If you understand this line, or better still can write this code you can pretty much say that you have understood the essence of Java 8 Lambdas. Certainly in as much as they can be used with collections.
I found this in a recent presentation by Peter Lawrey. (Definitely worth watching the whole presentation when you have a spare hour.)
Anyway the task was to find the 20 most frequent words in a file:
As you can see, with Java 8 this can actually be done in a single (albeit rather long) line.
If you're not used to lambdas the code might seem a little scary but actually it's pretty declarative and when you get past the logic reads fairly easily.