在java中使用while循环无法访问语句错误 [英] Unreachable statement error using while loop in java

问题描述

可能的重复:
为什么这段代码会给出Unreachable Statement"错误?

这似乎很简单的问题，我在一本书中找到了这个问题.如果有人帮我弄清楚为什么我会出错.

This seems very easy question, I found this question in one book. If anyone help me to figure out why I'm getting error.

    do {
        System.out.print("inside do");
    } while (false);
    while (false) { // error
        System.out.print("inside while");
    }
    System.out.print("outside");

我认为，据我所知，输出应该在inside dooutside.但是，它显示编译器错误:无法访问的语句.然后，我试图弄清楚为什么会显示 编译错误:无法访问语句*.所以，我把上面的代码改成这样

I thought, and according to me, output should be inside dooutside. But, it is showing Compiler Error : Unreachable Statement. Then, I tried to figure out, why, it is showing Compilation error : Unreachable Statement* . So, I change the above code like this

  boolean i = false;  
  do {
        System.out.print("inside do");
    } while (false);
    while (i) { // ok
        System.out.print("inside while");
    }
    System.out.print("outside");

现在，它显示了预期的输出，即 inside dooutside .所以，我的问题是 - 第一种和第二种情况有什么不同?另外，当我检查

Now, it is showing expected output i.e. inside dooutside . So, my question is - what makes difference in first and second case ? Also, when I check

if(false){ 
  //something here
   }

然后，上面的代码执行没有任何错误.

Then, above code executes without any error.

推荐答案

前两个示例的主要区别在于，在第一种情况下，条件是一个常量，而在第二种情况下则不是.

The main difference between the first two examples is that in the first case, the condition is a constant, whereas in the second it is not.

例如，如果您将 boolean i = false; 更改为 final boolean i = false;，您将得到相同的编译错误，因为 i 现在是一个常量.

For example, if you change boolean i = false; into final boolean i = false;, you will get the same compile error because i is now a constant.

不可达语句的规则定义在JLS 14.21.特别是对 if 有特殊处理，允许 if(DEBUG) 结构，其中 DEBUG 可能是一个常量.

The rules for unreachable statements are defined in the JLS 14.21. In particular there is a special treatment for if to allow if(DEBUG) structures where DEBUG might be a constant.

至于do/while，里面的语句会执行一次，所以没有问题.

As for the do / while, the statement inside will be executed once so there is no problem.

这篇相关帖子中有关常量的更多详细信息.

More details about the constants in this related post.

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