java switch 语句中的最终变量 case
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final variable case in switch statement
提问by Furlando
final int a = 1;
final int b;
b = 2;
final int x = 0;
switch (x) {
case a:break; // ok
case b:break; // compiler error: Constant expression required
}
/* COMPILER RESULT:
constant expression required
case b:break;
^
1 error
*/
Why am I getting this sort of error? If I would have done final int b = 2, everything works.
为什么我会收到这种错误？如果我做了final int b = 2，一切正常。
采纳答案by Bozho
bmay not have been initialized and it is possible to be assigned multiple values. In your example it is obviously initialized, but probably the compiler doesn't get to know that (and it can't). Imagine:
b可能尚未初始化，并且可以分配多个值。在您的示例中，它显然已初始化，但编译器可能不知道（并且不能）。想象：
final int b;
if (something) {
b = 1;
} else {
b = 2;
}
The compiler needs a constant in the switch, but the value of bdepends on some external variable.
编译器在 中需要一个常量switch，但 的值b取决于一些外部变量。
回答by Rahul Bobhate
The case in the switch statements should be constants at compile time. The command
switch 语句中的 case 在编译时应该是常量。命令
final int b=2
assigns the value of 2to b, right at the compile time. But the following command assigns the value of 2to bat Runtime.
在编译时为2to赋值b。但是下面的命令在运行时分配2to的值。b
final int b;
b = 2;
Thus, the compiler complains, when it can't find a constant in one of the cases of the switchstatement.
因此，编译器会抱怨，当它在switch语句的其中一种情况下找不到常量时。
回答by rozar
The final variable without value assigned to it is called a blank variable. A blank final can only be assigned once and must be unassigned when an assignment occurs or once in the program.
没有赋值的最终变量称为空变量。空白期末只能分配一次，并且必须在分配发生时或在程序中取消分配一次。
In order to do this, a Java compiler runs a flow analysis to ensure that, for every assignment to a blank final variable, the variable is definitely unassigned before the assignment; otherwise a compile-time error occurs
为了做到这一点，Java 编译器运行了一个流程分析，以确保对于空的 final 变量的每次赋值，该变量在赋值之前肯定是未赋值的；否则会发生编译时错误
That is why when the compiler compiles the switch construct it is throwing constant expression required because the value of b is unknown to the compiler.
这就是为什么当编译器编译 switch 构造时它会抛出所需的常量表达式，因为编译器不知道 b 的值。
