为什么最终变量不总是常量表达式? [英] Why isn't a final variable always a constant expression?

问题描述

在下面的代码中:

final int a;
a=2;
byte b=a;   // error: possible loss of precision

为什么会出现此错误? a最终变量不是编译时常量表达式，因此在赋值期间隐式地缩小为字节吗?

Why do I get this error? Isn't a final variable compile time constant expression and hence implicitly narrowed to byte during the assignment?

换句话说，上面的代码不等同于:

In other words isn't the above code equivalent to:

final int a=2;
byte b=a;

推荐答案

编译器不是那么聪明.

我们可以知道该值始终为2.但是如果我们有类似的东西怎么办?

We can tell that the value will always be 2. But what if we had something like this?

class ABC{
    final int a;

    public ABC(){
       if(Math.random() < .5){
          a = 2;
       }
       else{
          a = 12345;
       }

       byte b = a;
    }
}

编译器不够聪明，无法区分这两种情况，因此它会给您一个错误.

The compiler is not smart enough to tell these two cases apart, so it gives you an error instead.

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