[jvm-packages] Objectives starting with rank: are never classification (

dmlc#2837)

Training a model with the experimental rank:ndcg objective incorrectly
returns a Classification model. Adjust the classification check to
not recognize rank:* objectives as classification.

While writing tests for isClassificationTask also turned up that
obj_type -> regression was incorrectly identified as a classification
task so the function was slightly adjusted to pass the new tests.

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala

@@ -194,8 +194,8 @@ object XGBoost extends Serializable {
     val objective = params.getOrElse("objective", params.getOrElse("obj_type", null))
     objective != null && {
       val objStr = objective.toString
-      objStr == "classification" || (!objStr.startsWith("reg:") && objStr != "count:poisson" &&
-        objStr != "rank:pairwise")
+      objStr != "regression" && !objStr.startsWith("reg:") && objStr != "count:poisson" &&
+        !objStr.startsWith("rank:")
     }
   }
 

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/XGBoostGeneralSuite.scala

@@ -389,4 +389,28 @@ class XGBoostGeneralSuite extends FunSuite with PerTest {
     assert(eval.eval(xgBoostModel.booster.predict(testSetDMatrix, outPutMargin = true),
       testSetDMatrix) < 0.1)
   }
+
+  test("isClassificationTask correctly classifies supported objectives") {
+    import org.scalatest.prop.TableDrivenPropertyChecks._
+
+    val objectives = Table(
+      ("isClassificationTask", "params"),
+      (true, Map("obj_type" -> "classification")),
+      (false, Map("obj_type" -> "regression")),
+      (false, Map("objective" -> "rank:ndcg")),
+      (false, Map("objective" -> "rank:pairwise")),
+      (false, Map("objective" -> "rank:map")),
+      (false, Map("objective" -> "count:poisson")),
+      (true, Map("objective" -> "binary:logistic")),
+      (true, Map("objective" -> "binary:logitraw")),
+      (true, Map("objective" -> "multi:softmax")),
+      (true, Map("objective" -> "multi:softprob")),
+      (false, Map("objective" -> "reg:linear")),
+      (false, Map("objective" -> "reg:logistic")),
+      (false, Map("objective" -> "reg:gamma")),
+      (false, Map("objective" -> "reg:tweedie")))
+    forAll (objectives) { (isClassificationTask: Boolean, params: Map[String, String]) =>
+      assert(XGBoost.isClassificationTask(params) == isClassificationTask)
+    }
+  }
 }