补: autoboxing

网上搜NPE，发现了别人跟我碰到同样的问题.这些人还建议不要用autoboxing. 有句倒说的很到位：The root cause of the NPE-related weirdness is that Java doesn't distinguish between pointers and "pointers that can be null". 这也应该是autoboxing问题的根源。

P.S. NPE=NullPointerException(看来这个异常很常见呢)