How is the index structure implemented in ArrayList?

In ArrayList, the index structure is implemented through an internal array of objects (Object[] elementData). This array functions as a dynamic array, where each element is accessible by index in constant time.

When adding elements, ArrayList checks if there is enough space in the array. If not, a new larger array (usually 50% larger) is created, and the elements are copied into it.

Access to elements by index is done directly through array access:

E element = (E) elementData[index];

Thus, the index is simply the position in the array, which ensures quick retrieval and updating of elements. However, inserting or deleting in the middle of the list requires shifting elements, which takes linear time.