Remove unneeded no-arg constructor, and add additional note about cop…

…ied code in LICENSE

LICENSE

@@ -646,7 +646,8 @@ THE SOFTWARE.
 
 ========================================================================
 For Scala Interpreter classes (all .scala files in repl/src/main/scala
-except for Main.Scala, SparkHelper.scala and ExecutorClassLoader.scala):
+except for Main.Scala, SparkHelper.scala and ExecutorClassLoader.scala),
+and for SerializableMapWrapper in JavaUtils.scala:
 ========================================================================
 
 Copyright (c) 2002-2013 EPFL

core/src/main/scala/org/apache/spark/api/java/JavaUtils.scala

@@ -34,13 +34,12 @@ private[spark] object JavaUtils {
     new SerializableMapWrapper(underlying)
 
   // Implementation is copied from scala.collection.convert.Wrappers.MapWrapper,
-  // but implements java.io.Serializable and adds a no-arg constructor
+  // but implements java.io.Serializable. It can't just be subclassed to make it
+  // Serializable since the MapWrapper class has no no-arg constructor. This class
+  // doesn't need a no-arg constructor though.
   class SerializableMapWrapper[A, B](underlying: collection.Map[A, B])
     extends ju.AbstractMap[A, B] with java.io.Serializable { self =>
 
-    // Add no-arg constructor just for serialization
-    def this() = this(null)
-
     override def size = underlying.size
 
     override def get(key: AnyRef): B = try {