presto-orc/src/main/java/io/prestosql/orc/reader/MapColumnReader.java/MapColumnReader/MapColumnReader
class MapColumnReader:    public MapColumnReader(Type type, OrcColumn column, AggregatedMemoryContext systemMemoryContext, OrcBlockFactory blockFactory)
            throws OrcCorruptionException
    {
        requireNonNull(type, "type is null");
        verifyStreamType(column, type, MapType.class::isInstance);
        this.type = (MapType) type;

        this.column = requireNonNull(column, "column is null");
        this.blockFactory = requireNonNull(blockFactory, "blockFactory is null");
        this.keyColumnReader = createColumnReader(this.type.getKeyType(), column.getNestedColumns().get(0), systemMemoryContext, blockFactory);
        this.valueColumnReader = createColumnReader(this.type.getValueType(), column.getNestedColumns().get(1), systemMemoryContext, blockFactory);
    }