問題：
What's the difference between these two methods: Optional.flatMap() and Optional.map() ?這兩種方法有什麼區別： Optional.flatMap()和Optional.map() ？
An example would be appreciated.一個例子將不勝感激。
解決方案：
參考一： https://en.stackoom.com/question/25VHr參考二： https://stackoom.com/question/25VHr