What Is the Underlying Data Structure of ArrayList?


Larry Thompson

What Is the Underlying Data Structure of ArrayList?

An ArrayList is a dynamic array in Java that allows you to store and manipulate elements. It is one of the most commonly used data structures in Java programming due to its flexibility and ease of use.

The Basic Structure of ArrayList

An ArrayList is internally implemented as an array, which means it has a fixed size. However, unlike a regular array, an ArrayList can grow or shrink dynamically as elements are added or removed.

The underlying data structure of an ArrayList is an array, but it also includes additional methods and functionalities that make it more convenient to work with compared to a traditional array.

Dynamic Resizing

One of the key advantages of using an ArrayList is its ability to resize itself automatically. When you add elements to an ArrayList and it reaches its capacity, it will automatically create a new array with a larger size and copy all the elements from the old array into the new one.

  • This resizing process ensures that there is always enough space to accommodate new elements without causing any overflow or memory issues.
  • It also allows you to add and remove elements from any position within the ArrayList without worrying about shifting other elements manually.

Efficient Random Access

Since an ArrayList is implemented using an array, accessing elements by their index is very efficient. You can directly access any element in constant time (O(1)) by providing its index.

  • Note: Keep in mind that adding or removing elements at the beginning or middle of an ArrayList can be less efficient since it requires shifting all subsequent elements.

Flexibility in Element Types

An ArrayList can store elements of any type, including primitive types and custom objects. Unlike regular arrays, which are limited to a specific data type, an ArrayList allows you to mix different types of elements within the same list.

This flexibility makes ArrayLists extremely versatile and useful in a wide range of applications.


In summary, an ArrayList in Java is implemented using an underlying array data structure that provides dynamic resizing, efficient random access, and flexibility in element types. It is a powerful tool for managing collections of elements and is widely used in Java programming.

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