Data loss when passing objects via ObjectStream

Question

I am trying to pass an object from the server to the client using ObjectOutputStream . DataObject class:

public class DataObject implements Serializable{
    private byte dataType;
    private Object data;

public DataObject(byte dataType, Serializable data) {
    this.dataType = dataType;
    this.data = data;
}

Player class

public class Player implements Serializable{
    private int mana, health;
    private List<Integer> hand, grave, deck, used;
    private Effect healing, posioned, grievousWound, exhaust, resistance, power;
    private Player enemy;

I sent it away with this code

os = new ObjectOutputStream(server.getOutputStream());
os.writeObject(new DataObject((byte) 8, this.player));
System.out.println(this.player);
// It has printed out exact object

And get it with

DataObject d = (DataObject) is.readObject()
System.out.println(d.getDataType());
System.out.println(d.getData());
System.out.println((Player)d.getData());

It has printed out

8
null

What is the problem I'm having?

I fixed Object to Serializable

private byte dataType;
private Serializable data;

os.writeObject(new DataObject((byte) 8,(Serializable)this.player));

Answer 1

The code shown is okay(ish). The bug lies somewhere else. To demonstrate this I have expanded the original code to make an MRE, other than not reproducing and containing unnecessary detail.

import java.io.*;
import java.util.*;

class DataObject implements Serializable{
    private byte dataType;
    private Object data;
    public DataObject(byte dataType, Object data) {
        this.dataType = dataType;
        this.data = data;
    }
    public byte getDataType() { return dataType; }
    public Object getData() { return data; }

}
class Effect implements Serializable { }
class Player implements Serializable {
    private int mana, health;
    private List<Integer> hand, grave, deck, used;
    private Effect healing, posioned, grievousWound, exhaust, resistance, power;
    private Player enemy;
}
class Code {
    private final Player player = new Player();

    public static void main(String[] args) throws Throwable {
        new Code().go();
    }

    public void go() throws Throwable {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        try (ObjectOutputStream os = new ObjectOutputStream(out)) {
            os.writeObject(new DataObject((byte) 8, this.player));
            System.out.println(this.player);
        }
        ObjectInputStream is = new ObjectInputStream(
            new ByteArrayInputStream(out.toByteArray())
        );
        DataObject d = (DataObject) is.readObject();
        System.out.println(d.getDataType());
        System.out.println(d.getData());
        System.out.println((Player)d.getData());
    }
}

This prints:

Player@4fca772d
8
Player@49097b5d
Player@49097b5d