I tottaly don't get it

Now let's take care of another part of our archiver that is no less important.
Frequently, users want to create an archive from not just one file, but from an entire folder. In this case, zipping amounts to successively adding a ZipEntry for each file to the archive.
We need to write a FileManager class to get a list of all of the files in some folder. It must have a constructor that has a Path rootPath parameter that indicates the root (the folder containing the files we are interested in), and a List<Path> getFileList() method that should return a list of relative paths for all of the files in rootPath (including files in subfolders). The directory we will search can only be set once in the constructor. So, we can generate the list of files (relative paths) right in the constructor and save it in the class's List<Path> fileList variable.
We just have to decide how to gather up all of the files, even those that might be inside a folder, inside a folder, inside a folder, inside the folder we're looking at. Whoa, we almost entered a recursive loop. But maybe recursion can help us! Let's make a collectFileList(Path path) method that adds all the files inside the path parameter to the class's fileList variable, and calls itself recursively on subfolders.

1. Create a FileManager class with a FileManager(Path rootPath) throws IOException constructor
2. Declare and initialize the following private instance variables:
2.1. Path rootPath - The root path of the directory whose files we are interested in
2.2. List<Path> fileList - The list of relative paths of files inside rootPath
3. Create a getter for fileList
4. Implement a void collectFileList(Path path) throws IOException method that must:
4.1. Check to see if the path parameter is a regular file (use the Files.isRegularFile method), then get its path relative to rootPath and add it to fileList.
4.2. If the path parameter is a directory (determine this using the Files.isDirectory method), then go through all of the directory's contents and call collectFileList(Path path), passing in the discovered elements.
You can go through the all of the directory's contents by first getting a DirectoryStream using the Files class's newDirectoryStream method. Don't forget to close the DirectoryStream.
5. Call the collectFileList(rootPath) method inside the FileManager constructor.
6. Apply everything you've learned about encapsulation to this class.
In completing this task, you wrote an algorithm that traverses a file tree. But Java has a special FileVisitor interface for this. I highly recommend you learn how to use it.