Java Collection Framework – Quick Summary

The Java Collection Framework provides a unified architecture to store, manipulate, and process groups of objects. It includes interfaces, implementations, and utility classes.

Core Hierarchy

Collection (root interface for List, Set, Queue)
Map (separate hierarchy for key-value pairs)

1. List (ordered, allows duplicates, index-based access)

ArrayList → fast random access (O(1)), slow middle insert/remove
LinkedList → fast insert/delete at ends, slow random access, also implements Deque
Vector → synchronized (legacy, rarely used today)

2. Set (no duplicates)

HashSet → fastest (O(1)), no order
LinkedHashSet → maintains insertion order
TreeSet → sorted (natural order or Comparator), O(log n)

3. Map (key → value, unique keys)

HashMap → fastest (O(1) average), no order
LinkedHashMap → insertion order (or access order mode)
TreeMap → sorted by keys, O(log n)
Hashtable → legacy synchronized version (rarely used now)

4. Queue / Deque

→ priority-based ordering (heap)

Requirement	Best Choice(s)
Fast index access, occasional add/remove	ArrayList
Frequent add/remove at both ends	ArrayDeque or LinkedList
Uniqueness, no order needed	HashSet
Uniqueness + insertion order	LinkedHashSet
Uniqueness + sorted order	TreeSet
Key-value pairs, fastest access	HashMap
Key-value + insertion order	LinkedHashMap
Key-value + sorted keys	TreeMap
Thread-safe map (concurrent)	ConcurrentHashMap

import java.util.*; public class CollectionExamples { public static void main(String[] args) { // 1. ArrayList – ordered, allows duplicates List<String> names = new ArrayList<>(); names.add("Alice"); names.add("Bob"); names.add("Alice"); // duplicate allowed System.out.println("ArrayList: " + names); // [Alice, Bob, Alice] // 2. HashSet – no duplicates, no order Set<Integer> numbers = new HashSet<>(); numbers.add(42); numbers.add(7); numbers.add(42); // ignored System.out.println("HashSet: " + numbers); // e.g. [7, 42] // 3. LinkedHashSet – insertion order + uniqueness Set<String> orderedUnique = new LinkedHashSet<>(); orderedUnique.add("cat"); orderedUnique.add("dog"); orderedUnique.add("cat"); orderedUnique.add("bird"); System.out.println("LinkedHashSet: " + orderedUnique); // [cat, dog, bird] // 4. TreeSet – sorted Set<Integer> sorted = new TreeSet<>(); sorted.add(50); sorted.add(10); sorted.add(30); System.out.println("TreeSet: " + sorted); // [10, 30, 50] // 5. HashMap – key-value Map<String, Integer> ages = new HashMap<>(); ages.put("Alice", 28); ages.put("Bob", 34); ages.put("Charlie", 19); System.out.println("HashMap: " + ages); // 6. LinkedHashMap – insertion order Map<String, String> favorites = new LinkedHashMap<>(); favorites.put("color", "blue"); favorites.put("food", "pizza"); favorites.put("city", "Kyoto"); System.out.println("LinkedHashMap (order preserved): " + favorites); // 7. PriorityQueue – natural ordering Queue<Integer> priority = new PriorityQueue<>(); priority.offer(50); priority.offer(10); priority.offer(30); System.out.println("PriorityQueue poll: " + priority.poll()); // 10 (smallest) // 8. ArrayDeque as stack Deque<String> stack = new ArrayDeque<>(); stack.push("bottom"); stack.push("middle"); stack.push("top"); System.out.println("Pop from stack: " + stack.pop()); // top } }

Java Collection

Java Collection Framework – Quick Summary

Core Hierarchy

1. List (ordered, allows duplicates, index-based access)

2. Set (no duplicates)

3. Map (key → value, unique keys)

4. Queue / Deque

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Quick Decision Guide

Important Utilities

Thread Safety

Code Examples