Example of Scala injection of a singleton

injected-singleton.scala

// StopWatch class
abstract class StopWatch {

  def giveTime: Time

}

// Class is declared abstract as it is lacking a StopWatch which will be later injected.
// Note that we could have defined a default value here and thus made the class concrete.
abstract class FooBar {

   val watch: StopWatch

}

// This is a companion object, equivalent to static methods in Java,
// it is *not* a singleton and does not belong to the FooBar class hierarchy.
object FooBar {

  def doSomething ...

}

// This is a singleton -- note that it extends a type and thus is an instance
// of StopWatch.
object AtomicStopWatch extends StopWatch {

   override def giveTime = obtainTimeFromAtomicClockAndReturnIt

}

// This injects the dependency into an instance of FooBar
val myAtomicFooBar = new FooBar { override val watch = AtomicStopWatch }