NullPointerException通过Java三元运算符的自动装箱行为

问题描述

由于三元运算符中的意外类型转换，我在前一天遇到了一个非常奇怪的 NullPointerException 。鉴于此（无用的示例）函数：

 整数getNumber（）{
返回null; 
} 
  

我希望编译后以下两个代码段完全相同：

 整数; 
 if（condition）{
 number = getNumber（）; 
} else {
 number = 0; 
} 
  

vs。

 整数=（条件）？ getNumber（）：0; 
  

。

原来，如果条件是 true ，如果 -statement工作正常，第二个代码段中的三元操作会抛出 NullPointerException 。似乎三元操作决定在将结果自动装回到 Integer int 。 $ C>！？！实际上，如果我明确地将 0 转换为 Integer ，则异常就会消失。换句话说：

 整数=（条件）？ getNumber（）：0; 
  

与以下内容不同：

 整数=（条件）？ getNumber（）:(整数）0; 
  

。

所以，似乎三元运算符和等价的 if-else -statement（我没想到的）之间存在字节码差异。这提出了三个问题：为什么会有差异？这是三元实现中的错误还是有类型转换的原因？鉴于存在差异，三元运算的性能是否等同于，如果 -statement（我知道，差异不是很大，但仍然存在）？ JLS ： -

I tripped across a really strange NullPointerException the other day caused by an unexpected type-cast in the ternary operator. Given this (useless exemplary) function:

Integer getNumber() {
    return null;
}

I was expecting the following two code segments to be exactly identical after compilation:

Integer number;
if (condition) {
    number = getNumber();
} else {
    number = 0;
}

vs.

Integer number = (condition) ? getNumber() : 0;

.

Turns out, if condition is true, the if-statement works fine, while the ternary opration in the second code segment throws a NullPointerException. It seems as though the ternary operation has decided to type-cast both choices to int before auto-boxing the result back into an Integer!?! In fact, if I explicitly cast the 0 to Integer, the exception goes away. In other words:

Integer number = (condition) ? getNumber() : 0;

is not the same as:

Integer number = (condition) ? getNumber() : (Integer) 0;

.

So, it seems that there is a byte-code difference between the ternary operator and an equivalent if-else-statement (something I didn't expect). Which raises three questions: Why is there a difference? Is this a bug in the ternary implementation or is there a reason for the type cast? Given there is a difference, is the ternary operation more or less performant than an equivalent if-statement (I know, the difference can't be huge, but still)?

解决方案

According to JLS: -

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