📚 Java Collections Framework – Master Lists, Sets, Maps, and Queues

🔖 Introduction

When building real-world applications, we often need to store, retrieve, and manipulate groups of objects. Java provides a rich and powerful Collections Framework to make this easy.

In this lesson, we’ll explore:

List (ArrayList, LinkedList)
Set (HashSet, TreeSet)
Map (HashMap, TreeMap)
Queue (PriorityQueue, LinkedList)

🧱 What is the Java Collections Framework?

A Collection is a container object that holds a group of elements.

The Collections Framework provides:

Interfaces (List, Set, Map, Queue)
Implementations (ArrayList, HashMap, etc.)
Utility classes (Collections, Arrays)

📋 List – Ordered Collection (Duplicates Allowed)

🔹 ArrayList

import java.util.ArrayList;

public class ListExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple");

        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

✅ Preserves insertion order
✅ Allows duplicates
✅ Dynamic resizing

🔹 LinkedList

Similar to ArrayList, but optimized for frequent insertions/deletions.

LinkedList<String> items = new LinkedList<>();
items.add("One");
items.addFirst("Zero");
items.addLast("Two");

🚫 Set – No Duplicates Allowed

🔹 HashSet

import java.util.HashSet;

public class SetExample {
    public static void main(String[] args) {
        HashSet<String> cities = new HashSet<>();
        cities.add("Delhi");
        cities.add("Mumbai");
        cities.add("Delhi");

        System.out.println(cities);  // Duplicates ignored
    }
}

✅ No duplicates
❌ Unordered (no guarantee of insertion order)

🔹 TreeSet – Sorted Set

import java.util.TreeSet;

TreeSet<Integer> numbers = new TreeSet<>();
numbers.add(10);
numbers.add(5);
numbers.add(15);

System.out.println(numbers);  // Output: [5, 10, 15]

🗺️ Map – Key-Value Pairs

🔹 HashMap

import java.util.HashMap;

public class MapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 25);
        ages.put("Bob", 30);

        System.out.println(ages.get("Alice"));  // Output: 25
    }
}

✅ Fast lookup
✅ Keys must be unique
✅ Values can be duplicated

🔹 TreeMap – Sorted by Keys

import java.util.TreeMap;

TreeMap<String, Integer> scores = new TreeMap<>();
scores.put("Math", 90);
scores.put("English", 85);

System.out.println(scores);  // Sorted by keys

🛤️ Queue – FIFO (First In First Out)

🔹 PriorityQueue

import java.util.PriorityQueue;

PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.add(20);
pq.add(10);
pq.add(30);

while (!pq.isEmpty()) {
    System.out.println(pq.poll());  // Sorted order: 10, 20, 30
}

✅ Prioritizes elements (natural order by default)

🔧 Utility Class: `Collections`

import java.util.Collections;

ArrayList<String> names = new ArrayList<>();
names.add("John");
names.add("Alice");
Collections.sort(names);
System.out.println(names);  // [Alice, John]

🧠 Quick Summary

Interface	Allows Duplicates	Maintains Order	Sorted	Key-Value
List	✅ Yes	✅ Yes	❌ No	❌ No
Set	❌ No	❌ No (HashSet)	✅ (TreeSet)	❌ No
Map	❌ Keys	❌ No	✅ (TreeMap)	✅ Yes
Queue	✅ Yes	✅ FIFO / Priority	✅ (PriorityQueue)	❌ No

🧪 Mini Challenge

Create a HashMap<String, Integer> to store the names and scores of 3 students.
Use a TreeSet<Integer> to keep their scores sorted.

✅ What You Learned

Difference between List, Set, Map, and Queue
When to use ArrayList, HashSet, HashMap, etc.
How to choose the right data structure based on your needs