Collections Framework

Collection (Interface)
├── List (ordered, duplicates allowed)
│   ├── ArrayList
│   ├── LinkedList
│   └── Vector
├── Set (no duplicates)
│   ├── HashSet
│   ├── LinkedHashSet
│   └── TreeSet
└── Queue (FIFO)
    ├── PriorityQueue
    └── LinkedList

Map (Interface — separate hierarchy)
├── HashMap
├── LinkedHashMap
├── TreeMap
└── Hashtable

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.add("Charlie");
names.get(0);           // "Alice"
names.set(1, "Bobby");  // Replace at index 1
names.remove(2);        // Remove index 2
names.size();            // 2
names.contains("Alice"); // true

for (String name : names) {
    System.out.println(name);
}

import java.util.LinkedList;

LinkedList<String> queue = new LinkedList<>();
queue.addFirst("First");
queue.addLast("Last");
queue.removeFirst();

import java.util.HashMap;

HashMap<String, Integer> scores = new HashMap<>();
scores.put("Alice", 95);
scores.put("Bob", 87);
scores.put("Charlie", 92);

System.out.println(scores.get("Alice")); // 95
scores.containsKey("Bob");               // true
scores.remove("Charlie");
scores.size();                            // 2

for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

import java.util.HashSet;

HashSet<String> languages = new HashSet<>();
languages.add("Java");
languages.add("Python");
languages.add("Java"); // Ignored — duplicate!
System.out.println(languages.size()); // 2

import java.util.Iterator;

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

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    System.out.println(it.next());
}

import java.util.Collections;

ArrayList<Integer> nums = new ArrayList<>(List.of(5, 2, 8, 1, 9));
Collections.sort(nums);          // [1, 2, 5, 8, 9]
Collections.reverse(nums);       // [9, 8, 5, 2, 1]
Collections.shuffle(nums);       // Random order
Collections.max(nums);           // 9
Collections.min(nums);           // 1
Collections.frequency(nums, 5);  // Count of 5

import java.util.*;

public class ContactManager {
    public static void main(String[] args) {
        HashMap<String, String> contacts = new HashMap<>();
        Scanner sc = new Scanner(System.in);
        boolean running = true;

        while (running) {
            System.out.println("\n1. Add  2. Search  3. List All  4. Delete  5. Exit");
            System.out.print("Choice: ");
            int choice = sc.nextInt(); sc.nextLine();

            switch (choice) {
                case 1:
                    System.out.print("Name: "); String name = sc.nextLine();
                    System.out.print("Phone: "); String phone = sc.nextLine();
                    contacts.put(name, phone);
                    System.out.println("Contact added!");
                    break;
                case 2:
                    System.out.print("Search name: "); String search = sc.nextLine();
                    System.out.println(contacts.getOrDefault(search, "Not found"));
                    break;
                case 3:
                    contacts.forEach((k, v) -> System.out.println(k + ": " + v));
                    break;
                case 4:
                    System.out.print("Delete name: "); String del = sc.nextLine();
                    contacts.remove(del);
                    break;
                case 5: running = false; break;
            }
        }
        sc.close();
    }
}