Java throwing errors when client sends Object to server, with no errors/warnings in IDE

Question

Somebody please help, I have a client and a server and I'm sending an object from the client in this code where it says send packet of data:

   class Data implements Serializable {
        /**
         * 
         */
        // private static final long serialVersionUID = 1L;

        String a = "ok baby";
        String b = "hi there";
    }

    public void Connect(String[] args) throws IOException {

        Socket kkSocket = null;
        PrintWriter out = null;
        ObjectOutputStream oos = null;
        BufferedReader in = null;

        try {
            kkSocket = new Socket("192.168.0.5", 1337);
            out = new PrintWriter(kkSocket.getOutputStream(), true);
            oos = new ObjectOutputStream(kkSocket.getOutputStream());
            in = new BufferedReader(new InputStreamReader(
                    kkSocket.getInputStream()));
        } catch (UnknownHostException e) {
            System.err.println("Don't know about hostname, network problem.");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't connect to server.");
            System.exit(1);
        }

        BufferedReader stdIn = new BufferedReader(new InputStreamReader(
                System.in));
        String fromServer;
        String fromUser;

        // SEND PACKET OF DATA
        oos.writeObject(new Data());
        oos.flush();
        // SEND PACKET OF DATA

This code simply doesn't transmit and has this error pop-up when it tries to send, anyone mind helping me out here?

java.io.NotSerializableException: java.lang.Thread
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.access$300(Unknown Source)
    at java.io.ObjectOutputStream$PutFieldImpl.writeFields(Unknown Source)
    at java.io.ObjectOutputStream.writeFields(Unknown Source)
    at java.awt.Container.writeObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.access$300(Unknown Source)
    at java.io.ObjectOutputStream$PutFieldImpl.writeFields(Unknown Source)
    at java.io.ObjectOutputStream.writeFields(Unknown Source)
    at java.awt.Container.writeObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.access$300(Unknown Source)
    at java.io.ObjectOutputStream$PutFieldImpl.writeFields(Unknown Source)
    at java.io.ObjectOutputStream.writeFields(Unknown Source)
    at java.awt.Container.writeObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.access$300(Unknown Source)
    at java.io.ObjectOutputStream$PutFieldImpl.writeFields(Unknown Source)
    at java.io.ObjectOutputStream.writeFields(Unknown Source)
    at java.awt.Container.writeObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at citadelRPG.Main.Connect(Main.java:124)
    at citadelRPG.Main.<init>(Main.java:158)
    at citadelRPG.Main$2.run(Main.java:269)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at citadelRPG.Main$2.run(Main.java:277)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

part of my source was used from the second answer to this question , and I am a java newbie.

Answer 1

The Data class is a non-static inner class of your outer class, which extends Thread or has an attribute of type Thread. Every non-static inner class has an implicit reference to its outer object. To illustrate this, the following code

public class Outer extends Thread {
    class Data implements Serializable {
        String a = "ok baby";
        String b = "hi there";
    }

    ...
}

is equivalent to the following :

public class Outer extends Thread
    ...
}

class Data implements Serializable {
     String a = "ok baby";
     String b = "hi there";
     Outer outer;
}

So, when serializing an instance of Data, it also tries to serialize the outer object, and Thread being non-serializable, it breaks.

To fix the problem, make the Data inner class static, or make it a top-level class.