GAE - rpc and HashMap<K,V> field

Question

I tested my gwt app in eclipse and it worked fine; Then I tried to deploy my test app to gae... That what exception I can see in gae logs after each rpc call :

java.lang.RuntimeException: java.io.NotSerializableException

... so I am still looking the reason; but there is a point which make me confused cause one of my rpc-callback objects contains HashMap as one of its fields;

public abstract class Report implements Serializable{}
public abstract class Message implements Serializable{}
public class ErrorMessage extends Message{}

public final class Error extends Report
{
 private HashMap<String,ErrorMessage> msgs;//<---?

 //...
 public void setMsg(HashMap<String,ErrorMessage> msgs){this.msgs=msgs;}
 public HashMap<String,ErrorMessage> getMsg(){return this.msgs;}
}

...So my question is...

does gae supports java.util.HashMap ? And if not what is the most optimal collections alternative?

Thanks

Answer 1

It might be the "ErrorMessage" object that's not serializable.