[jvm-packages] change class to object in examples (#1703)

* change class to object in examples

* fix compilation error

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/BasicWalkThrough.scala

@@ -17,7 +17,6 @@
 package ml.dmlc.xgboost4j.scala.example
 
 import java.io.File
-import java.util
 
 import scala.collection.mutable
 
@@ -25,9 +24,8 @@ import ml.dmlc.xgboost4j.java.{DMatrix => JDMatrix}
 import ml.dmlc.xgboost4j.java.example.util.DataLoader
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class BasicWalkThrough {
+object BasicWalkThrough {
   def main(args: Array[String]): Unit = {
-    import BasicWalkThrough._
     val trainMax = new DMatrix("../../demo/data/agaricus.txt.train")
     val testMax = new DMatrix("../../demo/data/agaricus.txt.test")
 
@@ -82,9 +80,7 @@ class BasicWalkThrough {
     val predicts3 = booster3.predict(testMax2)
     println(checkPredicts(predicts, predicts3))
   }
-}
 
-object BasicWalkThrough {
   def checkPredicts(fPredicts: Array[Array[Float]], sPredicts: Array[Array[Float]]): Boolean = {
     require(fPredicts.length == sPredicts.length, "the comparing predicts must be with the same " +
       "length")

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/BoostFromPrediction.scala

@@ -20,7 +20,7 @@ import scala.collection.mutable
 
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class BoostFromPrediction {
+object BoostFromPrediction {
   def main(args: Array[String]): Unit = {
     println("start running example to start from a initial prediction")
 

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/CrossValidation.scala

@@ -19,7 +19,7 @@ import scala.collection.mutable
 
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class CrossValidation {
+object CrossValidation {
   def main(args: Array[String]): Unit = {
     val trainMat: DMatrix = new DMatrix("../../demo/data/agaricus.txt.train")
 

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/CustomObjective.scala

@@ -32,7 +32,7 @@ import org.apache.commons.logging.{LogFactory, Log}
  * function
  *
  */
-class CustomObjective {
+object CustomObjective {
 
   /**
    * loglikelihoode loss obj function

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/ExternalMemory.scala

@@ -20,7 +20,7 @@ import scala.collection.mutable
 
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class ExternalMemory {
+object ExternalMemory {
   def main(args: Array[String]): Unit = {
     // this is the only difference, add a # followed by a cache prefix name
     // several cache file with the prefix will be generated

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/GeneralizedLinearModel.scala

@@ -25,7 +25,7 @@ import ml.dmlc.xgboost4j.scala.example.util.CustomEval
  * this is an example of fit generalized linear model in xgboost
  * basically, we are using linear model, instead of tree for our boosters
  */
-class GeneralizedLinearModel {
+object GeneralizedLinearModel {
   def main(args: Array[String]): Unit = {
     val trainMat = new DMatrix("../../demo/data/agaricus.txt.train")
     val testMat = new DMatrix("../../demo/data/agaricus.txt.test")

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/PredictFirstNTree.scala

@@ -20,7 +20,7 @@ import scala.collection.mutable
 import ml.dmlc.xgboost4j.scala.example.util.CustomEval
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class PredictFirstNTree {
+object PredictFirstNTree {
 
   def main(args: Array[String]): Unit = {
     val trainMat = new DMatrix("../../demo/data/agaricus.txt.train")

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/PredictLeafIndices.scala

@@ -22,7 +22,7 @@ import scala.collection.mutable
 
 import ml.dmlc.xgboost4j.scala.{XGBoost, DMatrix}
 
-class PredictLeafIndices {
+object PredictLeafIndices {
 
   def main(args: Array[String]): Unit = {
     val trainMat = new DMatrix("../../demo/data/agaricus.txt.train")

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

@@ -35,7 +35,7 @@ case class Store(storeId: Int, storeType: String, assortment: String, competitio
                  competitionOpenSinceMonth: Int, competitionOpenSinceYear: Int, promo2: Int,
                  promo2SinceWeek: Int, promo2SinceYear: Int, promoInterval: String)
 
-object Main {
+object SparkModelTuningTool {
 
   private def parseStoreFile(storeFilePath: String): List[Store] = {
     var isHeader = true