In-memory file system

interface Node {
  String name();
  boolean isDirectory();
}

abstract class AbstractNode implements Node {
  private final String name;
  protected AbstractNode(String name) { this.name = Objects.requireNonNull(name); }
  public final String name() { return name; }
}

class FileNode extends AbstractNode {
  private byte[] content = new byte[0];
  FileNode(String name) { super(name); }
  public boolean isDirectory() { return false; }
  void write(byte[] bytes) { content = bytes.clone(); }
  byte[] read() { return content.clone(); }
}

class DirectoryNode extends AbstractNode {
  private final Map<String, Node> children = new LinkedHashMap<>();
  DirectoryNode(String name) { super(name); }
  public boolean isDirectory() { return true; }
  void add(Node child) { children.put(child.name(), child); }
  Optional<Node> get(String name) { return Optional.ofNullable(children.get(name)); }
  Collection<Node> children() { return Collections.unmodifiableCollection(children.values()); }
}

class Path {
  static List<String> parts(String raw) { /* split, reject empty, reject .. if out of scope */ }
}

class FileSystem {
  private final DirectoryNode root = new DirectoryNode("");
  void mkdir(String path) { /* walk, create dirs */ }
  void write(String path, byte[] content) { /* parent must be dir */ }
  byte[] read(String path);
  List<String> ls(String path);
}

@Test
void mkdirWriteLs() {
  FileSystem fs = new FileSystem();
  fs.mkdir("/a/b");
  fs.write("/a/b/c.txt", "hello".getBytes());
  assertEquals(List.of("b"), fs.ls("/a"));
  assertEquals("hello", new String(fs.read("/a/b/c.txt")));
}