new ArrayList<>() / list.add() / set() / get()

import java.util.ArrayList;

// Creates an empty ArrayList.
new ArrayList<Type>();

// Appends an element to the end of the list.
list.add(E e);

// Inserts an element at the specified position.
list.add(int index, E e);

// Replaces the element at the specified position.
list.set(int index, E element);

// Returns the element at the specified position.
list.get(int index);

import java.util.ArrayList;

// Create an ArrayList and add elements.
ArrayList<String> fruits = new ArrayList<>();
fruits.add("apple");
fruits.add("banana");
fruits.add("cherry");
System.out.println(fruits); // Prints "[apple, banana, cherry]".

// Insert an element at a specific position.
fruits.add(1, "blueberry"); // Inserts at index 1.
System.out.println(fruits); // Prints "[apple, blueberry, banana, cherry]".

// Retrieve elements using get().
System.out.println(fruits.get(0)); // Prints "apple".
System.out.println(fruits.get(2)); // Prints "banana".

// Update an element using set().
String old = fruits.set(0, "avocado"); // Replaces the first element.
System.out.println(old);    // Prints the previous value: "apple".
System.out.println(fruits); // Prints "[avocado, blueberry, banana, cherry]".

// Example using a list of numbers.
ArrayList<Integer> nums = new ArrayList<>();
for (int i = 1; i <= 5; i++) {
    nums.add(i * 10);
}
System.out.println(nums); // Prints "[10, 20, 30, 40, 50]".

// Accessing an out-of-bounds index throws an exception.
// System.out.println(nums.get(10)); // Throws IndexOutOfBoundsException.

ArrayList is the most commonly used collection class in Java. Internally it is backed by an array, so appending elements (add()) and retrieving elements (get()) run in O(1) time. However, inserting an element in the middle requires shifting all subsequent elements, so it runs in O(n) time.

Primitive types (int, double, etc.) cannot be stored directly. Use their wrapper classes (Integer, Double, etc.) instead. Thanks to Java's autoboxing feature, writing nums.add(1) automatically converts the value to Integer.

To remove elements or check the list size, see remove() / clear() / size(). To search for elements, see contains() / indexOf().