dirnav.java

package com.company;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {

    static class FileNode {
        final String path;
        final FileNode parent;
        final Map<String, FileNode> subNodes = new HashMap<>();

        FileNode(String path, FileNode parent) {
            this.path = path;
            this.parent = parent;
        }
    }

    private static FileNode rootDir = new FileNode("/", null);
    private static FileNode currentDir = rootDir;

    private static void mkdir(String path) {
        String _path = path.trim();
        var dirs = splitDirs(_path);
        if (_path.startsWith("/")) {
            makeDirectoriesFrom(rootDir, dirs);
        } else {
            makeDirectoriesFrom(currentDir, dirs);
        }
    }

    private static void makeDirectoriesFrom(FileNode startDir, List<String> dirs) {
        var cursor = startDir;
        for (String dir : dirs) {
            if (!cursor.subNodes.containsKey(dir)) {
                cursor.subNodes.put(dir, new FileNode(cursor.path + dir + "/", cursor));
            }
            cursor = cursor.subNodes.get(dir);
        }
    }

    private static void navigateDirectoriesFrom(FileNode startDir, List<String> dirs) {
        currentDir = startDir;
        for (String dir : dirs) {
            if (!currentDir.subNodes.containsKey(dir)) {
                throw new RuntimeException("Invalid directory: " + dir);
            }
            currentDir = currentDir.subNodes.get(dir);
        }
    }

    private static void cd(String path) {
        String _path = path.trim();
        var dirs = splitDirs(_path);
        if ("..".equals(_path) && currentDir.parent != null) {
            currentDir = currentDir.parent;
        } else {
            if (_path.startsWith("/")) {
                navigateDirectoriesFrom(rootDir, dirs);
            } else {
                navigateDirectoriesFrom(currentDir, dirs);
            }
        }
    }

    private static List<String> splitDirs(String _path) {
        return Arrays.stream(_path.split("/")).filter(d -> d.length() > 0).collect(Collectors.toList());
    }

    private static String pwd() {
        return currentDir.path;
    }

    public static void main(String[] args) {
        mkdir("/opt/home");
        cd("/opt/home");
        System.out.println(pwd());
        cd("..");
        System.out.println(pwd());
        mkdir("app");
        cd("app");
        System.out.println(pwd());
        mkdir("java8");
        cd("java8");
        System.out.println(pwd());
        cd("..");
        cd("..");
        System.out.println(pwd());
    }
}