ExternalStorage operation

ExternalStorage.java

private class ExternalStorage {

    public List<String> getDirPathList() {
        try {
            return getDirPathList(Environment.getExternalStorageDirectory().getCanonicalFile());
        } catch (IOException e) {
            return emptyList();
        }
    }

    public List<String>getDirPathList(File dir) {
        List<String> resultDirPathList = emptyList();
        if (dir != null) {
            List<String> dirPathList = getJustBelowDirPathList(dir);
            for (String path : dirPathList) {
                if (path != null) {
                    resultDirPathList.add(path);
                    resultDirPathList.addAll(getDirPathList(new File(path)));
                }
            }
        }
        return resultDirPathList;
    }

    public List<String>getJustBelowDirPathList(File dir) {
        List<String> dirPathList = new ArrayList<>();
        if (dir != null && dir.isDirectory() && !dir.isHidden() && dir.canRead()) {
            for (String nodeName: dir.list()) {
                if (nodeName != null) {
                    File node = new File(dir, nodeName);
                    if (isAvailableDir(node)) {
                        dirPathList.add(node.getPath());
                    }
                }
            }
        }
        return dirPathList;
    }

    private boolean isAvailableDir(File node) {
        return (node != null
                && node.isDirectory()
                && !node.isHidden()
                && node.canRead()
                && !hasNoMediaFile(node)
                && hasCanonicalPathName(node));
    }

    private boolean hasNoMediaFile(File node) {
        File file = new File(node, NO_MEDIA_FILE);
        return file.exists();
    }

    private boolean hasCanonicalPathName(File file) {
        try {
            return file.getPath().equals(file.getCanonicalPath());
        } catch (IOException e) {
            return false;
        }
    }

    private List<String> emptyList() {
        return new ArrayList<>();
    }
}