Create directories within directories (as many layers as you want).

Create text files in some of these directories while leaving some directories empty for more excitement.

We are going to create a static method that takes a File argument, which is eventually the root directory or the directory to start with. The method will return a set of files that are found within this root directory (and in all other subsequent directories):

        public static Set<File> listFiles(File rootDir) {  

First, create a HashSet that will contain the file information:

        Set<File> fileSet = new HashSet<File>(); 

Once the HashSet is created, we need to check whether the root directory or the directories within it are null. For such cases, we do not need to proceed further:

        if (rootDir == null || rootDir.listFiles() == null){ 
                     return fileSet; 
           } 

We consider one directory (or file) from the root directory at a time and check whether we are dealing with a file or with a directory. In the case of a file, we add that to our HashSet. In the case of a directory, we recursively call this method again by sending the path and name of the directory:

        for (File fileOrDir : rootDir.listFiles()) { 
                     if (fileOrDir.isFile()){ 
                       fileSet.add(fileOrDir); 
                     } 
                     else{ 
                       fileSet.addAll(listFiles(fileOrDir)); 
                     } 
             } 

Finally, we return the HashSet to the caller of this method:

       return fileSet; 
          } 

The complete method, with the class and the driver method to run it, is as follows:

import java.io.File; 
import java.util.HashSet; 
import java.util.Set; 
 
public class TestRecursiveDirectoryTraversal { 
   public static void main(String[] args){ 
      System.out.println(listFiles(new File("Path for root 
          directory")).size()); 
   } 
    
   public static Set<File> listFiles(File rootDir) { 
       Set<File> fileSet = new HashSet<File>(); 
       if(rootDir == null || rootDir.listFiles()==null){ 
           return fileSet; 
       } 
       for (File fileOrDir : rootDir.listFiles()) { 
             if (fileOrDir.isFile()){ 
               fileSet.add(fileOrDir); 
             } 
             else{ 
               fileSet.addAll(listFiles(fileOrDir)); 
             } 
     } 
 
       return fileSet; 
   } 
}