在条件中声明变量而没有花括号时编译器错误 [英] Compiler error when declaring a variable inside if condition and no curly braces

问题描述

为什么第一个如果编译好，第二个失败？

Why does this first if compile well and the second fail?

if(proceed) {int i;} // This compiles fine.
if(proceed) int i;// This gives an error. (Syntax error on token ")", { expected after this token)

推荐答案

因为语言规范如此：

http://docs.oracle.com/javase/specs/jls/se7/html/jls-6.html

声明将实体引入程序并包含可在名称中用于引用此实体的标识符（第3.8节）。声明的实体是以下之一：

...

一个局部变量，以下之一：

*在块中声明的局部变量（§14.4）

*在for语句中声明的局部变量（§14.14）

A declaration introduces an entity into a program and includes an identifier (§3.8) that can be used in a name to refer to this entity. A declared entity is one of the following:
...
A local variable, one of the following:
* A local variable declared in a block (§14.4)
* A local variable declared in a for statement (§14.14)

您的第一个示例是在块内声明 i （用花括号表示）。你的第二个不是，语句也不是。

Your first example is declaring i inside a block (denoted by curly braces). Your second isn't, nor is it a for statement.

编辑添加：这只会让公众有意义。如果它被允许，那将是无用的。它会立即超出范围。

Edited to add: Which just makes commons sense. If it were allowed, it would be useless. It would immediately fall out of scope.

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