Parse a YAML file

You could also use Jacksons YAML module.

In order to use that, you’ll need a few classes. The model classes which will carry the content of your file and the a class that takes care of reading the YAML file.

The root model class could look like this:

public class MyYamlFile {
    @JsonProperty
    private List<User> users;
    @JsonProperty
    private List<Group> groups;

    // getter methods ommitted
}

The User(*) class:

public class User {
    @JsonProperty
    private List<String> name;
    @JsonProperty
    private List<GroupType> groups;

    // getter methods ommitted
}

The GroupType could be an Enum containing all possible group types:

public enum GroupType {
    Premium, Mod, Default
}

Don’t forget that the enum entries are case sensitive. So “premium” won’t work.
You can build all your model classes that way. Every sub entry should get an own model class.

Now to the part where you can read that YAML file:

public MyYamlFile readYaml(final File file) {
    final ObjectMapper mapper = new ObjectMapper(new YAMLFactory()); // jackson databind
    return mapper.readValue(file, MyYamlFile.class);
}

As you can see, this part is really neat, because you don’t need much. The file instance contains your YAML file. You can create one like this:

File file = new File("path/to/my/yaml/usersAndGroups.yaml");

Instead of File the readValue method also supports InputStream, java.io.Reader, String (with the whole content), java.net.URL and byte array.
You should find something that suits you.

(*) You should consider changing the structure of your YAML file, because I don’t think it is possible to use dynamic keys with Jackson (maybe someone knows more about that):

users: 
    - name: user1
      groups:
        - Premium
    - name: user2
      groups:
        - Mod
    - name: user3
      groups:
        - Default
groups:
    ....