@org.apache.thrift.annotation.Nullable
  public _Fields fieldForId(int fieldId) {
    return _Fields.findByThriftId(fieldId);
  }
Returns true if field corresponding to fieldID is set (has been assigned a value) and false otherwise
hbase-thrift/src/main/java/org/apache/hadoop/hbase/thrift2/generated/TColumnFamilyDescriptor.java