Java UUID – Using Arbitrary Strings as UUID

Using rest to send/receive data to/from the server is a fine choice, its a well known and stable protocol for client-server communication. However, you're going to need a system to push updates (or notifications) to the clients as they change on the server, and to do that you need something other than http web connections.

On an Android device, you're best off using push technology - Google has a few sites that talk about implementing this kind of thing. When you receive the notification, if it doesn't already contain the data, it can be used as a trigger for the app to retrieve the data using your usual methods (ie automatically click the 'refresh' button)

If you don't want to use the push systems, then the other answer is to open a persistent tcp/ip socket to the server while the players are actively playing and receive notifications and data directly through that.

Data is now (after parsing) stored in just a list, searching for a specific peace of data is not very well optimized. Are there other structures I could use, which I can search using file(string) - level(integer) - sub-level(integer) so I can quickly get a specific data object.

Assuming that you want to search by File, Level, Sub-Level only, you have a clear hierarchical structure in that description, it sounds like you can divide one ArrayList into multiple steps. You could even make each hierarchy level a class itself to make it more clear, for example:

class SomeFileData {
   List<LevelData> levels;
}

class LevelData {
   List<SubLevelData> sublevels;
}

class SubLevelData {
   // Probably similar to your current `FileData` implementation
}

And if you need to save multiple files at once, you can use a Map with the file name as key.

class LotsOfFileData {
    Map<String, SomeFileData> files; 
}

Using this hierarchy, you can easily get file "mydata.dat", level 3, sublevel 4.

LotsOfFileData allData;
allData.getFile("mydata.dat").getLevel(3).getSublevel(4)

Each step in the get process looks at the current class' list or map to retrieve the data.

By the way, declare variables according to their interface, not their implementation. Declare your ArrayLists as List to allow for easy change of implementation.