ArrayList initial capacity and IndexOutOfBoundsException [duplicate]

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What I don’t understand is why it doesn’t create an “empty” list of size 7, with null values at each index, similar to what would happen if we declared String[] myArray = new String[7].

That would be useful in some cases… and not useful in others. Quite often you have an upper bound of the size of list you’re going to create (or at least a guess) but then you populate it… and you don’t want to have a list which then has the wrong size… so you’d have to maintain an index while you “set” values, and then set the size afterwards.

I recall learning that ArrayList is Java’s implementation of a dynamic array, so I’d expect a similar sort of behavior.

No, it’s really not. It’s a list which can be resized and uses an array behind the scenes. Try not to think of it as an array.

If I don’t actually have space for 7 Strings allocated when I declare new ArrayList<String>(7), what is actually happening?

You do have space for 7 string references. The buffer size (i.e. the capacity) is at least 7, but the logical size of the list is still 0 – you haven’t added anything to it. It’s like you’ve got a sheet of paper that’s long enough for 7 lines, but you haven’t written anything yet.

If you want a prefilled list, you can easily write a method to create one:

public static List<T> createPrefilledList(int size, T item) {
    ArrayList<T> list = new ArrayList<T>(size);
    for (int i = 0; i < size; i++) {
        list.add(item);
    }
    return list;
}

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