https://hungwenlin.com/blog/2014/%e4%b8%ad%e5%9c%8b%e5%a4%a7%e9%99%b8%e4%b8%80%e4%ba%8c-oo-%e5%84%84%e5%85%83%e4%ba%ba%e6%b0%91%e5%b9%a3%e8%b3%87%e9%87%91%e4%b8%8a%e8%86%9b-%e5%8f%b0%e7%81%a3%e5%8d%8a%e5%b0%8e%e9%ab%94%e6%a5%ad/