Java标签不规则(可能的bug?) [英] Java label irregularity (possible bug?)
问题描述
如果我们查看 Java标准§14.7,我们会看到这些声明可能有标签前缀,例如:
If we look at the Java standard §14.7, we see that statements may have label prefixes, e.g.:
LabeledStatement:
标识符:语句
理论上,标签应该能够标记任何后续语句。因此,例如,以下编译:
In theory, a label should be able to label any succeeding statement. So, for example, the following compiles accordingly:
public class Test {
public static void main(String[] args) {
hello:
return;
}
}
直观地,这也编译:
public class Test {
int i;
public static void main(String[] args) {
Test t = new Test();
label:
t.i = 2;
}
}
但以下是不 compile:
But the following does not compile:
public class Test {
public static void main(String[] args) {
oops:
int k = 3;
}
}
即使这样做(注意范围括号):
Even though this does (note the scoped brackets):
public class Test {
public static void main(String[] args) {
oops:
{
int k = 3;
}
}
}
所以问题取决于是否或不声明是声明。根据标准(以及在线文档):
So the question hinges on whether or not declarations are statements. According to the standard (and the online documentation):
除了表达式语句之外,还有另外两种语句:声明语句和控制流语句。声明语句声明一个变量。
In addition to expression statements, there are two other kinds of statements: declaration statements and control flow statements. A declaration statement declares a variable.
我在OSX和Windows上都注意到Java 7和8中的这种行为。这是一个错误还是我误解了标准?
I've noticed this behavior in Java 7 and 8 on both OSX as well as Windows. Is this a bug or am I misunderstanding the standard?
推荐答案
表达式
int k = 3;
是局部变量声明语句。
语句
标签声明
LabeledStatement
:
标识符
: Statement
不包含局部变量声明语句。因此,您不能直接在带标签的语句中使用它们。
does not contain local variable declaration statements. You therefore can't use them within a labeled statement directly.
本地变量声明语句可以在blocks 可以在标签声明中使用。
Local variable declaration statements can be used within blocks which can be used within labeled statements.
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