diff --git a/src/Pure/General/timing.scala b/src/Pure/General/timing.scala
--- a/src/Pure/General/timing.scala
+++ b/src/Pure/General/timing.scala
@@ -1,76 +1,76 @@
 /*  Title:      Pure/General/timing.scala
     Author:     Makarius
 
 Support for time measurement.
 */
 
 package isabelle
 
 
 import java.util.Locale
 
 
 object Timing
 {
   val zero: Timing = Timing(Time.zero, Time.zero, Time.zero)
 
   def timeit[A](
     message: String = "",
     enabled: Boolean = true,
     output: String => Unit = Output.warning(_))(e: => A): A =
   {
     if (enabled) {
       val start = Time.now()
       val result = Exn.capture(e)
       val stop = Time.now()
 
       val timing = stop - start
       if (timing.is_relevant) {
         output(
           (if (message == null || message == "") "" else message + ": ") +
             timing.message + " elapsed time")
       }
 
       Exn.release(result)
     }
     else e
   }
 }
 
 sealed case class Timing(elapsed: Time, cpu: Time, gc: Time)
 {
   def is_zero: Boolean = elapsed.is_zero && cpu.is_zero && gc.is_zero
   def is_relevant: Boolean = elapsed.is_relevant || cpu.is_relevant || gc.is_relevant
 
   def resources: Time = cpu + gc
 
   def factor: Option[Double] =
   {
     val t1 = elapsed.seconds
     val t2 = resources.seconds
     if (t1 >= 3.0 && t2 >= 3.0) Some(t2 / t1) else None
   }
 
   def + (t: Timing): Timing = Timing(elapsed + t.elapsed, cpu + t.cpu, gc + t.gc)
 
   def message: String =
     elapsed.message + " elapsed time, " + cpu.message + " cpu time, " + gc.message + " GC time"
 
   def message_resources: String =
   {
     val factor_text =
       factor match {
-        case Some(f) => String.format(Locale.ROOT, ", factor %.2f", new java.lang.Double(f))
+        case Some(f) => String.format(Locale.ROOT, ", factor %.2f", java.lang.Double.valueOf(f))
         case None => ""
       }
     if (resources.seconds >= 3.0)
       elapsed.message_hms + " elapsed time, " + resources.message_hms + " cpu time" + factor_text
     else
       elapsed.message_hms + " elapsed time" + factor_text
   }
 
   override def toString: String = message
 
   def json: JSON.Object.T =
     JSON.Object("elapsed" -> elapsed.seconds, "cpu" -> cpu.seconds, "gc" -> gc.seconds)
 }