### Question:
StatefulPreferencesUsageService implements UsageService { @Override public void clear() { dataStore.clear(); } @Inject  StatefulPreferencesUsageService(List<Module> modules, PreferencesUsageDataStore dataStore); @Override void incrementUsageFor(String moduleId); @Override List<Module> getMostUsed(); @Override List<Module> getMostRecentlyUsed(); @Override void clear(); @Override long getTotalUsage();  }### Answer:
@Test public void clean() { injector.instance(StatefulPreferencesUsageService.class).clear(); verify(injector.instance(PreferencesUsageDataStore.class)).clear(); }