use singleton objects for label parsers

mllib/src/main/scala/org/apache/spark/mllib/util/LabelParsers.scala

@@ -27,27 +27,23 @@ trait LabelParser extends Serializable {
  * Label parser for binary labels, which outputs 1.0 (positive) if the value is greater than 0.5,
  * or 0.0 (negative) otherwise. So it works with +1/-1 labeling and +1/0 labeling.
  */
-class BinaryLabelParser extends LabelParser {
+object BinaryLabelParser extends LabelParser {
+  /** Gets the default instance of BinaryLabelParser. */
+  def getInstance(): LabelParser = this
+
   /**
    * Parses the input label into positive (1.0) if the value is greater than 0.5,
    * or negative (0.0) otherwise.
    */
   override def parse(labelString: String): Double = if (labelString.toDouble > 0.5) 1.0 else 0.0
 }
 
-object BinaryLabelParser extends BinaryLabelParser {
-  /** Gets the default instance of BinaryLabelParser. */
-  def getInstance(): BinaryLabelParser = this
-}
-
 /**
  * Label parser for multiclass labels, which converts the input label to double.
  */
-class MulticlassLabelParser extends LabelParser {
-  override def parse(labelString: String): Double =  labelString.toDouble
-}
-
-object MulticlassLabelParser extends MulticlassLabelParser {
+object MulticlassLabelParser extends LabelParser {
   /** Gets the default instance of MulticlassLabelParser. */
-  def getInstance(): MulticlassLabelParser = this
+  def getInstance(): LabelParser = this
+
+  override def parse(labelString: String): Double =  labelString.toDouble
 }