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.

package util;

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * Lambdas in one line
 */
public class LambdaTest {
 public static List<String> parse(Path path) throws Exception{
 return Files.lines(path)
 .parallel()
 .flatMap(line -> Arrays.asList(line.split("\\b")).stream())
 .collect(Collectors.groupingBy(w -> w, Collectors.counting()))
 .entrySet().stream()
 .sorted(Comparator.comparing(Map.Entry<String, Long>::getValue).reversed())
 .limit(20)
 .map(Map.Entry::getKey)
 .collect(Collectors.toList());
 }

 public static void main(String... args) throws Exception{
 System.out.println(parse(Paths.get(args[0])));
 }
}