code: public boolean nextKey() throws IOException,InterruptedException {
      while (hasMore && nextKeyIsSame) {
        nextKeyValue();
      }
      if (hasMore) {
        if (inputKeyCounter != null) {
          inputKeyCounter.increment(1);
        }
        return nextKeyValue();
      } else {
        return false;
      }
    }
docs: Start processing next unique key.