There is an error message at every verification: "The server is unable to properly test your solution. Be sure you have all the classes you need. Be sure that your solution doesn't have any infinite loops." I guess it's because of List<Person>: public static class Person implements Externalizable { ... private List<Person> children; public void writeExternal(ObjectOutput out) throws IOException { .... out.writeObject(children); } public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { .... //children = (List)in.readObject(); children = (List<Person>) in.readObject(); } My Question is: What is the correct way to writExternal / readExternal a filled List ? Thanks for helping.