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Groups > comp.lang.java.programmer > #10119

Re: Size of an arraylist in bytes

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sara <sarasara82@gmail.com> wrote:
> Here is the code:
> 		ArrayList<Integer> tmp=new ArrayList<Integer>();
> 		tmp.add(-1);
> 		tmp.add(-1);
> 		System.out.println(DiGraph.GetBytes(tmp).length);
> 		tmp.set(0, 10);
> 		System.out.println(DiGraph.GetBytes(tmp).length);
>
> 	public static byte[] GetBytes(Object v) {
> 		ByteArrayOutputStream bos = new ByteArrayOutputStream();
> 		ObjectOutputStream oos;
> 		try {
> 			oos = new ObjectOutputStream(bos);
> 			oos.writeObject(v);

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.

> 			oos.flush();
> 			oos.close();
> 			bos.close();
> 		} catch (IOException e) {
> 			e.printStackTrace();
> 		}
> 		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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Size of an arraylist in bytes sara <sarasara82@gmail.com> - 2011-11-20 13:01 -0800
  Re: Size of an arraylist in bytes markspace <-@.> - 2011-11-20 13:05 -0800
    Re: Size of an arraylist in bytes sara <sarasara82@gmail.com> - 2011-11-20 13:11 -0800
      Re: Size of an arraylist in bytes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-11-20 21:58 +0000
      Re: Size of an arraylist in bytes markspace <-@.> - 2011-11-20 14:08 -0800
      Re: Size of an arraylist in bytes Patricia Shanahan <pats@acm.org> - 2011-11-20 14:50 -0800
      Re: Size of an arraylist in bytes Arne Vajhøj <arne@vajhoej.dk> - 2011-11-20 18:06 -0500
      Re: Size of an arraylist in bytes Lew <lewbloch@gmail.com> - 2011-11-20 20:28 -0800
  Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-20 16:30 -0500
    Re: Size of an arraylist in bytes sara <sarasara82@gmail.com> - 2011-11-20 13:35 -0800
      Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-20 17:19 -0500
    Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-20 16:58 -0500
      Re: Size of an arraylist in bytes Patricia Shanahan <pats@acm.org> - 2011-11-20 14:04 -0800
        Re: Size of an arraylist in bytes Arne Vajhøj <arne@vajhoej.dk> - 2011-11-20 17:18 -0500
          Re: Size of an arraylist in bytes Patricia Shanahan <pats@acm.org> - 2011-11-20 14:48 -0800
            Re: Size of an arraylist in bytes Arne Vajhøj <arne@vajhoej.dk> - 2011-11-25 22:12 -0500
      Re: Size of an arraylist in bytes Lew <lewbloch@gmail.com> - 2011-11-20 20:44 -0800
        Re: Size of an arraylist in bytes Arne Vajhøj <arne@vajhoej.dk> - 2011-11-25 22:16 -0500
          Re: Size of an arraylist in bytes Lew <lewbloch@gmail.com> - 2011-11-25 20:15 -0800
  Re: Size of an arraylist in bytes Roedy Green <see_website@mindprod.com.invalid> - 2011-11-20 22:25 -0800
    Re: Size of an arraylist in bytes Arne Vajhøj <arne@vajhoej.dk> - 2011-11-25 22:11 -0500

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