public interface Persistent{
        public Key getKey();
    public void setKey(Key key);
}
public class PersistentObject implements Persistent{
    Key key;
    public Key getKey() {
        return key;
    }
    public void setKey(Key key) {
        this.key=key;
    }
}

public @interface Persistent {
}
@Persistent
public class PersistentObject {
    Key key; //the coder must create this variable or the system doesn't work
}