无法到达的return语句仍会引发错误 [英] Unreachable return statement still throws error

问题描述

我有一个非常简单的代码段:

I have this very simple code snippet:

static String getInput() throws IOException{
  if(in.ready()){
      return in.readLine().trim();
  }
  System.err.println("Please provide more input in order to execute the program.");
  System.exit(0);
  return "";
}

据我所知，JVM无法在代码末尾执行return语句.但是，如果我将这一行注释掉，java将抱怨缺少return语句. JVM为什么不能识别System.exit(0)不允许执行任何其他代码，但是如果返回将不允许执行代码，则会抱怨无法到达的语句? 我认为最后的return语句是多余的，可能会使其他开发人员感到困惑，所以java为什么不让我摆脱它呢?

By what I think I know, there is no possible way that the JVM will execute the return statement at the end of the code. But if I comment this line out, java will complain about a missing return statement. Why doesn't the JVM recognize that a System.exit(0) will not allow any other code to execute, but complains about unreachable statements if a return will not allow code to be executed? I think the return statement at the end is redundant and might be confusing to other devs, so why won't java let me get rid of it?

推荐答案

为什么JVM不能识别System.exit(0)不允许执行任何其他代码，但是如果返回将不允许执行代码，则会抱怨语句不可达?

Why doesn't the JVM recognize that a System.exit(0) will not allow any other code to execute, but complains about unreachable statements if a return will not allow code to be executed?

不是JVM，而是编译器.而且编译器不知道 library 调用会做什么-它只知道 language 规则. (特别是 JLS第14.21节，无法访问的语句 )

It's not the JVM - it's the compiler. And the compiler doesn't know what library calls will do - it only knows the language rules. (In particular, JLS section 14.21, Unreachable Statements.)

例如:

public int foo() {
    alwaysThrow();
    // This is required.
    return 10;
}

private static void alwaysThrow() {
    throw new RuntimeException();
}

vs

public int foo() {
    throw new RuntimeException();
    // Error: unreachable statement
    return 10;
}

这种简单的内联改变了代码的意义，甚至影响了编译器.

That simple inlining changes the meaning of the code as far as the compiler's concerned.

这可以通过返回类型为从不"来修复"，以表示此方法从不正常返回-它挂起或抛出异常"，但这根本不是该语言的一部分(并且将具有自己的语言)并发症).如果您有兴趣，埃里克·利珀特(Eric Lippert)关于C#(位置相似)有一些关于该主题的博客文章:部分两个.

This could be "fixed" by having a return type of "never" - to indicate "this method never returns normally - it either hangs or throws an exception" but that's simply not part of the language (and would have its own complications). If you're interested, Eric Lippert has a couple of blog posts on this topic with regard to C# (which is in a similar position): part one, part two.

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