Java ArrayList: Internal Working, Dynamic Resizing and Methods

ArrayList is the most frequently used collection class in Java. It implements the List interface, backed internally by a dynamically resizing array allocated on the Heap.

How ArrayList Works Internally (Under the Hood)

When you instantiate ArrayList<String> list = new ArrayList<>();, Java creates a reference to an empty internal array: elementData = {}.

• First Addition: Calling list.add("Java") expands the internal array to the default capacity of 10.
• Capacity Exhaustion & 1.5x Growth: When the 11th element is added, the internal array is full. Java calculates:
  int newCapacity = oldCapacity + (oldCapacity >> 1); // 10 + 5 = 15!
  A new 15-element array is allocated on the Heap, elements are copied via native System.arraycopy(), and the old array is reclaimed by the Garbage Collector.

Practical Code Demonstration

import java.util.ArrayList;
import java.util.Collections;

public class ArrayListDemo {
    public static void main(String[] args) {
        // 1. Instantiating a generic ArrayList of Strings
        ArrayList<String> frameworks = new ArrayList<>();

        // 2. Adding elements (O(1) amortized time)
        frameworks.add("Spring Boot");
        frameworks.add("Hibernate");
        frameworks.add("Micronaut");
        frameworks.add("Quarkus");

        // 3. Fast random access by index (O(1) time)
        System.out.println("First framework: " + frameworks.get(0)); // Spring Boot

        // 4. Updating an element
        frameworks.set(2, "Jakarta EE");

        // 5. Removing an element
        frameworks.remove("Quarkus");

        // 6. Checking existence and size
        System.out.println("Contains Hibernate? " + frameworks.contains("Hibernate")); // true
        System.out.println("Current Size: " + frameworks.size()); // 3

        // 7. Sorting using Collections utility
        Collections.sort(frameworks);
        System.out.println("Sorted List: " + frameworks);
    }
}

Time Complexity Matrix