FileBrowser.java

import org.apache.commons.io.filefilter.DirectoryFileFilter;
import org.apache.commons.io.filefilter.WildcardFileFilter;

import java.io.File;
import java.io.FileFilter;
import java.io.FilenameFilter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class FileBrowser {

    public static void main(String[] args) {
        List<String> folderList = findFoldersInDirectory("/home/prokash/Downloads");
        System.out.println(folderList.size());

        List<String> directoryList = findFilesInDirectory("/home/prokash/Downloads/Wallpapers");
        System.out.println(directoryList.size());

        List<File> fileList = findAllFoldersAndFilesInDirectory("/home/prokash/Downloads");
        System.out.println(fileList.size());
    }

    private static List<File> findAllFoldersAndFilesInDirectory(String directoryPath) {

        File topDirectory = new File(directoryPath);

        List<File> foldersInDirectory = new ArrayList<>();
        foldersInDirectory.add(topDirectory);

        List<File> filesInDirectory = new ArrayList<>();

        List<String> filterWildcards = new ArrayList<>();
        filterWildcards.add("*.png");
        filterWildcards.add("*.jpeg");

        FileFilter typeFilter = new WildcardFileFilter(filterWildcards);

        while (!foldersInDirectory.isEmpty()) {
            List<File> subDirectories = new ArrayList<>();

            for (File f : foldersInDirectory) {
                subDirectories.addAll(Arrays.asList(f.listFiles((FileFilter) DirectoryFileFilter.INSTANCE)));
                filesInDirectory.addAll(Arrays.asList(f.listFiles(typeFilter)));
            }

            foldersInDirectory.clear();
            foldersInDirectory.addAll(subDirectories);
        }

        return filesInDirectory;
    }

    private static List<String> findFoldersInDirectory(String directoryPath) {
        File directory = new File(directoryPath);

        FileFilter directoryFileFilter = File::isDirectory;

        File[] directoryListAsFile = directory.listFiles(directoryFileFilter);
        List<String> foldersInDirectory = new ArrayList<>();
        if (directoryListAsFile != null) {
            for (File directoryAsFile : directoryListAsFile) {
                foldersInDirectory.add(directoryAsFile.getName());
            }
        }

        return foldersInDirectory;
    }

    private static List<String> findFilesInDirectory(String directoryPath) {
        File directory = new File(directoryPath);

        FilenameFilter fileNameFilter = new FilenameFilter() {
            @Override
            public boolean accept(File file, String name) {
                if (name.toLowerCase().endsWith(".png")) {
                    return true;
                } else if (name.toLowerCase().endsWith(".jpeg")) {
                    return true;
                } else if (name.toLowerCase().endsWith(".pdf")) {
                    return true;
                }
                return false;
            }
        };

        File[] directoryListAsFile = directory.listFiles(fileNameFilter);
        List<String> filesInDirectory = new ArrayList<>();
        if (directoryListAsFile != null) {
            for (File directoryAsFile : directoryListAsFile) {
                filesInDirectory.add(directoryAsFile.getName());
            }
        }

        return filesInDirectory;
    }

}