🚀 Java Collections Framework - Complete Guide

“The Collections Framework provides a unified architecture for representing and manipulating collections, enabling you to focus on what you want to accomplish rather than how to accomplish it.” - Oracle Java Documentation

🎯 What is Java Collections Framework?

The Java Collections Framework is a unified architecture for representing and manipulating collections of objects. It provides a set of interfaces, implementations, and algorithms to handle groups of objects efficiently, eliminating the need to write custom data structures for most use cases.

🌳 Core Interface Hierarchy

📈 Performance Characteristics Quick Reference

Implementation	Access	Search	Insertion	Deletion	Ordering	Thread-Safe
ArrayList	O(1)	O(n)	O(1) amortized	O(n)	Insertion	❌
LinkedList	O(n)	O(n)	O(1)	O(1)	Insertion	❌
HashSet	N/A	O(1)	O(1)	O(1)	No	❌
TreeSet	N/A	O(log n)	O(log n)	O(log n)	Sorted	❌
HashMap	O(1)	O(1)	O(1)	O(1)	No	❌
TreeMap	O(log n)	O(log n)	O(log n)	O(log n)	Sorted	❌

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🎯 When to Use Which Collection?

📝 Decision Framework

🤔 What type of data are you storing?

├── 🔑 Key-Value pairs?
│   ├── Need sorting? → TreeMap
│   ├── Need insertion order? → LinkedHashMap
│   └── Best performance? → HashMap
│
├── 📋 Unique elements only?
│   ├── Need sorting? → TreeSet
│   ├── Need insertion order? → LinkedHashSet
│   └── Best performance? → HashSet
│
├── 📊 Ordered elements with duplicates?
│   ├── Frequent random access? → ArrayList
│   ├── Frequent insertion/deletion? → LinkedList
│   └── Thread safety required? → Vector or Collections.synchronizedList()
│
└── 🎯 Processing queue?
    ├── Priority-based? → PriorityQueue
    ├── FIFO operations? → ArrayDeque
    └── Both ends access? → ArrayDeque

🛠️ Common Patterns and Idioms

🔍 Initialization Patterns

// Diamond operator (Java 7+)
List<String> list = new ArrayList<>();
Map<String, Integer> map = new HashMap<>();

// Factory methods (Java 9+)
List<String> immutableList = List.of("a", "b", "c");
Set<Integer> immutableSet = Set.of(1, 2, 3);
Map<String, Integer> immutableMap = Map.of("one", 1, "two", 2);

// Arrays.asList() for quick initialization
List<String> list = Arrays.asList("a", "b", "c");

// Collection copy constructors
List<String> copy = new ArrayList<>(original);
Set<String> uniqueItems = new HashSet<>(listWithDuplicates);

🔄 Iteration Patterns

// Enhanced for-loop
for (String item : collection) {
    process(item);
}

// Iterator with removal
Iterator<String> it = collection.iterator();
while (it.hasNext()) {
    String item = it.next();
    if (shouldRemove(item)) {
        it.remove(); // Safe removal during iteration
    }
}

// Stream API
collection.stream()
    .filter(predicate)
    .map(mapper)
    .collect(Collectors.toList());

🔧 Utility Operations

// Sorting
Collections.sort(list);
Collections.sort(list, comparator);

// Searching
int index = Collections.binarySearch(sortedList, key);

// Min/Max
String min = Collections.min(collection);
String max = Collections.max(collection, comparator);

// Frequency and disjoint checks
int count = Collections.frequency(collection, item);
boolean noCommon = Collections.disjoint(collection1, collection2);