private void assertContainsExpectedLines(String separator) throws IOException {
    String expected = expectedLines.isEmpty() ? "" : Joiner.on(separator).join(expectedLines);
    if (!lines.isEmpty()) {
      // if we wrote any lines in writeLines(), there will be a trailing newline
      expected += separator;
    }
    assertEquals(expected, factory.getSinkContents());
  }

  private void assertContainsExpectedLines(String separator) throws IOException {
    String expected = expectedLines.isEmpty() ? "" : Joiner.on(separator).join(expectedLines);
    if (!lines.isEmpty()) {
      // if we wrote any lines in writeLines(), there will be a trailing newline
      expected += separator;
    }
    assertEquals(expected, factory.getSinkContents());
  }