为什么我可以在三元操作中将原始类型设置为null

I always thought that primitive types in Java cannot be null, as it is a compile time error if i attempt to do something like this:

我一直认为Java中的原始类型不能为空，因为如果我尝试这样做的话，这是一个编译时错误:

int test = null;

However in a ternary operation, it seems to be allowed:

但在三元手术中，似乎可以:

int test = something != 0 ? 5 : null;

Isn't a ternary operation just short for (in this case):

(在本例中)不是三元操作(在本例中):

int test;
if (something != 0){
    test = 5;
} else {
    test = null
}

which of course should not be allowed. if that condition fails, It will automaticly throw a NullPointerException due to autoboxing. So why the java-compiler doesn't fetch nonsense like this?

当然，这是不允许的。如果这个条件失败，它将自动抛出一个NullPointerException由于自动装箱。那么为什么java编译器不像这样取无意义的东西呢?

1 个解决方案

#1

What happens is that the Java compiler first tries to make the types of the expressions on either side of the : equal. In this case, it autoboxes the 5 to an Integer; note that null is a valid value for Integer. The result of the whole ternary expression is Integer. You assign that to an int, so the Integer is then autounboxed.

发生的情况是，Java编译器首先尝试使:等号两边的表达式的类型相等。在这种情况下，它将5自动填入整数;注意，null是整型的有效值。整个三元表达式的结果是整数。你把它赋给一个整数，那么这个整数就是autounbox。

Essentially the compiler applies autoboxing and -unboxing so that the line is going to look like this:

本质上，编译器应用自动装箱和-反装箱，这样行就会是这样的:

int test = (something != 0 ? Integer.valueOf(5) : null).intValue();

Indeed, autounboxing null leads to a NullPointerException.

实际上，autounboxing null会导致NullPointerException。

So why the java-compiler doesn't fetch nonsense like this?

那么为什么java编译器不像这样取无意义的东西呢?

Because the designers of the Java language defined the language in such a way that it works like this and didn't decide that this has to be treated as an error...

因为Java语言的设计者定义语言的方式是这样的，它的工作方式是这样的，而不是认为必须将其视为一个错误……

Section 15.25 of the Java Language Specification explains how the type of the whole expression is determined.

Java语言规范的15.25节解释了如何确定整个表达式的类型。

#1