Convert Json to List Java

I have a JSON array like this :

    [
      {"_id": {"$oid":"57e9e4b1f36d281c4b330509"}, "user": "edmtdev" },
      {"_id": {"$oid":"57e9e4cec2ef164375c4c292"}, "user": "admin1234" },
      {"_id": {"$oid":"57ea1b0ac2ef164375c5ff1e"}, "username": "admin34" }
   ]

This is my User.class, used to store all data:

 public class User{
       private Id id;
       private String user;

       public Id getId() {
          return id;
       }

       public void setId(Id id) {
          this.id = id;
       }

       public String getUser() {
          return user;
       }

       public void setUser(String user) {
          this.user = user;
       }
    }

And my Id.class, used to store the ID:

public class Id {

   private String $oid;

   public String get$oid() {
     return $oid;
   }


   public void set$oid(String $oid) {
     this.$oid = $oid;
   }
}

I am using GSON in Java to get my users as a List<User>:

   List<User> users = new ArrayList<User>();
   Gson gson = new Gson();
   Type listType = new TypeToken<List<User>>(){}.getType();
   users = gson.fromJson(s,listType);

The problem is, I get users with username but no ID, $oid is not registered. Can someone help me understanding what does not work in this piece of code ?