This week's book giveaway is in the Reactive Progamming forum. We're giving away four copies of Reactive Streams in Java: Concurrency with RxJava, Reactor, and Akka Streams and have Adam Davis on-line! See this thread for details.
I believe the compilation fails because it is unable to determine the order of the elements. It has no rules on how objects of type "Compare" are supposed to be ordered. So for this to compile, you'd need to implement "Comparable" and its mandatory method "compareTo". Happy reading!