presto-orc/src/main/java/io/prestosql/orc/reader/MapColumnReader.java/MapColumnReader/startStripe
class MapColumnReader:    @Override
    public void startStripe(ZoneId timeZone, InputStreamSources dictionaryStreamSources, ColumnMetadata<ColumnEncoding> encoding)
            throws IOException
    {
        presentStreamSource = missingStreamSource(BooleanInputStream.class);
        lengthStreamSource = missingStreamSource(LongInputStream.class);

        readOffset = 0;
        nextBatchSize = 0;

        presentStream = null;
        lengthStream = null;

        rowGroupOpen = false;

        keyColumnReader.startStripe(timeZone, dictionaryStreamSources, encoding);
        valueColumnReader.startStripe(timeZone, dictionaryStreamSources, encoding);
    }