After examining Java 8 lambdas, I came to a conclusion that they tip the scales in favor of Java; the reasons for picking one of the alternative JVM languages are not that obvious to me anymore. Not to mention the hacks required to maintain binary compatibility, Java 8 Stream API is more well-thought-out than I expected.
How to make the code more concise and readable? How do the closures work? How are the lambda types being described in method signatures? What had to be sacrificed in order to maintain binary compatibility? How the collection operations can be parallelized? Is it always beneficial?
If you want to get to know the answers, take a look at the slides: http://lukaszwrobel.pl/blog/java-8-lambdas-and-collection-handling