System.out.println(myList.size());

Since no elements are in the list

output => 0

myList.add("newString");  // use myList.add() to insert elements to the arraylist
System.out.println(myList.size());

Since one element is added to the list

output => 1

There is no such method in the ArrayList API. The capacity of an ArrayList is hidden by design.

However, I think that your question is based on a misunderstanding.

How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.

In fact, the size of a List, the number of elements in a List, and the number of indexes (i.e. indexable positions) for a List ... are all the same thing.

The capacity of an ArrayList is something different. It is the number of elements that the object could contain, without reallocating the list's backing array. However, the fact that the list has a capacity N does NOT mean that you can index up to N - 1. In fact, you can only index up to size() - 1, irrespective of the capacity.

Now to deal with your examples:

ArrayList list = new ArrayList(5);
System.out.println(list.size());

This prints out zero because the list has zero elements. The ArrayList() and ArrayList(int) constructors both create and return lists that are empty. The list currently has space for 5 elements (because you gave it an initial capacity of 5) but you can't index those slots.

System.out.println(list.toArray().length);

This prints zero because when you copy the list's contents to an array (using toArray()), the array is the same size as the list. By definition.

This does not mean that the list's backing array has changed. On the contrary, it is still big enough to hold 5 elements without reallocation ... just like before.

But ... I hear you say ... the array's length is zero!

Yes, but that is not the backing array! The toArray() method allocates a new array and copies the List contents into that array. It does NOT return the actual backing array.