What Data Structure Does Java Use to Implement an ArrayList?


Larry Thompson

Java provides a powerful data structure called ArrayList that is widely used in programming. It is a part of the java.util package and offers a dynamic array that can grow or shrink as needed. In this article, we will explore the underlying data structure used by Java to implement an ArrayList and understand how it works.

The Array-Based Implementation

The ArrayList in Java uses an array-based implementation to store its elements. It internally maintains an array that holds the actual elements added to the list. This allows for efficient random access to elements based on their index.

The size of the internal array dynamically adjusts as elements are added or removed from the ArrayList. When the number of elements exceeds the capacity of the current internal array, a new larger array is created and all existing elements are copied into it.

This resizing process ensures that there is always enough space for new elements and avoids running out of memory or wasting memory when there are fewer elements in the list.

Growth Factor

To determine how much larger the new array should be when resizing, Java uses a growth factor algorithm. The growth factor determines how much extra space is added to accommodate new elements.

In Java’s implementation, when the internal array needs to be resized, a new array with a size equal to (oldCapacity + (oldCapacity >> 1)) is created. This means that the new array will be 50% larger than its previous size.

Rapid Insertion and Deletion at End

An ArrayList provides fast insertion and deletion operations at the end of the list. Since it uses an array-based implementation, appending an element at the end simply involves adding it to the internal array. Similarly, removing an element from the end is a straightforward operation.

Slower Insertion and Deletion in Middle

Insertion or deletion at any position other than the end of the ArrayList requires shifting elements within the internal array. This can be a relatively slow operation as it involves iterating over and moving several elements.

When an element is inserted or deleted in the middle of the ArrayList, all subsequent elements need to be shifted to fill in or close the gap. This process can become time-consuming for large lists or frequent modifications in non-end positions.


In summary, Java uses an array-based implementation for ArrayLists. It provides fast random access using index-based operations but slower insertion and deletion at non-end positions. The dynamic resizing of the internal array ensures efficient memory utilization and prevents running out of space.

  • ArrayList is a powerful data structure in Java that uses an array-based implementation.
  • The internal array dynamically grows or shrinks as elements are added or removed.
  • A growth factor algorithm determines how much larger the new array should be when resizing.
  • Appending an element at the end of the list is efficient, but insertion or deletion in non-end positions can be slower due to element shifting.

Note: It’s important to consider these characteristics when choosing between ArrayList and other data structures based on specific requirements and performance considerations.

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