How to recursively traverse and store a dir structure?

Question

How would I go around recursively traversing and storing a directory structure? I'm writing an app to load up two folders, traverse their structure, show the differences in a GUI, and synchronize the two if prompted to do so.

Background: I get the basic principle(use a tree; if a file, append it to the calling dir, if a dir, append and recursively call the function on it), but all the implementations I found are just a little bit off from what I need, or they spit out a NullPointerException which I can't fix, as I don't understand what exactly they do. I think I'm stuck on a technicality or a quirk somewhere in the recursive calls or in creating child nodes.

Edit: This is a node class i copied from somewhere. I recently deleted my old putIntoTree function, and I'm currently working on a new one. I'm stuck on how exactly the children/parent creation works.

import java.nio.file.Path;
import java.util.*;

abstract class TreeNode implements Iterable<TreeNode> {

    private Set<TreeNode> children;
    public Path path;

    public TreeNode(Path path) {
        children = new HashSet<TreeNode>();
        this.path = path;
    }

    public boolean addChild(TreeNode n) {
        return children.add(n);
    }

    public boolean removeChild(TreeNode n) {
        return children.remove(n);
    }

    public Iterator<TreeNode> iterator() {
        return children.iterator();
    }
}

Edit: I'm trying to get this tree structure to work in there, but I'm a bit baffled at how exactly pointy brackets and generics work. It says "cannot resolve symbol[whatever is after the in my own attempt at defining a method]"

public class Tree<T> {
    private Node<T> root;

    public Tree(T rootData) {
        root = new Node<T>();
        root.data = rootData;
        root.children = new ArrayList<Node<T>>();
    }

    public static class Node<T> {
        private T data;
        private Node<T> parent;
        private List<Node<T>> children;
    }
}