• Let see how to copy file named File1.txt from Dir3 directory in Dir2 to a folder named Dir3 wihin DirCopy within Dir2.Please note DirCopy still does not exist in Dir2. it will be created when we copy files.
• So relative path of file in both source and destination will be Dir3\file1.txt
• So we going to call relativised method to get relative path to the source root

So an example is as below

 sourceFolder = "FileTree\Dir2\Dir3\File1.txt";
 sourceRoot = "FileTree\Dir2";
 targetRoot = "FileTree\Dir4\DirCopy;
 relativizedPath = sourceRoot.relativize(sourcePath);  which will be "Dir3\File1.txt"
 relativizedPathForCopy = targetRoot.resolve(relativizedPath); which will be "FileTree\Dir4\Dir\DirCopy\Dir3\File1.txt

• So what be are doing above basically is we are figuring out relative path to source Root and appending it to target root because when doing a copy of relative path to both source and target, roots will be the same
• We have also seen we need both sourceRoot and targetRoot path to figure out path for the copy but preVisitDirectory method and visitFile method are only passed a reference to the directory or file being visited.So we have to save the source and destination roots as instance variables in CopyFile class that we have created.So we will add those fields in this class and constructor to handle this.

Complete code for a class implementing SimpleFileVisitor class is below. We will use this class to copy complete of a directory from one place to another

public class CopyFiles extends SimpleFileVisitor<Path> {
   private Path sourceRoot;
   private Path targetRoot;

   public CopyFiles(Path sourceRoot, Path targetRoot) {
       this.sourceRoot = sourceRoot;
       this.targetRoot = targetRoot;
   }

   @Override
   public FileVisitResult visitFileFailed(Path file, IOException exc) throws IOException {
       System.out.println("Error access file :"+file.toAbsolutePath()+ "  "+exc.getMessage());
       return FileVisitResult.CONTINUE;
   }

   @Override
   public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
       //To construct the path we use Path.relativize method .This method constructs a relative path which resolves to a given path
       //For example if path to relativize against is C:\path1 and given path is C:\path1\path2\path3
       // then the relativised path is \path2\path3 which is given path relative to C:\path1
       //Now once we got the relative path  we then need to resolve it against copied directories locations by calling
       //by calling path.resolve method and that will turn the relative path we go from relativised method into the full path for the copied file.


       //So over here we are getting relativized path for source directory and target directory
       Path relativizedPath = sourceRoot.relativize(dir);
       System.out.println("RelativisedPath = "+relativizedPath);
       Path copyDir = targetRoot.resolve(relativizedPath);
       System.out.println("RelativisedPath = "+copyDir);


       try{
           //We are then copying the files
           Files.copy(dir,copyDir);
       }
       catch(Exception e ){
           e.printStackTrace();
           //If an exception occurs  we will skip the subtree by using the below line of code i.e we will stop processing files for that subtree
           return FileVisitResult.SKIP_SUBTREE;
       }
       return FileVisitResult.CONTINUE;

   }

   //Here we override visitFile method
   //In this method just as in directory method we use relativised method to find paths for copied files and then we can use copy method to copy those 
 files.
   @Override
   public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {

       //So over here we are getting relativized path to find paths for copied files.
       Path relativizedPath = sourceRoot.relativize(file);
       System.out.println("RelativisedPath = "+relativizedPath);
       Path copyDir = targetRoot.resolve(relativizedPath);
       System.out.println("RelativisedPath = "+copyDir);


       try{
           //We are then copying the files
           Files.copy(file,copyDir);
       }
       catch(Exception e ){
           System.out.println(e.getMessage());
       }
       return FileVisitResult.CONTINUE;
   }
}

We can then simply copy the complete contents of a directory in main method of a class using the code below

 Path copyPath = FileSystems.getDefault().getPath("FileTree1"+File.separator+"Dir4"+File.separator+"Dir2Copy");
  //so path will be like FileTree1\\Dir4\\Dir2Copy
     try{
         Files.walkFileTree(dir2Path,new CopyFiles(dir2Path,copyPath));
      }
     catch(Exception e){
       System.out.println(e.getMessage());
    }