Re: Size of an arraylist in bytes

Andreas Leitgeb <>
20 Nov 2011 21:58:19 GMT
sara <> wrote:

Here is the code:
        ArrayList<Integer> tmp=new ArrayList<Integer>();
        tmp.set(0, 10);

    public static byte[] GetBytes(Object v) {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos;
        try {
            oos = new ObjectOutputStream(bos);

The serialization output size of an ArrayList<Integer> depends on
more than just the number of Integer elements in the array. There
is the capacity, which may be larger than the size, but what really
spoils it for you is the Integer-objects, which get serialized along
with the array. If both are same, only one Integer-object gets saved,
but if you change the value for one, then you get two different
Integer-objects serialized along with the actual array, and thus
you get more bytes.

If you need fixed-size records for your arrays (assuming a fixed
size() ), you might be more lucky with arrays of primitives:

If you had:
     int[] = new int[2]; tmp[0]=-1; tmp[1]=-1;
and dump that array onto oos, then change tmp[0]=0;
it's very likely, you'll see the same number of bytes
dumped, afterwards.

        } catch (IOException e) {
        byte[] data = bos.toByteArray();
        return data;

The problem is I need to write multiple arraylists on disk and later
on I update the elements of them. I store the starting location of
arraylists and their size such that later I can refer to them. If the
size of objects change then it messes up! Could you please help?

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