private void validateKey(String key) {
        if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {
            throw new IllegalArgumentException(
                    "keys must not contain spaces or newlines: \"" + key + "\"");
        }
    }
Closes the cache and deletes all of its stored values. This will delete
all files in the cache directory including files that weren't created by
the cache.
app/src/main/java/com/duan/musicoco/cache/DiskLruCache.java