文件過濾器

    /**
     * A filename filter to use to identify the cache filenames which have CACHE_FILENAME_PREFIX
     * prepended.
     */
    private static final FilenameFilter cacheFileFilter = new FilenameFilter() {
        @Override
        public boolean accept(File dir, String filename) {
            return filename.startsWith(CACHE_FILENAME_PREFIX);
        }
    };

獲取文件夾下 ，以特定字符串開頭的文件。