Github user HyukjinKwon commented on a diff in the pull request: https://github.com/apache/spark/pull/20884#discussion_r176897268 --- Diff: sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/json/JacksonGeneratorSuite.scala --- @@ -56,7 +56,7 @@ class JacksonGeneratorSuite extends SparkFunSuite { val gen = new JacksonGenerator(dataType, writer, option) gen.write(input) gen.flush() - assert(writer.toString === """[{}]""") + assert(writer.toString === """[{"a":null}]""") --- End diff -- I think you should compare this: ```scala scala> sql(""" select array(cast(null as struct<k:string>)) as my_array""").toJSON.collect().foreach(println) {"my_array":[null]} scala> sql(""" select array(struct(cast(null as string))) as my_array""").toJSON.collect().foreach(println) {"my_array":[{}]} ```
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