Starting with JDK 8, the Files class has been enriched with two walk() methods. These methods return a Stream that is lazily populated with Path. It does this by walking the file tree that's rooted at a given starting file using the given maximum depth and options:

public static Stream<Path> walk​(
  Path start, FileVisitOption...options) 
    throws IOException

public static Stream<Path> walk​(
  Path start, int maxDepth, FileVisitOption...options) 
    throws IOException

For example, let's display all the paths from D:/learning that start with D:/learning/books/cdi:

Path directory = Paths.get("D:/learning");

Stream<Path> streamOfPath = Files.walk(
  directory, FileVisitOption.FOLLOW_LINKS);

streamOfPath.filter(e -> e.startsWith("D:/learning/books/cdi"))
  .forEach(System.out::println);

Now, let's compute the size in bytes for a folder (for example, D:/learning):

long folderSize = Files.walk(directory)
  .filter(f -> f.toFile().isFile())
  .mapToLong(f -> f.toFile().length())
  .sum();