@Override
  public Cell transformCell(Cell cell) throws IOException {
    if (cell instanceof ExtendedCell) {
      return createKeyOnlyCell((ExtendedCell) cell);
    }
    throw new DoNotRetryIOException(
      "Customized cell implementation is not support: " + cell.getClass().getName());
  }
A filter that will only return the key component of each KV (the value will be rewritten as
empty).
<p>
This filter can be used to grab all of the keys without having to also grab the values.
hbase-client/src/main/java/org/apache/hadoop/hbase/filter/KeyOnlyFilter.java