英文:
Does if condition/ for loop makes a new variable scope in java?
问题
以下是您要翻译的内容：
我有一个类似这样的问题：
考虑以下Java程序
class main_class
{
public static void main(String args[])
{
int x = 9;
if (x == 9)
{
int x = 8;
System.out.println(x);
}
}
}
A. 9
B. 8
C. 编译错误
D. 运行时错误
我选择了B，但我在本地环境中运行时发现我错了，if条件中不允许重复的变量。但是我想的是，我在编码时总是使用类似于for(int i=0;i<len;i++)，但是“i”没有重复，难道不是“{}”表示一个新的变量作用域吗？这让我有点困惑。
我在我的VS Code环境中进行了本地操作，如下所示：
点击此处查看图像描述
英文:
I got a question like this:
Consider the following Java program
class main_class
{
public static void main(String args[])
{
int x = 9;
if (x == 9)
{
int x = 8;
System.out.println(x);
}
}
}
A.9
B.8
C.Compilation Error
D.Runtime Error
I chose B, but I operated in my local environemnt and I find I'm wrong, the duplicated variable is not allowed in if condition. But what I thought is I always use for(int i=0;i<len;i++) when I'm coding, but the "i" is not duplicated, doesn't "{}" means a new variable scope? it makes me a little confused.
The local operation is conducted as below on my VS code env
enter image description here
答案1
得分: 2
是的，每个块在Java中都是一个新的作用域 - 但根据JLS 6.4的规定：
> 如果在v的作用域内使用局部变量v的名称来声明一个新变量，除非新变量在v的作用域内声明在类或接口声明中，否则将在编译时出现错误。
因此，拥有一个名为v的字段，然后声明一个名为v的局部变量是可以的，但你不能声明两个名为v的嵌套局部变量声明（除了局部类声明等）。
在你的另一段代码中：
for (int i = 0; i < 100; i++) {
}
for (int i = 0; i < 100; i++) {
}
...每个i变量的作用域是for循环本身。根据JLS 6.3的规定：
> 在基本for语句的ForInit部分（§14.14.1）中声明的局部变量的作用域包括以下所有内容：
>
> - 它自己的初始化程序
> - 在for语句的ForInit部分中更多的声明符
> - for语句的Expression和ForUpdate部分
> - 包含的Statement
因此，这类似于编写：
{
for (int i = 0; i < 100; i++) {
}
}
{
for (int i = 0; i < 100; i++) {
}
}
...显然，没有嵌套的i声明。这两个变量都不是在另一个变量的作用域内声明的。
英文:
Yes, each block is a new scope in Java - but as per JLS 6.4:
> It is a compile-time error if the name of a local variable v is used to declare a new variable within the scope of v, unless the new variable is declared within a class or interface declaration appearing within the scope of v.
So it's fine to have a field called v and then declare a local variable called v, but you can't declare nest two local variable declarations of v (aside from local class declarations etc).
In your other code of:
for (int i = 0; i < 100; i++) {
}
for (int i = 0; i < 100; i++) {
}
... the scope of each i variable is the for loop itself. As per JLS 6.3 :
> The scope of a local variable declared in the ForInit part of a basic for statement (§14.14.1) includes all of the following:
>
> - Its own initializer
> - Any further declarators to the right in the ForInit part of the for statement
> - The Expression and ForUpdate parts of the for statement
> - The contained Statement
So it's similar to writing:
{
for (int i = 0; i < 100; i++) {
}
}
{
for (int i = 0; i < 100; i++) {
}
}
... clearly there's no nesting of the declarations of i. Neither variable is declared within the scope of the other.
答案2
得分: 0
你可能也会混淆以下两个示例。
class main_class
{
public static void main(String args[])
{
int x = 9;
{
// 在第二次声明期间，x仍然在范围内。
int x = 10; // 编译问题，因为另一个x仍然在此范围内
}
// 相对于
{
int y = 9;
}
int y = 10; // 没问题。第二次声明没问题，因为范围不共享。
// 第一个y在第二次声明之前超出范围。
}
<details>
<summary>英文:</summary>
You also may be confusing the following two examples.
class main_class
{
public static void main(String args[])
{
int x = 9;
{
// x still in scope during second re-declaration.
int x = 10; // compilation issue other x still in this scope
}
// vs
{
int y = 9;
}
int y = 10; // OK. Second declaration fine since scope is not shared.
// first y goes out of scope prior to second declaration.
}
答案3
得分: 0
这里涉及Java中的变量作用域概念。更多详细信息请查看您的问题链接：https://softwareengineering.stackexchange.com/questions/271683/why-can-we-use-the-same-name-for-local-variable-in-different-scopes
英文:
There is this concept of variable scope in Java.
More details to your question here https://softwareengineering.stackexchange.com/questions/271683/why-can-we-use-the-same-name-for-local-variable-in-different-scopes
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