What Is the Underlying Data Structure of ArrayList in Java?


Heather Bennett

The underlying data structure of ArrayList in Java is a topic that every Java developer should have a clear understanding of. The ArrayList class in Java is a resizable array implementation that provides dynamic storage for elements. It is part of the Java Collections Framework and is widely used in Java programming.

ArrayList class: The ArrayList class is defined in the java.util package and extends the AbstractList class. It implements the List interface, which means that it supports all the methods defined in the List interface such as adding, removing, and accessing elements at specified positions.

Data Structure: The underlying data structure of ArrayList is an array. An array is a collection of elements of the same type stored at contiguous memory locations. In the case of ArrayList, this array can dynamically grow or shrink depending on the number of elements it contains.

Growth Strategy:

When an ArrayList is created without specifying an initial capacity, it starts with a default capacity. By default, an empty ArrayList has an initial capacity of 10 elements. As elements are added to the ArrayList and its size exceeds its current capacity, it automatically increases its capacity to accommodate more elements.

The growth strategy of ArrayList involves creating a new array with a larger size and copying all existing elements to the new array. This process can be computationally expensive if performed frequently, so it’s recommended to specify an initial capacity if you know how many elements you will be storing in advance.


The performance characteristics of an ArrayList depend on several factors such as the number of elements stored, operations performed (addition, removal, access), and available memory.

  • Addition: Adding an element to an ArrayList has an average time complexity of O(1). However, if resizing is required due to reaching maximum capacity, it can take O(n) time.
  • Removal: Removing an element from an ArrayList has an average time complexity of O(n) because it requires shifting subsequent elements to fill the gap.
  • Access: Accessing an element by index in an ArrayList has a time complexity of O(1) as it directly retrieves the element using the index.

Use Cases:

ArrayList is commonly used when dynamic resizing of the collection is required and random access to elements is frequent. It provides flexibility in adding, removing, and accessing elements without worrying about managing the underlying array’s size manually.


In summary, the underlying data structure of ArrayList in Java is based on a dynamic resizing array. It provides a flexible and efficient way to store and manipulate collections of elements. Understanding the underlying data structure helps developers make informed decisions about when and how to use ArrayList effectively.

So, next time you need a collection that can grow or shrink dynamically while providing fast random access to elements, consider using the ArrayList class in Java!

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