javaprivate ArrayList<File> mFiles = new ArrayList<File>();

private void traverseFiles(String rootPath){
    File rootDir = new File(rootPath);
    File childFiles[] = rootDir.listFiles();
    if (childFiles != null){
        for(File childFile:childFiles){
            if (childFile.isDirectory()){
                traverseFiles(childFile.getPath());
            } else {
                mFiles.add(childFile);
            }
    }
}

public void test(){
    traverseFiles(Environment.getExternalStorageDirectory().getPath());
}