How can I determine the byte length of an object that I serialize to a stream?

There are a couple of things you can do. First, you can pipe the ObjectOutputStream into a ByteArrayOutputStream, then examine the length of the byte array:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream out = ObjectOutputStream(baos);
out.writeObject(new MyObject());
out.flush();
//
// Subtract 4 bytes from the length, because the serialization
// magic number (2 bytes) and version number (2 bytes) are
// both written to the stream before the object
//
int objectsize = baos.toByteArray().length - 4;
System.out.println("MyObject occupies " + objectsize 
  + " bytes when serialized");

Or, if you want to stream to a destination other than a byte array, you can write a subclass of DataOutputStream which allows you to access the protected variable written. This subclass can be used to monitor the data sent through the stream to any destination source:

import java.io.*;

public class ByteCounterOutputStream extends DataOutputStream {
    public ByteCounterOutputStream(OutputStream out) {
        super(out);
    }

    public int bytesWrittenSoFar() {
        return written;
    }
}

FileOutputStream file = new FileOutputStream("junk");
ByteCounterOutputStream bcos = new ByteCounterOutputStream(file);
ObjectOutputStream out = new ObjectOutputStream(bcos);
int start = bcos.bytesWrittenSoFar();
out.writeObject(new MyObject());
out.flush();
int objectsize = bcos.bytesWrittenSoFar() - start;
System.out.println("MyObject occupies " + objectsize 
    + " bytes when serialized");