Java: ObjectOutputStream with Serializable problem

Question

Hello,

I have these three classes:

Command:

package pack;

public abstract class Command impements java.io.Serializable
{
    public abstract void execute();
}

Client:

package pack;

// imports....

public class Client
{
    Socket socket;

    // Constructor...

    public void sendCommand(Command c)
    {
         try
         {
              new ObjectOuputStream(socket.getOutputStream()).writeObject(c);
         } catch (Exception e) {e.printStackTrace()};
    }
}

MyKeyListener:

This keylistener is added to a component in a JFrame.

public class MyKeyListener implements KeyListener
{

     private Client client;

     public MyKeyListener(Client c)
     { client = c; }


     public void keyTyped(....)......; // This method does nothing

     public void keyPressed(KeyEvent e)
     {
          client.sendCommand(new Command() {
               public void execute()
               {
                    new Robot().keyPress(e.getKeyCode());
               }
          });
     }

     // The same for keyRelease()....
}

The problem is: if I run the code and he wants to send a Command. The streams stops writing because "MyKeyListener is not Serializable" ???!! But I never try to send MyKeyListener

Answer 1

Hint: Try typing MyKeyListener.this.toString() in the (anonymous) Command class execute() method.

Answer 2

Nested classes in Java actually don't exist at the byte code level - the compiler fakes them by inserting hidden fields, access methods and constructors. In your case, the anonymous subclass of Command probably has a compiler-generated reference to the MyKeyListener instance in which it was created, even if that reference is not used for anything.

To fix this, use a top-level subclass of Command instead of an anonymous class.