Java label irregularity (possible bug?)

问题

If we look at the Java standard §14.7, we see that statements may have label prefixes, e.g.:

LabeledStatement:

     Identifier : Statement

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;
    }

}

Intuitively, this also compiles:

public class Test {
    int i;
    public static void main(String[] args) {
        Test t = new Test();
    label:
        t.i = 2;        
    }
}

But the following does not compile:

public class Test {
    public static void main(String[] args) {
    oops:
        int k = 3;  
    }
}

Even though this does (note the scoped brackets):

public class Test {
    public static void main(String[] args) {
    oops:
        {
            int k = 3;
        }
    }
}

So the question hinges on whether or not declarations are statements. According to the standard (and the online documentation):

In addition to expression statements, there are two other kinds of statements: declaration statements and control flow statements. A declaration statement declares a variable.

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?

回答1:

The expression

int k = 3; 

is a local variable declaration statement.

The statement used in the syntax of a label statement

LabeledStatement:

  Identifier : Statement

does not contain local variable declaration statements. You therefore can't use them within a labeled statement directly.

Local variable declaration statements can be used within blocks which can be used within labeled statements.

来源：https://stackoverflow.com/questions/30609301/java-label-irregularity-possible-bug