Minor][MLLIB] Format toString method in MLLIB

Change to string interpolation way

Author: Alain

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -526,7 +526,7 @@ class SparseVector(
     s" ${values.size} values.")
 
   override def toString: String =
-    "(%s,%s,%s)".format(size, indices.mkString("[", ",", "]"), values.mkString("[", ",", "]"))
+    s"($size,${indices.mkString("[", ",", "]")},${values.mkString("[", ",", "]")})"
 
   override def toArray: Array[Double] = {
     val data = new Array[Double](size)

mllib/src/main/scala/org/apache/spark/mllib/regression/LabeledPoint.scala

@@ -32,7 +32,7 @@ import org.apache.spark.SparkException
 @BeanInfo
 case class LabeledPoint(label: Double, features: Vector) {
   override def toString: String = {
-    "(%s,%s)".format(label, features)
+    s"($label,$features)"
   }
 }
 

mllib/src/main/scala/org/apache/spark/mllib/tree/model/InformationGainStats.scala

@@ -39,8 +39,7 @@ class InformationGainStats(
     val rightPredict: Predict) extends Serializable {
 
   override def toString: String = {
-    "gain = %f, impurity = %f, left impurity = %f, right impurity = %f"
-      .format(gain, impurity, leftImpurity, rightImpurity)
+    s"gain = $gain, impurity = $impurity, left impurity = $leftImpurity, right impurity = $rightImpurity"
   }
 
   override def equals(o: Any): Boolean = o match {

mllib/src/main/scala/org/apache/spark/mllib/tree/model/Node.scala

@@ -51,8 +51,8 @@ class Node (
     var stats: Option[InformationGainStats]) extends Serializable with Logging {
 
   override def toString: String = {
-    "id = " + id + ", isLeaf = " + isLeaf + ", predict = " + predict + ", " +
-      "impurity = " + impurity + ", split = " + split + ", stats = " + stats
+    s"id = $id, isLeaf = $isLeaf, predict = $predict, impurity = $impurity, " +
+      s"split = $split, stats = $stats"
   }
 
   /**

mllib/src/main/scala/org/apache/spark/mllib/tree/model/Split.scala

@@ -39,8 +39,8 @@ case class Split(
     categories: List[Double]) {
 
   override def toString: String = {
-    "Feature = " + feature + ", threshold = " + threshold + ", featureType = " + featureType +
-      ", categories = " + categories
+    s"Feature = $feature, threshold = $threshold, featureType = $featureType, " +
+      s"categories = $categories"
   }
 }