presto-main/src/main/java/io/prestosql/util/JsonUtil.java/MapBlockBuilderAppender/MapBlockBuilderAppender
class MapBlockBuilderAppender:        MapBlockBuilderAppender(BlockBuilderAppender keyAppender, BlockBuilderAppender valueAppender, Type keyType)
        {
            this.keyAppender = keyAppender;
            this.valueAppender = valueAppender;
            this.keyType = keyType;
        }