Java Collection Framework (JCF)

List, Set, Queue, Deque, and Map.

The Java Collection Framework (JCF) is a set of classes and interfaces that implement commonly reusable data structures like List, Set, Queue, Deque, and Map.

These are ready-made data structures with powerful algorithms, allowing developers to store, manipulate, and access groups of objects efficiently.

🔷 Why Use the Collection Framework?

• Reduces development time.
• Provides performance-optimized data structures.
• Supports algorithms like sorting, searching, shuffling.
• Interfaces + polymorphism for flexible design.

✅ Major Interfaces in the Collection Framework

📌 1. Collection Interface (java.util.Collection)

Parent interface of List, Set, Queue.

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Interface DescriptionListOrdered collection, allows duplicatesSetUnordered, no duplicates allowedQueueFIFO order, useful for scheduling

🔷 2. List Interface and Implementations

➤ List<E>

Maintains insertion order and allows duplicate elements.

✅ Implementations:

• ArrayList
• LinkedList
• Vector (legacy, thread-safe)

📌 Example:

import java.util.*;

public class ListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Java");
        list.add("C++");
        list.add("Python");
        list.add("Java"); // Duplicates allowed
        for (String lang : list)
            System.out.println(lang);
    }
}

🔷 3. Set Interface and Implementations

➤ Set<E>

Stores unique elements. No duplicates.

✅ Implementations:

• HashSet — Fast, unordered
• LinkedHashSet — Maintains insertion order
• TreeSet — Sorted set using red-black tree

📌 Example:

Set<Integer> set = new HashSet<>();
set.add(5);
set.add(10);
set.add(5); // Ignored (duplicate)

System.out.println(set); // No order guaranteed

🔷 4. Queue Interface and Implementations

➤ Queue<E>

Used for FIFO (first-in-first-out) operations.

✅ Implementations:

• LinkedList
• PriorityQueue — Min-heap (by default), sorted queue

📌 Example:

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

System.out.println(queue); // [B]

🔷 5. Deque Interface

➤ Deque<E> (Double Ended Queue)

Can insert/remove elements from both ends.

✅ Implementations:

• ArrayDeque

📌 Example:

Deque<String> deque = new ArrayDeque<>();
deque.addFirst("Front");
deque.addLast("Back");

System.out.println(deque); // [Front, Back]

🔷 6. Map Interface (Not part of Collection, but in framework)

➤ Map<K, V>

Stores key-value pairs.

✅ Implementations:

• HashMap
• TreeMap
• LinkedHashMap

📌 Example:

Map<String, Integer> map = new HashMap<>();
map.put("Java", 10);
map.put("Python", 8);

System.out.println(map.get("Java")); // 10

✅ Hierarchy of Java Collections

Iterable
           │
        Collection
     ┌────┼────┬───────┐
   List  Set  Queue  Deque
           │     │
     SortedSet PriorityQueue
           │
        NavigableSet
           │
         TreeSet

Map (separate interface)
 ├── HashMap
 ├── TreeMap
 └── LinkedHashMap

✅ Summary Table

📌 Common Real-World Use Cases

Scenario — — — > Recommended Class

Store student marks by ID — — →HashMap<Integer, Integer>

Store tasks to process in order — — →Queue or Deque

Get top 3 scores — — →PriorityQueue

Remove duplicate items from list — — →HashSet

Need sorted list of names — — →TreeSet

Navigation or autocomplete system — — →TreeMap

🛠 Example: Using All in One

import java.util.*;

public class CollectionDemo {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(List.of("Java", "Python", "C++", "Java"));
        Set<String> uniqueSet = new HashSet<>(list);
        Queue<String> queue = new LinkedList<>(list);
        Map<String, Integer> map = new HashMap<>();
        for (String lang : list)
            map.put(lang, map.getOrDefault(lang, 0) + 1);
        System.out.println("List: " + list);
        System.out.println("Unique Set: " + uniqueSet);
        System.out.println("Queue Front: " + queue.poll());
        System.out.println("Map Frequency: " + map);
    }
}