在 if 条件中声明变量且没有大括号时出现编译器错误 [英] Compiler error when declaring a variable inside if condition and no curly braces

问题描述

为什么第一个 if 编译良好而第二个失败?

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

因为语言规范是这样说的:

Because the language spec says so:

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

声明将实体引入程序并包含一个标识符(第 3.8 节)，可在名称中使用该标识符来引用该实体.声明的实体是以下之一:
...
局部变量，以下之一:
* 在块中声明的局部变量(第 14.4 节)
* 在 for 语句中声明的局部变量(第 14.14 节)

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，也不是 for 语句.

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:这只是常识.如果允许，那就没用了.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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