Github user HyukjinKwon commented on a diff in the pull request: https://github.com/apache/spark/pull/18875#discussion_r138058695 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/json/JacksonGenerator.scala --- @@ -26,20 +26,53 @@ import org.apache.spark.sql.catalyst.expressions.SpecializedGetters import org.apache.spark.sql.catalyst.util.{ArrayData, DateTimeUtils, MapData} import org.apache.spark.sql.types._ +/** + * `JackGenerator` can only be initialized with a `StructType` or a `MapType`. + * Once it is initialized with `StructType`, it can be used to write out a struct or an array of + * struct. Once it is initialized with ``MapType``, it can be used to write out a map or an array + * of map. An exception will be thrown if trying to write out a struct if it is initialized with + * a `MapType`, and vice verse. + */ private[sql] class JacksonGenerator( - schema: StructType, + dataType: DataType, writer: Writer, options: JSONOptions) { // A `ValueWriter` is responsible for writing a field of an `InternalRow` to appropriate // JSON data. Here we are using `SpecializedGetters` rather than `InternalRow` so that // we can directly access data in `ArrayData` without the help of `SpecificMutableRow`. private type ValueWriter = (SpecializedGetters, Int) => Unit + // `JackGenerator` can only be initialized with a `StructType` or a `MapType`. + dataType match { --- End diff -- I'd just do a `require(... .isInstanceOf... || ..., "...")` instead.
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