Size

The you can access the number of elements in an ArrayList with the .size() method.

import java.util.ArrayList;

void main() {
    ArrayList<String> names = new ArrayList<>();

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

    names.add("Vincent Bisset de Gramont");
    System.out.println(names.size());

    names.add("Mr. Nobody");
    System.out.println(names.size());
}

This will tell you the number of elements in the ArrayList but not the amount of space allocated by the underlying array.

This is fine because, if you aren't the one making a growable array, it doesn't matter. All you need to concern yourself with is what is actually there, not any book keeping.