Question

I want to figure out sometihng.

is this:

    List <String> list = new ArrayList<String>();
    list.add("abc");
    list.add(null);

equals to this

    List <String> list = new ArrayList<String>();
    list.add("abc");

in memory usage?

thanks!

Answer 1

The initial capacity of an ArrayList is ten (references). That is to say, the underlying array will be of size ten, even if you’ve only got one entry in your collection. Those references will default to null, and consequently setting the second reference to null will neither affect the internal state of the arraylist (in terms of the underlying array) nor its memory consumption.

If you’d added an eleventh item (set to null), the ArrayList would expand its capacity, and consequently you’d consume more memory, but rather because the ArrayList had created extra buckets for your String references.

From the doc linked above:

Each ArrayList instance has a capacity. The capacity is the size of
the array used to store the elements in the list. It is always at
least as large as the list size. As elements are added to an
ArrayList, its capacity grows automatically. The details of the growth
policy are not specified beyond the fact that adding an element has
constant amortized time cost.