The ArrayList Class (Dynamic Arrays)

On Day 27, we learn about the ArrayList class in Java, which provides a dynamic array that can grow and shrink automatically. Unlike normal arrays, where the size is fixed at creation, ArrayList adjusts its size as elements are added or removed.

1. Creating an ArrayList

import java.util.ArrayList;

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

        // Add elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Mango");

        System.out.println(fruits);
    }
}

Explanation: We import java.util.ArrayList, then create a list of String objects. The add() method inserts new items.

2. Accessing and Modifying Elements

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> colors = new ArrayList<>();
        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");

        // Access elements
        System.out.println("First color: " + colors.get(0));

        // Modify element
        colors.set(1, "Yellow");

        // Remove element
        colors.remove(2);

        System.out.println("Updated list: " + colors);
    }
}

Explanation: get() retrieves an element, set() updates it, and remove() deletes it.

3. Looping Through an ArrayList

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);

        // For loop
        for (int i = 0; i < numbers.size(); i++) {
            System.out.println("Index " + i + ": " + numbers.get(i));
        }

        // Enhanced for loop
        for (int n : numbers) {
            System.out.println("Number: " + n);
        }
    }
}

Explanation: We can loop using a traditional for loop with size() or the enhanced for-each loop.

4. Useful Methods

• add(element) – Add item
• get(index) – Get item
• set(index, element) – Update item
• remove(index) – Remove item
• size() – Returns list size
• clear() – Removes all items
• contains(element) – Checks if item exists

Summary

ArrayList is a dynamic array in Java that makes it easy to store, access, and modify a collection of elements without worrying about fixed sizes.