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What is ArrayList?

ArrayList is a resizable array implementation in Java that belongs to the java.util package. Unlike regular arrays that have a fixed size, ArrayList can grow and shrink automatically as you add or remove elements.

Think of it like a backpack that expands when you put more items in it and shrinks when you take items out!

Dynamic Size: Automatically adjusts its size when elements are added or removed
Indexed Access: Access elements quickly using index positions (just like arrays)
Flexible Operations: Easy insertion, deletion, and searching of elements
Type Safety: Use generics to ensure type-safe operations

💡 Key Point: ArrayList maintains the order of insertion, meaning elements stay in the sequence you added them.

Creating and Using ArrayList

Here's how to create an ArrayList and perform basic operations:

Example: ArrayList Basic Operations

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        // Creating an ArrayList of Strings
        ArrayList fruits = new ArrayList<>();
        
        // Adding elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        fruits.add("Mango");
        
        // Display ArrayList
        System.out.println("Fruits: " + fruits);
        
        // Accessing an element
        System.out.println("First fruit: " + fruits.get(0));
        
        // Changing an element
        fruits.set(1, "Grapes");
        System.out.println("After change: " + fruits);
        
        // Removing an element
        fruits.remove("Orange");
        System.out.println("After removal: " + fruits);
        
        // Size of ArrayList
        System.out.println("Total fruits: " + fruits.size());
    }
}

Output:

Fruits: [Apple, Banana, Orange, Mango]
First fruit: Apple
After change: [Apple, Grapes, Orange, Mango]
After removal: [Apple, Grapes, Mango]
Total fruits: 3

Explanation:

ArrayList<String> - Creates an ArrayList that stores String objects only
add() - Adds elements to the end of the list
get(index) - Retrieves the element at the specified position
set(index, element) - Replaces the element at the specified position
remove() - Removes the specified element or element at index
size() - Returns the number of elements in the list

Common ArrayList Methods

1. Adding Elements

You can add elements at the end or at a specific position in the ArrayList.

Methods: add(element), add(index, element), addAll(collection)

2. Accessing Elements

Retrieve elements using their index position or check if an element exists.

Methods: get(index), contains(element), indexOf(element)

3. Modifying Elements

Update existing elements or remove them from the list.

Methods: set(index, element), remove(element), remove(index), clear()

4. Checking and Iterating

Check the size, if empty, or loop through all elements.

Methods: size(), isEmpty(), for-each loop, iterator()

Looping Through ArrayList

There are multiple ways to iterate through an ArrayList. Here are the most common methods:

Example: Different Ways to Loop

import java.util.ArrayList;

public class LoopExample {
    public static void main(String[] args) {
        ArrayList numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(40);
        
        // Method 1: For-each loop (Most common)
        System.out.println("Using for-each loop:");
        for (Integer num : numbers) {
            System.out.println(num);
        }
        
        // Method 2: Traditional for loop
        System.out.println("\nUsing traditional for loop:");
        for (int i = 0; i < numbers.size(); i++) {
            System.out.println(numbers.get(i));
        }
        
        // Method 3: forEach with Lambda (Java 8+)
        System.out.println("\nUsing forEach method:");
        numbers.forEach(num -> System.out.println(num));
    }
}