How do I convert a WrappedArray column in spark dataframe to Strings?

WrappedArray is not an Array (which is plain old Java Array not a natve Scala collection). You can either change signature to:

import scala.collection.mutable.WrappedArray

(arrayCol: WrappedArray[String]) => arrayCol.mkString(",")

or use one of the supertypes like Seq:

(arrayCol: Seq[String]) => arrayCol.mkString(",")

In the recent Spark versions you can use concat_ws instead:

import org.apache.spark.sql.functions.concat_ws

df.select(concat_ws(",", $"arrayCol"))

The code work for me:

df.select("wifi_ids").rdd.map(row =>row.get(0).asInstanceOf[WrappedArray[WrappedArray[String]]].toSeq.map(x=>x.toSeq.apply(0)))

In your case,I guess it is:

val mkString = udf(arrayCol=>arrayCol.asInstanceOf[WrappedArray[String]].toArray.mkString(","))  
val dfWithString=df.select($"arrayCol").withColumn("arrayString",mkString($"arrayCol"))