@Override
  protected CellFlatMap<T> createSubCellFlatMap(int min, int max, boolean descending) {
    return new CellArrayMap<>(comparator(), this.block, min, max, descending);
  }
CellArrayMap is a simple array of Cells and cannot be allocated off-heap. As all java arrays
CellArrayMap's array of references pointing to Cell objects.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CellArrayMap.java