@SharedPref(SharedPref.Scope.UNIQUE)
public interface PrefsClass {
    /* Documentation of @DefaultStringSet states:
     * "Use on methods in SharedPref annotated class.
     * The generated method will return an empty java.util.Set of Strings by default."
     */
    @DefaultStringSet
    public Set<String> stringSetPref();
}

public final class PrefsClass_ extends SharedPreferencesHelper{
    // ...
    public StringSetPrefField stringSetPref() {
        // returns null instead of empty Set
        return stringSetField("stringSetPref", null);
    }
}

public final class PrefsClass_ extends SharedPreferencesHelper{
    // ...
    public StringSetPrefField stringSetPref() {
        // returns a new empty set (not necessarily HashSet)
        return stringSetField("stringSetPref", new HashSet<String>());
    }
}