Why Does Initializing ArrayList Size Matter If You Can\'t Access the Allocated Space Immediately?

Understanding the Purpose of Initializing ArrayList Size

The Java ArrayList class offers the flexibility to specify an initial size upon creation. However, it's often confusing why this is necessary if you can't access the allocated space immediately. This question delves into the distinction between the size and capacity of an ArrayList to clarify this matter.

Initial Size vs. Capacity

When initializing an ArrayList with new ArrayList<>(10), you are not defining its size but its capacity. Capacity refers to the number of elements the ArrayList can hold without having to reallocate its underlying data structure. By setting an initial capacity, you optimize performance by avoiding unnecessary reallocation as you add elements.

Adding Elements

Contrary to your assumption, add(int index, Object element) indeed adds an element to the specified index. However, this requires the list to have sufficient capacity to accommodate the new element. When you try arr.add(5, 10), you receive an out-of-bounds exception because the ArrayList has no elements yet, resulting in an empty capacity.

Example: Initializing and Filling an ArrayList

To initialize an ArrayList with a capacity of 10 and populate it with elements, you can use a loop:

<code class="java">ArrayList<Integer> arr = new ArrayList<>(10);
for (int i = 0; i < 10; i++) {
  arr.add(0);
}</code>

Now, you can modify or retrieve elements at indices 0 to 9, utilizing the initial capacity you defined.

In summary, setting an initial capacity for an ArrayList helps optimize performance by avoiding reallocation overhead as your list grows. While the initial size of an ArrayList is zero, the capacity ensures that enough space is available to efficiently accommodate subsequent elements.

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