Getting Sub-folders

import java.io.File;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 *
 * @author Aamir khan
 */
public class EbookShop {

    private static  List<File> FinalData;
    public List<File> allFilesAndFolders, SubFolders;
    private final File ROOT = new File("PATH TO THE ROOT FOLDER");


    public EbookShop() {
        EbookShop.FinalData = new ArrayList();
        this.allFilesAndFolders = new ArrayList();
        this.SubFolders = new ArrayList();
    }

    //Fire Up
    public void Fire() {

        //get All Files and Folders
        allFilesAndFolders.addAll(Arrays.asList(ROOT.listFiles()));
        //Extract Folders
        allFilesAndFolders.iterator().forEachRemaining(x -> {
            try {
                if (x.isDirectory()) {
                    SearchFiles(x);
                    if (hasSubFolders(x)) {
                        getSubFolders(x);
                    }

                }else{
                //Take the Files out
                CheckAndAdd(x);
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        });
        //Get Files From SubFolders
        SubFolders.iterator().forEachRemaining(y -> SearchFiles(y));

    }


    public final File[] listFolders(File path) {
        File[] listFiles = null;
        try {
            //list the Folders only those have Folders/Files inside
            listFiles = path.listFiles((File pathname)
                    -> {
                        return pathname.isDirectory()//make sure its Directory
                        && !(pathname.isHidden())//Do not include any hidden File/Folder
                        //Make Sure its not Empty
                        && pathname.getAbsoluteFile().listFiles(
                                (File pathname1) -> pathname1.isDirectory() || pathname1.isFile()).length != 0;
                    }
            );

        } catch (Exception e) {
            e.printStackTrace();
        }
        return listFiles;
    }

    public final void SearchFiles(File folder) {
        if (folder.isDirectory()) {
            File[] listFiles = folder.listFiles(path -> CheckForExtension(path));
            for (File temp : listFiles) {
                CheckAndAdd(temp);
            }
        }

    }

    private boolean CheckAndAdd(File folder) {
        boolean flag = false;
        if (CheckForExtension(folder)) {
            flag = FinalData.add(folder);

        }

        return flag;
    }

    private boolean CheckForExtension(File file) {
        return (file.getName().toLowerCase().endsWith(".pdf"));
//                || file.getName().toLowerCase().endsWith(".7z")
//                || file.getName().toLowerCase().endsWith(".zip")
//                || file.getName().toLowerCase().endsWith(".rar"));
    }

    private boolean hasSubFolders(File x) {
        File[] listFolders = listFolders(x);
        return (listFolders.length != 0);
    }

    private void getSubFolders(File x) {
        File[] listFolders = listFolders(x);
        SubFolders.addAll(Arrays.asList(listFolders));
    }
    public static void main(String[] args)throws java.io.IOException {
        new EbookShop().Fire();
        //get the Final Result
        System.out.println("Total Files Found "+FinalData.size());
        //Length Base Sort
        Collections.sort(FinalData, ( a,  b) -> Integer.compare(a.getAbsolutePath().length(), b.getAbsolutePath().length()));
        //print Files
        FinalData.iterator().forEachRemaining(System.out::println);
    }

}