### Question:
PartitionedIndexStorage { public List<File> listFolders() { return listFolders( getIndexFolder() ); } PartitionedIndexStorage( DirectoryFactory directoryFactory, FileSystemAbstraction fileSystem, File rootFolder ); Directory openDirectory( File folder ); File getPartitionFolder( int partition ); File getIndexFolder(); void reserveIndexFailureStorage(); void storeIndexFailure( String failure ); String getStoredIndexFailure(); void prepareFolder( File folder ); void cleanupFolder( File folder ); Map<File,Directory> openIndexDirectories(); List<File> listFolders();  }### Answer:
@Test void listFolders() throws IOException { File indexFolder = storage.getIndexFolder(); fs.mkdirs( indexFolder ); createRandomFile( indexFolder ); createRandomFile( indexFolder ); File folder1 = createRandomFolder( indexFolder ); File folder2 = createRandomFolder( indexFolder ); List<File> folders = storage.listFolders(); assertEquals( asSet( folder1, folder2 ), new HashSet<>( folders ) ); }