我的疑問為何不是2，答案這樣解釋：
3) Compile time error
The wrapper classes cannot be used like primitives.
Depending on your compiler you will get an error that says someting like “Error: Can’t convert java lang Integer”. Wrapper classes have similar names to primitives but all start with upper case letters.
Thus in this case we have int as a primitive and Integer as a wrapper. The objectives do not specifically mention the wrapper classes but don’t be surprised if they come up.
但是我編譯及執行的結果是2沒錯，我是直覺它可以autobox，所以不會錯，問題真是讓人費解！