@Override
  public Long add(Long l1, Long l2) {
    if (l1 == null ^ l2 == null) {
      return (l1 == null) ? l2 : l1; // either of one is null.
    } else if (l1 == null) // both are null
      return null;
    return l1 + l2;
  }
a concrete column interpreter implementation. The cell value is a Long value and its promoted
data type is also a Long value. For computing aggregation function, this class is used to find
the datatype of the cell value. Client is supposed to instantiate it and passed along as a
parameter. See TestAggregateProtocol methods for its sample usage. Its methods handle null
arguments gracefully.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/coprocessor/LongColumnInterpreter.java