### Question:
MailBackendFoldersBuilder { public BackendFolders build() { Map<MailboxPath, Node> pathToNode = Maps.newHashMap(); Node root = new Node(); for (Entry<MailboxPath, FolderType> folder : specialMailboxes) { appendNode(pathToNode, root, folder.getKey(), folder.getValue()); } for (MailboxPath folder : Sets.difference(mailboxes, pathToNode.keySet())) { appendNode(pathToNode, root, folder, FolderType.USER_CREATED_EMAIL_FOLDER); } return new TreeBackendFolders(root); } MailBackendFoldersBuilder(); MailBackendFoldersBuilder addFolders(MailboxFolders folders); MailBackendFoldersBuilder addSpecialFolder(String path, FolderType type); MailBackendFoldersBuilder addSpecialFolders(List<Entry<MailboxPath, FolderType>> folders); BackendFolders build();  }### Answer:
@Test public void buildShouldReturnEmptyWhenEmpty() { assertThat(new MailBackendFoldersBuilder().build()).isEmpty(); }