Java - Java Collections Framework (In Depth)

 

The Java Collections Framework (JCF) is a unified architecture for storing and manipulating groups of objects.

It includes:

  1. Interfaces

  2. Implementations (classes)

  3. Algorithms

1. Core Interfaces

1. Collection (Root Interface)

Extended by:

  • List

  • Set

  • Queue

Map is separate (not a subtype of Collection).

2. List

  • Ordered collection

  • Allows duplicates

  • Access by index

Common Implementations:

  • ArrayList – dynamic array, fast access, slow insert/delete in middle

  • LinkedList – doubly linked list, fast insert/delete, slower access

  • Vector – synchronized (legacy)

Example:

 
List<String> list = new ArrayList<>(); list.add("A"); list.add("B");

3. Set

  • No duplicates

  • No index-based access

Implementations:

  • HashSet – unordered, fast

  • LinkedHashSet – maintains insertion order

  • TreeSet – sorted (uses Red-Black Tree)

4. Queue

  • Follows FIFO (First In First Out)

Implementations:

  • PriorityQueue – elements sorted by priority

  • ArrayDeque – faster than Stack for LIFO operations

5. Map (Key-Value Pairs)

Not part of Collection interface.

Characteristics:

  • Keys are unique

  • Values can duplicate

Implementations:

  • HashMap – unordered, fast

  • LinkedHashMap – insertion order

  • TreeMap – sorted by key

  • Hashtable – synchronized (legacy)

Example:

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

2. Iteration Methods

  • for-each loop

  • Iterator

  • ListIterator

  • forEach (Java 8+)

Example:

 
for(String s : list) { System.out.println(s); }

3. Performance Overview

  • ArrayList → Fast random access (O(1))

  • LinkedList → Fast insert/delete (O(1))

  • HashMap → Fast lookup (O(1) average)

  • TreeMap → Sorted, O(log n)

Choosing the right collection depends on:

  • Do you need ordering?

  • Do you need sorting?

  • Do you allow duplicates?

  • Do you need thread safety?

4. Utility Class: Collections

Provides algorithms:

  • sort()

  • reverse()

  • shuffle()

  • binarySearch()

  • min(), max()

Example:

 
Collections.sort(list);

5. Key Concepts for Interviews

  • Difference between ArrayList and LinkedList

  • HashMap internal working (hashing, buckets)

  • Fail-fast vs fail-safe iterators

  • Comparable vs Comparator

  • Load factor and initial capacity

In short:

Java Collections Framework provides ready-made data structures with different performance characteristics. Choosing the correct implementation is critical for efficient programs.