Java Implementation of String Path Changer (cd <dir>)

StringPathChanger.java

package com.codility;

import java.util.EmptyStackException;
import java.util.Stack;

public class StringPathChanger {

    public static final String PARENT_DIR_NOTATION = "..";
    public static final String SINGLE_SPACE = " ";
    public static final String DIR_PARTITION_NOTATION = "/";
    public static final String CD_COMMAND = "cd ";

    public String changeDirectoryString(String currentDirectory, String command) {
        // currentDirectory : absolute current path
        // command: cd ../..

        if(currentDirectory == null || command == null) {
            throw new IllegalStateException();
        }

        String[] listSource = validateCurrentDirectory(currentDirectory);
        String[] listDest = validateCommand(command);

        boolean absoluteDestination = isAbsoluteDestination(command);


        if(absoluteDestination) {
            return getAbsoluteDestination(listDest);
        }

        return getRelativeDestination(listSource, listDest);
    }

    private String getRelativeDestination(String[] listSource, String[] listDest) {
        Stack<String> directoryStack = new Stack<>();
        for(String dir : listSource) {
            directoryStack.push(dir);
        }

        traverseRelativeDirectory(listDest, directoryStack);

        if(directoryStack.isEmpty()) {
            return DIR_PARTITION_NOTATION;
        }

        return DIR_PARTITION_NOTATION + String.join(DIR_PARTITION_NOTATION, directoryStack);
    }

    private String getAbsoluteDestination(String[] listDest) {
        Stack<String> directoryStack = new Stack<>();

        traverseRelativeDirectory(listDest, directoryStack);
        if(directoryStack.isEmpty()) {
            return DIR_PARTITION_NOTATION;
        }

        return DIR_PARTITION_NOTATION + String.join(DIR_PARTITION_NOTATION, directoryStack);
    }

    private void traverseRelativeDirectory(String[] listDest, Stack<String> directoryStack) {
        for (String dir : listDest) {
            // if '..' we pop the directory from stack
            if ((dir.equals(PARENT_DIR_NOTATION))) {
                // if no where to go, we stick to root
                try {
                    directoryStack.pop();
                } catch (EmptyStackException e) {
                }
            }
            // else we push it into stack
            else {
                directoryStack.push(dir);
            }
        }
    }

    private boolean isAbsoluteDestination(String command) {
        String[] commandPartitions = command.split(SINGLE_SPACE);
        return commandPartitions[1].startsWith(DIR_PARTITION_NOTATION);
    }

    private String[] validateCurrentDirectory(String currentDirectory) {
        if(currentDirectory.isEmpty()) {
            throw new IllegalStateException();
        }

        if(!currentDirectory.startsWith(DIR_PARTITION_NOTATION)) {
            throw new IllegalStateException();
        }

        // remove initial '/'
        return currentDirectory.substring(1).split(DIR_PARTITION_NOTATION);
    }

    private  String[] validateCommand(String command) {
        if(command.isEmpty()) {
            throw new IllegalStateException();
        }

        if(!command.startsWith(CD_COMMAND)) {
            throw new IllegalStateException();
        }

        String[] commandPartitions = command.split(SINGLE_SPACE);
        if(commandPartitions.length != 2) {
            throw new IllegalStateException();
        }

        if(commandPartitions[1].startsWith(DIR_PARTITION_NOTATION)) {
            commandPartitions[1] = commandPartitions[1].substring(1);
        }

        return commandPartitions[1].split(DIR_PARTITION_NOTATION);
    }
}

Here's my code:

package com.codility;

import java.util.*;

public class StringPathChanger {
public String changeDirectoryString(String dir, String command) {
System.out.print("dir: " + dir + " command:" + command);

    if(dir == null || command == null || !command.startsWith("cd ")) throw new IllegalStateException();

    command = command.substring(3).trim();
    
    if(command.startsWith("/")) return command;
    
    Deque<String> stack  = new ArrayDeque<>();

    for(String part: dir.split("/")){
        stack.push(part);
    }


    //System.out.print(Arrays.toString(command.split("/"))); 

    for(String part: command.split("/")){
        if(part.equals(".."))
            stack.pop();
        
        else if(!part.equals("."))
            stack.push(part);
    }


    StringBuilder res = new StringBuilder();
    List<String> list = new ArrayList<>();
    while(!stack.isEmpty()) {
        list.add(stack.pop());
    }
    Collections.reverse(list);


    for(String part: list){
        res.append(part);
        res.append("/");
    }

    res.setLength(res.length()-1);
    System.out.print(" ans: " + res.toString() + "\n");
    return res.toString();

}

}