Creates a list with files from a selected path. These data will be used on a GridView. The adapter and the FileItem can change according the information that must be shown on the GridView

FillFilesGridView.java

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.support.v7.widget.RecyclerView;
import android.view.View;

import com.vspallas.npflocker.R;
import com.vspallas.npflocker.general.FileType;
import com.vspallas.npflocker.listeners.RecyclerItemClickListener;
import com.vspallas.npflocker.models.FileItem;

import java.io.File;
import java.io.FilenameFilter;
import java.util.ArrayList;
import java.util.Collections;

/**
 * Created by vassilis on 5/19/16.
 */
public class FillFilesGridView implements RecyclerItemClickListener.OnItemClickListener {

    Context context;
    private ArrayList<FileItem> imageItems;
    // Check if the first level of the directory structure is the one showing
    private Boolean isTopParent = true;
    private File rootPath;
    private File selectedFile;
    // Stores names of traversed directories
    private ArrayList<String> dirNames = new ArrayList<>();
    private Bitmap bitmap;
    private RecyclerView recyclerView;
    private final String root;
    private SomeAdapter adapter;
    
    public FillFilesGridView(Context context, RecyclerView recyclerView) {
        this.context = context;
        this.recyclerView = recyclerView;
        root = Actions.getInternalPath(context);
        rootPath = new File(root);
    }

    public void loadFiles() {
        rootPath.mkdirs();

        if (rootPath.exists()) {
            FilenameFilter filter = new FilenameFilter() {
                @Override
                public boolean accept(File dir, String filename) {
                    File sel = new File(dir, filename);
                    // Filters based on whether the file is hidden or not
                    return (sel.isFile() || sel.isDirectory());

                }
            };
            String[] fileList = rootPath.list(filter);
            imageItems = new ArrayList<>();

            for (int i = 0; i < fileList.length; i++) {
                File selected = new File(rootPath, fileList[i]);
                if (selected.isDirectory()) {
                    bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_folder_white);
                    imageItems.add(new FileItem(bitmap, fileList[i], selected.length()));
                } else {
                    extensionImage(fileList[i], selected);

                    if (!isTopParent) {
                        ArrayList temp = new ArrayList();
                        bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_up);
                        temp.add(0, new FileItem(bitmap, "..", selected.length()));
                        for (int ii = 0; ii < imageItems.size(); ii++) {
                            temp.add(ii + 1, imageItems.get(ii));
                        }

                        imageItems = temp;
                    }
                }
            }

            Collections.sort(imageItems);
            adapter = new SomeAdapter(imageItems);
            recyclerView.setAdapter(adapter);

            fillList();
        } else {
            // inform user that the path does not exists
        }
    }

    private void fillList() {
        if (imageItems == null || imageItems.size() == 0) {
            return;
        }
        recyclerView.addOnItemTouchListener(new RecyclerItemClickListener(context, recyclerView, this));
    }

    private void backButtonPressed() {
        if (!isTopParent) {
            // present directory removed from list
            String s = dirNames.remove(dirNames.size() - 1);

            // rootPath modified to exclude present directory
            rootPath = new File(rootPath.toString().substring(0,
                    rootPath.toString().lastIndexOf(s)));
            imageItems = null;

            if (dirNames.isEmpty() || rootPath.equals(root)) {
                isTopParent = true;
            }

            loadFiles();
        }
    }

    @Override
    public void onItemClick(View view, int position) {
        String chosenFile = imageItems.get(position).getTitle();
        selectedFile = new File(rootPath + "/" + chosenFile);

        if (selectedFile.isDirectory() && !chosenFile.equalsIgnoreCase("..")) {

            isTopParent = false;
            // Adds chosen directory to list
            dirNames.add(chosenFile);
            imageItems = null;
            rootPath = new File(selectedFile + "");
            loadFiles();
        } // Checks if '..' was clicked
        else if (chosenFile.equalsIgnoreCase("..")) {
            backButtonPressed();
        } else {
            // open file
        }
    }

    @Override
    public void onItemLongClick(View view, int position) {

    }

    private void extensionImage(String file, File selected) {
        FileType type = Actions.getAttachmentType(file);
        switch (type) {
            case TXT:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_white);
                break;
            case PDF:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_pdf);
                break;
            case WORD:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_word);
                break;
            case EXCEL:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_excel);
                break;
            case POWERPOINT:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_powerpoint);
                break;
            case AUDIO:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_audio);
                break;
            case VIDEO:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_video);
                break;
            case ZIP:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_zip);
                break;
            default:
                bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_file_white);
                break;
        }

        imageItems.add(new FileItem(bitmap, file, selected.length()));
    }
}

listener for RecycleView is here. Images can be found here and here.

public static AttachmentType getAttachmentType(String file) {
            String extension = file.substring(file.lastIndexOf('.') + 1).toLowerCase();

            switch (extension.toLowerCase()) {
                case "txt":
                case "srt":
                case "rtf":
                case "nfo":
                case "html":
                case "css":
                case "xml":
                case "json":
                    return AttachmentType.TXT;
                case "pdf":
                    return AttachmentType.PDF;
                case "doc":
                case "dot":
                case "docx":
                case "docm":
                case "dotx":
                case "dotm":
                case "docb":
                    return AttachmentType.WORD;
                case "xls":
                case "xlt":
                case "xlm":
                case "xlsx":
                case "xlsm":
                case "xltx":
                case "xltm":
                case "xlsb":
                case "xla":
                case "xlam":
                case "xll":
                case "xlw":
                    return AttachmentType.EXCEL;
                case "ppt":
                case "pot":
                case "pps":
                case "pptx":
                case "pptm":
                case "potx":
                case "potm":
                case "ppam":
                case "ppsx":
                case "ppsm":
                case "sldx":
                case "sldm":
                    return AttachmentType.POWERPOINT;
                case "wma":
                case "wav":
                case "mp3":
                case "mid":
                case "m4a":
                    return AttachmentType.AUDIO;
                case "gif":
                case "jpeg":
                case "jpg":
                case "jif":
                case "jfif":
                case "jp2":
                case "jpx":
                case "j2k":
                case "j2c":
                case "fpx":
                case "png":
                case "dng":
                    return AttachmentType.IMAGE;
                case "mkv":
                case "flv":
                case "ogv":
                case "ogg":
                case "avi":
                case "mov":
                case "wmv":
                case "mp4":
                case "m4p":
                case "m4v":
                case "mpg":
                case "mp2":
                case "mpeg":
                case "raw":
                    return AttachmentType.VIDEO;
                case "rar":
                case "zip":
                case "tar":
                    return AttachmentType.ZIP;
                default:
                    return AttachmentType.UNKwOWN;
            }
}