How many space is used by an object

Question

Hi @ all,
my english isn't very well but I hope you will understand me.
I have to find out how many space is used by an object at runtime. The goal is to get information about the actual used space by an objectbuffer. (I mustn't use jvmpi or something else)
Do you have an idea?
best regards

Answer 1

Try playing with the following code:
import java.io.*;
class IWantToBeMeasured implements Serializable {
    String[] stringArray = new String[1000]; //-- you'll waste at least 4 * 1000 bytes here...
    byte[] byteArray = new byte[1000]; //-- you'll waste at least 1000 bytes here...
    public IWantToBeMeasured() {
        //-- generate 1000 distinct strings "12345" // so you'll waste at least 5 * 1000 * 2 bytes here...
        for (int i = 0; i < stringArray.length; ++i) {
            stringArray[i] = String.valueOf (12345);
        //-- just fill 1000 bytes, and make all different, to avoid some optimizations
        for (int i = 0; i < byteArray.length; ++i) {
            byteArray[i] = (byte)i;
class Test114 {
    public static void main(String[] args) {
        int estimatedObjectSize = -1;
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream ();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            IWantToBeMeasured iwtbm = new IWantToBeMeasured ();
            oos.writeObject (iwtbm);
            oos.flush();
            estimatedObjectSize = baos.size();
            baos.close();
        } catch (IOException ex) {
        System.out.println ("Estimated object size = " + estimatedObjectSize);
}It prints "Estimated object size = 9154".
Well, do you expect a number that's greater than 15000 here. Why my program printed 9154?
There is a problem here. I know that a string is stored in Unicode in memory, so each character must occupy 2 bytes. But strings are serialized in UTF-8 encoding, that use only 1 byte for each ASCII character. Then the program printed 9154 , that's yet smaller than 4000 + 5000 + 1000.
I leave the problem to you.