It's great that Java 8 finally added lambdas, streams, and method references to the language. Being Java, they managed to do it in as verbose a fashion as possible, but so be it. The thing is, Groovy works fine with Java 8 right now.
For example, I can write:
Wherever Java expects a lambda, I can supply a Groovy closure. In Java 8, there is no class called "Lambda", so instead they use functional interfaces like Predicate and Consumer and assign lambdas to them. I just used Groovy closures instead, but the rest worked as expected.
What isn't supported (yet) is the new syntax for lambda or method references. Groovy uses closures for lambdas, of course, and it has its own syntax for method references (an & symbol). Still, all the new features in Java 8 are automatically available in Groovy as well.
Incidentally, the corresponding Groovy approach would be:
If you run that at the Groovy console you get [6, 18].
Even with the new Java 8 additions, it still doesn't have a meta-object protocol, which is necessary for metaprogramming. Still, everything else they added (static and default methods in interfaces, the new java.time package, and so on), you can use in Groovy without a problem.