public interface Folder<T> extends Node<Folder<T>>, Name {
    List<T> files();
    void travel(Consumer<? super Folder<T>> folder);
}

List<T> files() - return list of files folder.

void travel(Consumer<? super Folder<T>> folder) - traversal of the file tree, starting at the root. 

FolderPure<T> - main implementation.

FolderText - implementation to create a folder from a raw list of files (raw full filename list).

FolderFile - implementation to create a folder with java.io.File files from a raw list of files 
from unix-like system (raw full filename list).

FolderFileWin - implementation to create a folder with java.io.File files from a raw list of files 
from Windows (raw full filename list).

Folder contains files and children. Children is another Folder, which can be obtained with children() method from Node interface, or can be reached with travel() method.

Files in Folder interface is generic type. For example, implementation FolderText has type String for files, i.e. files in FolderText is a short filename.

Folder extends interface Node<T> and Name.