java - How to serialize a class with an interface? -

i have never done serialization, trying use google's gson serialize java object file. here example of issue:

public interface animal {     public string getname(); }    public class cat implements animal {      private string mname = "cat";     private string mhabbit = "playing yarn";      public string getname() {         return mname;     }      public void setname(string pname) {         mname = pname;     }      public string gethabbit() {         return mhabbit;     }      public void sethabbit(string phabbit) {         mhabbit = phabbit;     }  }  public class exhibit {      private string mdescription;     private animal manimal;      public exhibit() {         mdescription = "this public exhibit.";     }      public string getdescription() {         return mdescription;     }      public void setdescription(string pdescription) {         mdescription = pdescription;     }      public animal getanimal() {         return manimal;     }      public void setanimal(animal panimal) {         manimal = panimal;     }  }  public class gsontest {  public static void main(string[] argv) {     exhibit exhibit = new exhibit();     exhibit.setanimal(new cat());     gson gson = new gson();     string jsonstring = gson.tojson(exhibit);     system.out.println(jsonstring);     exhibit deserializedexhibit = gson.fromjson(jsonstring, exhibit.class);     system.out.println(deserializedexhibit); } } 

so serializes nicely -- understandably drops type information on animal:

{"mdescription":"this public exhibit.","manimal":{"mname":"cat","mhabbit":"playing yarn"}} 

this causes real problems deserialization, though:

exception in thread "main" java.lang.runtimeexception: no-args constructor interface com.atg.lp.gson.animal not exist. register instancecreator gson type fix problem. 

i why happening, having trouble figuring out proper pattern dealing this. did in guide didn't address directly.

put animal transient, not serialized.

or can serialize implementing defaultwriteobject(...) , defaultreadobject(...) (i think thats called...)

edit see part writing instance creator here http://sites.google.com/site/gson/gson-user-guide

gson cant deserialize interface since doesnt know implementing class used, need provide instance creator animal , set default or similar.