Gwt JSNI error

This is my JSNI code where I need to call the onFlip() method in the JSNI code

        public static native void copyFrom(String s) /*-{

     this.@com.dhinakar.TestProj::onFlip(Ljava/lang/String;)(s);

}-*/;

 private void onFlip(String flippedName) {

   Window.alert(flippedName);

 }

I am facing the below exception. Please help me to fix this. Thanks in advance.

com.google.gwt.event.shared.UmbrellaException: Exception caught: Cannot cast com.google.gwt.core.client.JavaScriptObject$ to 

com.dhinakar.TestProj

--
You received this message because you are subscribed to the Google Groups "GWT Users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to google-web-toolkit+unsubscribe@googlegroups.com.
To post to this group, send email to google-web-toolkit@googlegroups.com.
Visit this group at https://groups.google.com/group/google-web-toolkit.
For more options, visit https://groups.google.com/d/optout.