code: public Set<String> getRequiredColumns() {
      Set<String> set = new HashSet<String>();
      for (FieldMapping fieldMapping : fieldMappings) {
        if (FieldMapping.MappingType.KEY == fieldMapping.getMappingType()) {
          continue;
        } else if (FieldMapping.MappingType.KEY_AS_COLUMN == fieldMapping.getMappingType()) {
          set.add(fieldMapping.getFamilyAsString() + ":");
        } else {
          set.add(fieldMapping.getFamilyAsString() + ":"
              + fieldMapping.getQualifierAsString());
        }
      }
      return set;
    }
docs: Get the columns required by this schema.
  
  @return The set of columns