private void checkReadonly() {
    if (readonly == true) {
      throw new UnsupportedOperationException("Attempting to modify readonly EMPTY_RESULT!");
    }
  }
All methods modifying state of Result object must call this method to ensure that special
purpose immutable Results can't be accidentally modified.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/Result.java