code: @SuppressWarnings("unchecked") // No values from static EMPTY class
    public void accept(CompositeRecordReader.JoinCollector jc, K key)
        throws IOException {
      if (hasNext() && 0 == cmp.compare(key, key())) {
        fillJoinCollector(createKey());
        jc.add(id, getDelegate());
        return;
      }
      jc.add(id, EMPTY);
    }
docs: If key provided matches that of this Composite, give JoinCollector
  iterator over values it may emit.