Android Communication – Sending Data to a Specific Device

As you say, typically it is very difficult or impossible to establish a socket connection originating at a central server and attempting to connect to mobile devices. Instead, what you can do is originate the connection at the mobile device, connect to the central server, and leave the socket open waiting for a command from the central server. Of course the first thing your Android code would do on connection is identify itself to the central server, so the central server knows who it is talking to.

Fortunately, Google has already done this for you in Google Cloud Messaging for Android:

Google Cloud Messaging for Android (GCM) is a service that allows you to send data from your server to your users' Android-powered device. This could be a lightweight message telling your app there is new data to be fetched from the server (for instance, a movie uploaded by a friend), or it could be a message containing up to 4kb of payload data (so apps like instant messaging can consume the message directly).

If you need to pass only the URL, then pass only the URL. No need to over think it.

There are several disadvantages of passing the whole object:

• the performance cost of json serialization and deserialization
• weaker encapsulation and weaker information hiding: the source fragment reveals more information to the other than necessary

If you only need to pass one field of many, then most certainly it's best to pass just the one field.

If you need to pass a few fields of many, then it's probably still better to pass only the needed fields, for the above reasons.

If you need to pass two product fields out of three, then it might be acceptable to pass the entire product.