ArrayList is a resizable-array implementation of the List interface. It grows dynamically as elements are added and allows random access to elements.

Use ArrayList when you need frequent access to elements by index and the number of elements can change but removals from the middle are rare.

• Access/Get: $O(1)$
• Add (at end): $O(1)$ (amortized)
• Add/Remove (at specific index): $O(n)$
• Search (by value): $O(n)$

java Standard
import java.util.ArrayList;
import java.util.List;

public class ListExample {
    public static void main(String[] args) {
        List<String> cities = new ArrayList<>();
        cities.add("New York");
        cities.add("London");
        cities.get(0); // "New York"
        cities.size(); // 2
    }
}

ArrayList is backed by an internal array. When the array is full, it creates a new, larger array (usually 1.5x the old size) and copies all elements to it.

• Ignoring Capacity: Not providing an initial capacity when the final size is known (leads to multiple resize operations).
• Using for removals: Using ArrayList for frequent removals from the beginning or middle (use LinkedList if removals are the primary operation).
• Concurrent Modification: Modifying an ArrayList while iterating over it with an Iterator or for-each loop incorrectly.