What Data Structure Is Used to Implement the ArrayList in Java?


Scott Campbell

When working with Java, the ArrayList class is a commonly used data structure that provides a dynamic array-like implementation. It allows us to store and manipulate a collection of elements in a flexible manner.

But have you ever wondered what data structure lies behind the scenes to make this possible? In this article, we will explore the inner workings of the ArrayList in Java and examine the data structure it uses for efficient element storage and retrieval.

The Array-Based Structure

The ArrayList class in Java is implemented using an underlying array-based structure. This means that internally, an array is used to store the elements of the list. The size of this array can dynamically grow or shrink as elements are added or removed from the ArrayList.

The advantage of using an array-based structure is that it provides constant-time random access to elements. In other words, we can directly access any element in the list by its index position. This makes element retrieval fast and efficient.

Dynamically Resizing Array

To handle the dynamic resizing of the underlying array, the ArrayList class uses a strategy known as doubling and halving. Initially, when we create an empty ArrayList, it starts with an internal array of a default size (typically 10).

As we add elements to the list and it reaches its capacity, the ArrayList automatically increases its internal array’s size by doubling it. This ensures that sufficient space is available for additional elements without having to resize frequently.

If we remove elements from the ArrayList and it becomes significantly less than half full, then it shrinks its internal array by halving it. This helps in conserving memory when the list has fewer elements.

ArrayList Operations and Performance

The ArrayList class provides several operations for element manipulation, such as adding, removing, and accessing elements. Let’s take a look at some of these operations and their performance characteristics:

Adding Elements

When we add an element to the ArrayList using the add() method, it checks if the internal array is already full. If it is, the ArrayList resizes its array by creating a new array with twice the size and copying all existing elements into it. This operation takes linear time complexity (O(n)), where n is the number of elements currently in the list.

Removing Elements

The remove() method in ArrayList removes an element from a specific index position. After removing an element, it shifts all subsequent elements to fill the empty space. This operation also takes linear time complexity (O(n)) since shifting requires iterating over some or all elements.

Accessing Elements

To access an element at a given index position, we can use the get() method provided by ArrayList. Since it uses direct indexing into the internal array, accessing an element has constant time complexity (O(1)). This makes ArrayList efficient for random access operations.

In Conclusion

In this article, we explored how the ArrayList class in Java is implemented using an underlying array-based structure. We learned about its dynamic resizing strategy and how it allows for efficient element storage and retrieval. Understanding these implementation details can help us make informed decisions when working with ArrayLists in Java.

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