Review

class TermsHashPerThread {
  public DocumentsWriter.DocWriter finishDocument() throws IOException {
    final DocumentsWriter.DocWriter doc = consumer.finishDocument();

    final DocumentsWriter.DocWriter doc2;
    if (nextPerThread != null)
      doc2 = nextPerThread.consumer.finishDocument();
    else
      doc2 = null;
    if (doc == null)
      return doc2;
    else {
      doc.setNext(doc2);
      return doc;
    }
  }

}