import java.io.*;
import java.util.*;
class  ListAllFiles{
     static int  size = 20;         
    public static void main(String[] args) {
        System.out.print("Enter the name of a directory for listing:");
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) {
            String root = in.next();
            File file = new File(root);
            if (file != null && file.isDirectory()) {
                displayList(file, 0);
            }      
            }
    }
    public static void displayList(File f, int depth) {
        if(f.isFile()){
        System.out.println(f.getName()); 
        }
        if (f.isDirectory()  && depth <= size) {
            File[] dir = f.listFiles(); 
            for (File ff : dir) {
            displayList(ff, depth+1); 
            }
        }
    }

For more information, please visit the following links:

http://www.roseindia.net/java/example/java/io/TraversingFileAndDirectories.shtml

http://www.roseindia.net/tutorial/java/core/files/filelistfiles.html