List<ExtendedCell> getCellList(byte[] family) {
    return familyMap.computeIfAbsent(family, k -> new ArrayList<>());
  }
Creates an empty list if one doesn't exist for the given column family or else it returns the
associated list of Cell objects.
@param family column family
@return a list of Cell objects, returns an empty list if one doesn't exist.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/Mutation.java