address comments

Author: WeichenXu123

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

@@ -84,7 +84,7 @@ private[feature] trait StringIndexerBase extends Params with HasHandleInvalid wi
 
     require((isSet(inputCol) && isSet(outputCol) && !isSet(inputCols) && !isSet(outputCols)) ||
       (!isSet(inputCol) && !isSet(outputCol) && isSet(inputCols) && isSet(outputCols)),
-      "Only allow to set either inputCol/outputCol, or inputCols/outputCols"
+      "StringIndexer only supports setting either inputCol/outputCol or inputCols/outputCols."
     )
 
     if (isSet(inputCol)) {

mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala

@@ -37,6 +37,29 @@ class StringIndexerSuite
     val modelWithoutUid = new StringIndexerModel(Array("a", "b"))
     ParamsSuite.checkParams(model)
     ParamsSuite.checkParams(modelWithoutUid)
+
+    val stringIndexerSingleCol = new StringIndexer()
+      .setInputCol("in").setOutputCol("out")
+    val inOutCols1 = stringIndexerSingleCol.getInOutCols
+    assert(inOutCols1._1 === Array("in"))
+    assert(inOutCols1._2 === Array("out"))
+
+    val stringIndexerMultiCol = new StringIndexer()
+      .setInputCols(Array("in1", "in2")).setOutputCols(Array("out1", "out2"))
+    val inOutCols2 = stringIndexerMultiCol.getInOutCols
+    assert(inOutCols2._1 === Array("in1", "in2"))
+    assert(inOutCols2._2 === Array("out1", "out2"))
+
+    intercept[IllegalArgumentException] {
+      new StringIndexer().setInputCol("in").setOutputCols(Array("out1", "out2")).getInOutCols
+    }
+    intercept[IllegalArgumentException] {
+      new StringIndexer().setInputCols(Array("in1", "in2")).setOutputCol("out1").getInOutCols
+    }
+    intercept[IllegalArgumentException] {
+      new StringIndexer().setInputCols(Array("in1", "in2"))
+        .setOutputCols(Array("out1", "out2", "out3")).getInOutCols
+    }
   }
 
   test("StringIndexer") {