Contains only static methods
• Private constructor (cannot be instantiated)

Provides:

• Sorting
• Searching
• Reversing
• Synchronizing

Concept:
"Toolbox for working with collections"

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🔄 ITERATION MECHANISMS

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🔄 ITERATOR

Introduced: J DK 1.2

Features:

• Forward direction only
• Works with all collections

Methods:

• hasNext()
• next()
• remove()

Use Case:

• Simple forward traversal

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🔁 LISTITERATOR

Features:

• Forward and backward traversal
• Index-based navigation
• Works only with List

Use Case:

• Full control over List traversal

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⭐ FINAL INTERVIEW SUMMARY

Collection → Stores single objects
Map → Stores key–value pairs
List → Duplicates + Index
Set → No duplicates
Queue → FIFO
Iterator → Forward only
ListIterator → Forward & Backward
Vector,
Hashtable → Legacy