[SPARK-9915] [ML] stopWords should use StringArrayParam

hhbyyh

Author: Xiangrui Meng <meng@databricks.com>

Closes #8141 from mengxr/SPARK-9915.

mllib/src/main/scala/org/apache/spark/ml/feature/StopWordsRemover.scala

@@ -19,12 +19,12 @@ package org.apache.spark.ml.feature
 
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.ml.Transformer
+import org.apache.spark.ml.param.{BooleanParam, ParamMap, StringArrayParam}
 import org.apache.spark.ml.param.shared.{HasInputCol, HasOutputCol}
-import org.apache.spark.ml.param.{ParamMap, BooleanParam, Param}
 import org.apache.spark.ml.util.Identifiable
 import org.apache.spark.sql.DataFrame
-import org.apache.spark.sql.types.{StringType, StructField, ArrayType, StructType}
 import org.apache.spark.sql.functions.{col, udf}
+import org.apache.spark.sql.types.{ArrayType, StringType, StructField, StructType}
 
 /**
  * stop words list
@@ -100,7 +100,7 @@ class StopWordsRemover(override val uid: String)
    * the stop words set to be filtered out
    * @group param
    */
-  val stopWords: Param[Array[String]] = new Param(this, "stopWords", "stop words")
+  val stopWords: StringArrayParam = new StringArrayParam(this, "stopWords", "stop words")
 
   /** @group setParam */
   def setStopWords(value: Array[String]): this.type = set(stopWords, value)