http://zhidao.baidu.com/question/130229186.html
如果你不小心写成了
userId = null
报错的时候一般不会报这里，因为赋值又没有错。
但是要是写成了
null = userId
报错的时候立即定位到这
还有一种情况，比如a是一个String变量,要比较a是否等于"hello"就要这样写：
"hello".equals(a)，不要a.equals("hello")，后面一种容易空指针异常，前面没有
higher141125
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null = userId与userId = null
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- JAVA-EE
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