Java ArrayList

The Java ArrayList Class

The ArrayList class is the most commonly used implementation of the List interface. It uses a dynamic array to store the elements.

A standard Java array is of a fixed size. Once created, you cannot change its size. An ArrayList, however, can grow and shrink automatically as you add or remove elements.

How ArrayList Works

Internally, an ArrayList maintains an array of objects. When you create an ArrayList, it starts with an initial capacity (e.g., 10 elements).

When you add elements and the internal array becomes full, the ArrayList automatically creates a new, larger array (typically 1.5 times the old size), copies all the elements from the old array to the new one, and then adds the new element. This resizing process is handled automatically, giving the impression of a "dynamic" array.

Key Characteristics of ArrayList

• Fast Random Access: Since ArrayList is backed by an array, retrieving an element at a specific index using the get(index) method is very fast. This is an O(1) operation.
• Slow Insertion/Deletion: Adding or removing elements from the middle of an ArrayList is slow. This is because all subsequent elements need to be shifted to fill the gap or make space. This is an O(n) operation.
• Allows Duplicates and null values: Like any List, it can store duplicate elements and null values.

When to Use ArrayList

ArrayList is the go-to choice for a List when your main requirement is storing and accessing data. If you frequently need to retrieve elements by their index and don't often add or remove elements from the middle of the list, ArrayList is the most efficient option.

ArrayList Example

Here is an example demonstrating common operations on an ArrayList.

import java.util.ArrayList;
import java.util.Collections; // For sorting

public class Main {
  public static void main(String[] args) {
    ArrayList<String> cars = new ArrayList<String>();
    // Add items
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    System.out.println(cars);
    // Access an item
    System.out.println("Element at index 1: " + cars.get(1));
    // Change an item
    cars.set(0, "Opel");
    System.out.println("After changing index 0: " + cars);
    // Remove an item
    cars.remove(2);
    System.out.println("After removing index 2: " + cars);
    // Get the size
    System.out.println("Size of the list: " + cars.size());
    // Loop through an ArrayList
    System.out.println("--- Looping through items ---");
    for (String car : cars) {
      System.out.println(car);
    }
    // Sort the ArrayList
    Collections.sort(cars);
    System.out.println("Sorted list: " + cars);
    // Clear all items
    cars.clear();
    System.out.println("After clearing: " + cars);
  }
}