Why Can\'t I Access the Initial Capacity of an ArrayList in Java?

Initial Size of an ArrayList: Delving into Capacity vs. Size

In Java, creating an ArrayList with an initial size is a common practice to optimize performance. However, it's crucial to understand the distinction between size and capacity when using the add() method.

Question:

When creating an ArrayList with an initial size, why can't you access the space allocated?

Answer:

The initial size value you provide when creating an ArrayList using the constructor ArrayList<>(initialSize) sets the internal capacity of the ArrayList. The size, on the other hand, represents the number of elements currently in the ArrayList.

When you add an element to an ArrayList using the add(index, element) method, you specify the position (index) where the element will be inserted. If you attempt to add an element to an index that exceeds the current size of the ArrayList, you will encounter an out of bounds exception.

This is because the add() method modifies the size of the ArrayList by incrementing it by one. It does not affect the capacity, which remains unchanged.

To add multiple elements to an ArrayList's initial capacity, you can use a loop:

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

This loop iteratively adds elements to the ArrayList, effectively utilizing the initial capacity you set during its creation. Once the loop has completed, you can modify elements at indices 0 to (initialSize - 1).

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