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JVM / Java / Collections / Framework

1. Java Collections Framework (JCF)

In Java, Collections are a full arhitecture of interfaces, classes, utilities. They are part of the Java Collection Framework (JCF), located in java.util package. The JCF provides: - Interfaces (List, Set, Map, Queue) - Implementations (ArrayList, HashSet, HashMap, LinkedList) - Algorithms (sorting, searching) 
 
               Iterable
                  |
              Collection
      ┌───────────┼──────────┐
      |           |          |
     List        Set       Queue
      |           |          |
   ArrayList   HashSet    ArrayDeque
   LinkedList  TreeSet    PriorityQueue
 
                Map
                 |
              HashMap
              TreeMap

2. Collection Interface

The Collectioin Interface represents a group of objects (like a bag). Main subinterfaces: - List (ordered, indexed access): ArrayList, LinkedList - Set (no duplicates): HashSet (unordered), TreeSet (ordered) - Queue/Deque (FIFO/LIFO): LinkedList, ArrayDeque

3. Map Interace

Map is NOT a subtype of Collection. Stores key-value pairs: - HashMap - LinkedHashMap - TreeMap

4. Collections vs Arrays

Arrays: - fixed size - can store primitives - no built-in utilities - hard to manipulate Collections: - dynamic size - store objects (but wrappers allow primitives) - rich functionality - easy add/remove/find 
 
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class JCF {
    public static void main(String[] args) {

        // LIST (ordered, allows duplicates)
        // =================================
        List<String> names = new ArrayList<>();

        names.add("Alex");
        names.add("Maria");

        for (String n : names) {
            System.out.println(n); // Alex, Maria
        }

        // SET (no duplicates, unordered)
        // ==============================
        Set<String> cities = new HashSet<>();

        cities.add("Bucharest");
        cities.add("Cluj");
        cities.add("Bucharest"); // duplicate ignored

        for (String c : cities) {
            System.out.println(c);  // Bucharest, Cluj
        }

        // MAP (key-value pairs)
        // =====================
        Map<String, Integer> ages = new HashMap<>();

        ages.put("Ana", 25);
        ages.put("Bogdan", 32);
        ages.put("Ana", 26);  // overwrites previous value
        
        for (Map.Entry<String, Integer> entry : ages.entrySet()) {
            System.out.println(
                entry.getKey() + " -> " + entry.getValue()  // Ana -> 26  Bogdan -> 32
            );
        }
    }
}




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