Overhead in java ObjectOutputStream?

Question

I am puzzled by the behavior of ObjectOutputStream. It seems like it has an overhead of 9 bytes when writing data. Consider the code below:

float[] speeds = new float[96];
float[] flows = new float[96];

//.. do some stuff here to fill the arrays with data

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos=null;
try {
    oos = new ObjectOutputStream(baos);
    oos.writeInt(speeds.length);
    for(int i=0;i<speeds.length;i++) {
        oos.writeFloat(speeds[i]);
    }
    for(int i=0;i<flows.length;i++) {
        oos.writeFloat(flows[i]);
    }
    oos.flush();
} catch (IOException e) {
    e.printStackTrace();
} finally {
    try {
        if(oos!=null) {
            oos.close();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

byte[] array = baos.toByteArray();

The length of the array is always 781, while I would expect it to be (1+96+96)*4 = 772 bytes. I can't seem to find where the 9 bytes go.

Thanks!

--edit: added if(oos!=null) { ... } to prevent NPE

Answer 1

ObjectOutputStream is used to serialize objects. You shouldn't make any assumptions how the data is stored.

If you want to store just the raw data use DataOutputStream instead.