How to Read Java File Structure using Java?

File dir = new File("directoryName");
File temp;
String[] children = dir.list();
if (children !=null) {
    for (int i=0; i<children.length; i++) {
        temp = new File(children[i]);
        if(temp.isDirectory()) //add children[i] to arrayList
    }
}

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.Stack;

public class DirStuctureReader {

  private static final int INDENTATION = 4;

  public static void main(String[] args) throws Exception {
    Directory result = new DirStuctureReader().read(new File("testfile.txt"));
    System.out.println(result);
  }

  public Directory read(File file) throws IOException {
    Scanner scanner = new Scanner(file);
    Stack<Directory> directoryStack = new Stack<>();
    Directory root = new Directory("/"); // a root directory for everything
    directoryStack.add(root);
    while (scanner.hasNextLine()) {
      processLine(scanner.nextLine(), directoryStack);
    }

    return root;
  }

  private void processLine(String line, Stack<Directory> directoryStack) {
    int nLeadingSpaces = getNumberOfLeadingSpaces(line);
    if (nLeadingSpaces == -1) return;
    int depth = nLeadingSpaces / INDENTATION;
    while (directoryStack.size() > depth + 1) {
      directoryStack.pop(); // discard elements from the stack when we are deep and jump up one or more levels
    }
    String dirName = line.substring(nLeadingSpaces + 1);
    Directory directory = new Directory(dirName);
    directoryStack.peek().getChildren().add(directory); // add the directory to the children of the proper parent dir
    directoryStack.push(directory);
  }

  private int getNumberOfLeadingSpaces(String line) {
    for (int i = 0; i < line.length(); i++) {
      if (line.charAt(i) != ' ') return i;
    }

    return -1;
  }

  public static class Directory {
    private List<Directory> children = new ArrayList<>();

    private final String name;

    public Directory(String name) {
      this.name = name;
    }

    public String getName() {
      return name;
    }

    public List<Directory> getChildren() {
      return children;
    }
  }
}

PrintWriter p = new PrintWriter("reservations.txt");
p.write("your text goes here");

Passenger p1 = new Passenger(0001, "John Smith", "1A", "AA12");
Passenger p2 = new Passenger(0002, "Annah Smith", "1B", "AA12");
p.write(p1.toString());
p.write(p2.toString());

    Scanner keywords = new Scanner(new File("keywords.txt"));

    ArrayList<String> stack = new ArrayList<String>();
    ArrayList<String> csvLines = new ArrayList<String>();

    // stores the number of elements of the last line processed
    int lastSize = -1;

    while (keywords.hasNext()) {
        String line = keywords.nextLine();

        int tabs = 0;
        // Count tabs of current line
        while (line.length() > tabs // to avoid IndexOutOfBoundsException in charAt()
                && line.charAt(tabs) == '\t') {
            tabs++;
        }

        line = line.substring(tabs); // delete the starting tabs

        if (tabs <= lastSize) {
            // if the current line has the same number of elements than the previous line, 
            // then we can save the previous processed line as CSV 
            String csvLine = "";
            for (String element : stack) {
                if (csvLine.length() > 0) {
                    csvLine += ", ";
                }
                csvLine += element;
            }
            csvLines.add(csvLine);
        }

        // if the current line has less tabs than the previous, then cut the stack 
        for (int i = stack.size() - 1; i >= tabs; i--) {
            stack.remove(i);
        }

        // if the current line has more tabs than the previous, then add the new element to the stack
        if (tabs >= stack.size()) {
            stack.add(line);
        }

        // save the number of tabs of the current line
        lastSize = tabs;
    }
    keywords.close();

    // we have to save the last line processed
    if (lastSize >= 0) {
        // save line
        String csvLine = "";
        for (String element : stack) {
            if (csvLine.length() > 0) {
                csvLine += ", ";
            }
            csvLine += element;
        }
        csvLines.add(csvLine);
    }

    // print out CSV
    for (String string : csvLines) {
        System.out.println(string);
    }