英文:
URL in Java code compiles without error: Why is the javac compiler ignoring it?
问题
我在测试一些代码时犯了一个错误，忘记删除只包含URL的一行。代码看起来像这样：
class Example {
public static void main(String[] args) {
https://example.com
System.out.println("Example");
}
}
我假设编译器会失败，因为main方法的第一行包含非Java代码。然而，代码编译并正常工作。
我以为//符号后的所有内容都是注释，并预期以下代码将无法编译通过：
https:
System.out.println("Example");
令我惊讶的是，它竟然可以编译通过。
然而，以下代码产生了编译错误，如预期的那样：
class Example {
public static void main(String[] args) {
https://example.com
}
}
编译器错误信息如下：
/Example.java:4: error: illegal start of statement
}
^
1 error
有人可以解释为什么以及在什么情况下javac编译器会忽略包含URL的行吗？我在线编译代码在https://www.jdoodle.com/上，也在本地手动编译，行为总是相同的。我使用了Java8和Java17。
我注意到这行被忽略是因为在IntelliJ中我可以在反编译的代码中检查到该行不存在。
英文:
I made a mistake while testing some code and forgot to delete a line that contained only a URL. The code looked something like this:
class Example {
public static void main(String[] args) {
https://example.com
System.out.println("Example");
}
}
I assumed that the compiler would fail because a non-Java code was present in the first line of the main method. However, the code compiled and worked fine.
I thought that everything after the // symbol was a comment and expected the following code to fail compilation:
https:
System.out.println("Example");
To my surprise, it compiled without error.
However, the following code produced a compile error, as expected:
class Example {
public static void main(String[] args) {
https://example.com
}
}
Compiler error
/Example.java:4: error: illegal start of statement
}
^
1 error
Can anyone explain why and under what circumstances the javac compiler ignores a line that contains a URL?
I compile the code online in https://www.jdoodle.com/ locally by hand and the behavior is the same always. I used Java8 and Java17.
I saw that the line is ignored because in intellij I could check in the decompiled code that the line is not there.
答案1
得分: 1
以下是已翻译的内容：
这实际上是有效的Java代码：
https://example.com
System.out.println("Example");
https: 是一个标签
// 是一行的注释
System.out.println("Example"); 是带有 https: 标签的语句
带标签的语句是Java中的一种不常见的构造，通常与带标签的break语句一起使用。以下是一个（略微）有用的示例：
out: while (true) {
int i = Random.nextInt(10);
while (true) {
int j = Random.nextInt(10);
if (i == j) break out;
}
}
break out; 中断了两个while循环。
你的第二个例子不是有效的Java，因为标签后面没有语句。
英文:
It is actually valid Java code:
https://example.com
System.out.println("Example");
The https: is a label
The // is a comment ... to the end of the line
The System.out.println("Example"); is the statement that is labelled with https:
Labelled statements are an obscure Java construct that is used in conjunction with the break to label statement. Here's a (marginally) useful example:
out: while (true) {
int i = Random.nextInt(10);
while (true) {
int j = Random.nextInt(10);
if (i == j) break out;
}
}
The break out; breaks the two while loops.
Your second example is not valid Java because there isn't a statement after the label.
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