public abstract boolean isEmpty() throws IOException;
@Test
  public void testIsEmpty() throws Exception {
    File tmpFile = tmpFolder.newFile();
    List<IsmRecord<byte[]>> data = new ArrayList<>();
    writeElementsToFile(data, tmpFile);

    IsmReader<byte[]> reader =
        new IsmReaderImpl<byte[]>(
            FileSystems.matchSingleFileSpec(tmpFile.getAbsolutePath()).resourceId(), CODER, cache);
    assertFalse(reader.isInitialized());
    assertTrue(reader.isEmpty());
    assertTrue(reader.isInitialized());
  }