在许多空指针异常中，真正的错误出现在变量被赋为空值的地方。
In many null-pointer exceptions, the true bug occurs where the variable is actually assigned to null.
由于空指针能够被改变，但通常它意味着你不能初始化一个对象或者你不能检查一个函数中的返回值。
The cause of null pointers can be varied, but generally it means that either you haven't initialized an object, or you haven't checked the return value of a function.
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