if语句中的Java变量范围 [英] Java variable scope in if statement

问题描述

我收到以下代码的编译错误:

I received a compilation error for the following code:

if(true)
    int a = 10;
else
    int b = 20;

如果将其更改为以下代码，则没有编译错误:

If I change it to the following code, then there is no compilation error:

if(true) {
    int a = 10;
}
else {
    int b = 20;
}

为什么第一个语法错误，以及从哪种语言标准开始?

Why is the first syntax wrong, and from what language standard?

推荐答案

Java规范指出，if-then-else语句具有以下形式:

The Java specification says that an if-then-else statement is of the following form:

IfThenElseStatement:
    if ( Expression ) StatementNoShortIf else Statement

Statement和StatementNoShortIf可以是各种各样的东西，包括块(用大括号括起来的代码)，赋值(已声明的变量)，其他if语句等.

Where Statement and StatementNoShortIf can be various things including blocks (code surrounded with braces), assignments (to already declared variables), other if statements etc.

值得注意的是，该列表中缺少声明语句(例如int a;或int a = 10;)，因此会出现编译错误.

Of note is that declaration statements (e.g. int a; or int a = 10;) are missing from that list, thus you get a compilation error.

有关完整列表，您可以在此处阅读Java规范: http://docs.oracle.com/javase/specs/

For the full list, you can read the Java specification here: http://docs.oracle.com/javase/specs/

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