三元运算符，使用赋值时语法错误 [英] Ternary operator, syntax error when using assignment

问题描述

下面的下面3行代码编译OK。 （请注意，该代码是人工Java编码的一个示例，因此在专业编写的代码中不会出现。）

The following 3 lines of code below compile OK. (Please note that this code is an example of "artificial Java coding", and consequently wouldn't be seen in professionally written code.)

int x, y;
boolean b=true;

x = b ? y=1 : 2;  // Compiles OK.

如果我现在更改上面第3行中的代码，则看起来像下面的代码行

If I now change the code in line #3 above, so that it looks like the following code line below, the compiler generates an error.

// Change the position of the "y assignment", and now the code doesn't compile.
x = b ? 1 : y=2;  

以下是语法错误消息：

有人可以解释吗？这种行为（对于菜鸟Java学习者）？谢谢。

Can someone please explain this behaviour (to a rookie Java learner)? Thank you.

推荐答案

短：

这是由于运算符的优先级。第一种情况等于：

This is because of operator precedence. The first case is equal to this:

x = (b ? (y=1) : 2);  // Compiles OK.

而第二个是：

x = (b ? 1 : y) = 2;  

第一个编译的确不错，因为赋值被赋值为新值。因此，如果 b 为true，则将同时导致 x 和 y 等于1。第二个像说： x = 1 = 2 。因此，是的，要解决此编译器错误，请在您的语句中添加括号：

The first one compiles indeed fine, because an assignment gets evaluated to the new value. So, if b is true, it will cause to be both x and y equal to 1. The second one is like saying: x = 1 = 2. So, yes, to fix this compiler error, add paratheses to your statement:

x = (b ? 1 : (y = 2));  // Outer () are not needed, but is clearer, IMHO.

---

更长：

首先， Java中的运算符优先级说，赋值运算符的优先级低于条件三进制操作员。这意味着您的第二个表达式等效于：

First of all, operator precedence in Java says that the assignment operators have lower priority than the conditional ternary operator. This means that your second expression is equivalent to:

x = (b ? 1 : y) = 2;

您可以看到，这显然是错误的。实际上， JLS§15.26这样说：

As you can see, this looks plain wrong. Indeed, JLS §15.26 says this:

有12个赋值运算符；在语法上都是 right-associative （它们从右到左分组）。因此， a = b = c 表示 a =（b = c），该值分配了 c 到 b ，然后将 b 的值分配给 a 。

There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

赋值运算符的第一个操作数的结果必须为变量或编译时发生错误。 _{（这说明了您遇到的编译时错误）}

The result of the first operand of an assignment operator must be a variable, or a compile-time error occurs. _{(This explains the compile time error you face)}

在运行时，赋值表达式的结果为分配发生后的变量。赋值表达式的结果本身并不是变量。

At run time, the result of the assignment expression is the value of the variable after the assignment has occurred. The result of an assignment expression is not itself a variable.

应用右关联：

x = ((b ? 1 : y) = 2);

最后我们可以看到为什么会生成编译器错误：三元条件运算符的结果是不是变量（据我所知，它实际上不在JLS中，但是编译器会在一个简单的测试用例中告诉您： https://ideone.com/kMr7Xv ）。

Finally we can see why this generates a compiler error: the result of a ternary conditional operator is not a variable (which is actually not in the JLS as far as I can find, however the compiler tells you in a simple test case like this one: https://ideone.com/kMr7Xv).

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