কালেকশন ফ্রেমওয়ার্ক (Collections Framework)

import java.util.ArrayList;
import java.util.Collections;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();

        // Add items
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");
        fruits.add("Banana");  // duplicates are OK!

        System.out.println("Fruits: " + fruits);
        System.out.println("Size: " + fruits.size());
        System.out.println("First: " + fruits.get(0));

        // Remove by value
        fruits.remove("Banana");  // removes first occurrence
        System.out.println("After remove: " + fruits);

        // Check if it contains something
        System.out.println("Has Cherry? " + fruits.contains("Cherry"));

        // Sort alphabetically
        fruits.add("Avocado");
        Collections.sort(fruits);
        System.out.println("Sorted: " + fruits);

        // Loop through
        System.out.print("Each: ");
        for (String f : fruits) {
            System.out.print(f + " ");
        }
        System.out.println();
    }
}

Fruits: [Apple, Banana, Cherry, Banana]
Size: 4
First: Apple
After remove: [Apple, Cherry, Banana]
Has Cherry? true
Sorted: [Apple, Avocado, Banana, Cherry]
Each: Apple Avocado Banana Cherry

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<String> visitors = new HashSet<>();

        visitors.add("Anika");
        visitors.add("Rafi");
        visitors.add("Anika");  // duplicate — ignored!
        visitors.add("Sadia");
        visitors.add("Rafi");    // duplicate — ignored!

        System.out.println("Visitors: " + visitors);
        System.out.println("Count: " + visitors.size());  // 3, not 5

        System.out.println("Has Anika? " + visitors.contains("Anika"));
        System.out.println("Has Dave? " + visitors.contains("Dave"));

        visitors.remove("Rafi");
        System.out.println("After removing Rafi: " + visitors);

        // Great for removing duplicates from a list!
        String[] words = {"cat", "dog", "cat", "bird", "dog", "cat"};
        HashSet<String> unique = new HashSet<>();
        for (String w : words) {
            unique.add(w);
        }
        System.out.println("Unique words: " + unique);
    }
}

Visitors: [Anika, Rafi, Sadia]
Count: 3
Has Anika? true
Has Dave? false
After removing Rafi: [Anika, Sadia]
Unique words: [cat, bird, dog]

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> scores = new HashMap<>();

        // Put key-value pairs
        scores.put("Anika", 95);
        scores.put("Rafi", 82);
        scores.put("Sadia", 91);

        // Get by key
        System.out.println("Anika's score: " + scores.get("Anika"));

        // Check if key exists
        System.out.println("Has Rafi? " + scores.containsKey("Rafi"));

        // Update a value
        scores.put("Rafi", 88);  // same key = replace value
        System.out.println("Rafi's new score: " + scores.get("Rafi"));

        // getOrDefault — safe lookup
        System.out.println("Dave's score: " + scores.getOrDefault("Dave", 0));

        // Loop through all entries
        System.out.println("\nAll scores:");
        for (String name : scores.keySet()) {
            System.out.println("  " + name + ": " + scores.get(name));
        }

        // Count word frequency — classic HashMap pattern
        String sentence = "the cat sat on the mat the cat";
        HashMap<String, Integer> freq = new HashMap<>();
        for (String word : sentence.split(" ")) {
            freq.put(word, freq.getOrDefault(word, 0) + 1);
        }
        System.out.println("\nWord counts: " + freq);
    }
}

Anika's score: 95
Has Rafi? true
Bob's new score: 88
Dave's score: 0

All scores:
  Rafi: 88
  Anika: 95
  Sadia: 91

Word counts: {the=3, cat=2, sat=1, on=1, mat=1}