labels

[Steven Squeers]

Hi,

Why does this compile

        x:
        System.out.println("x");

but this result in a compilation error:

        x:
        int i=10; 
        

Okay, one is an assignment, but they're both a statement, no? There's some subtlety here that I'm missing.

Thanks.

[fred rosenberger]

It's kind of hard to say without seeing the rest of the code, and what the EXACT compilation error is.

[Tony Docherty]

In all the years I've used Java (and it's a lot of years) I have never used a label so can't be certain on this but I suspect the label must be followed by an assignment or a statement and the second one is a declaration which isn't allowed.

[Steven Squeers]

Sorry; the following code:

class C1 {    
    public static void main(String...args) {
        x:
        int i=10;          
    }    
}

generates the following error:

C1.java:5: error: '.class' expected
int i=10;
^
C1.java:5: error: not a statement
int i=10;
^
C1.java:5: error: illegal start of expression
int i=10;
^
C1.java:5: error: ';' expected
int i=10;
^
4 errors

[Henry Wong]

Steven Squeers wrote:
Why does this compile

        x:
        System.out.println("x");

but this result in a compilation error:

        x:
        int i=10; 
        

Okay, one is an assignment, but they're both a statement, no? There's some subtlety here that I'm missing.

Short answer. It is defined that way in the Java Language Specification. Longer answer, as to why it is defined that way -- not sure, perhaps the designers forgot about that case. There are just so many statement types, I am not surprised that the case is missing.

Henry

[Steve Luke]

So here is my read on why it is illegal:

According to JLS (here: http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html):

14.4 Local Variable Declaration Statements wrote:Every local variable declaration statement is immediately contained by a block.

While

14.7 Labeled Statements wrote:A labeled statement is executed by executing the immediately contained Statement.

A local variable declaration must be in a block. A labeled statement is not a block, but contains the statement that is part of the labeled statement. So a local variable declaration after a label would be a variable declaration not immediately contained in a block.

The error messages I see:

D:\test>"C:\Program Files\Java\jdk1.7.0\bin\javac" -cp . pack/Test.java
pack\Test.java:7: error: '.class' expected
        LabelMe: int assignMe = 100;
                     ^
pack\Test.java:7: error: not a statement
        LabelMe: int assignMe = 100;
                 ^
pack\Test.java:7: error: illegal start of expression
        LabelMe: int assignMe = 100;
                              ^
pack\Test.java:7: error: ';' expected
        LabelMe: int assignMe = 100;
                               ^
4 errors

Are a bit non-expressive, but sore of make sense if the above is the reason.
Here is the sample code:

package pack;

public class Test
{
    public static void main(String[] arge)
    {
        LabelMe: int assignMe = 100;
    }
}

And if you change it by enclosing the assignment in braces (creating a block) the error goes away:

package pack;

public class Test
{
    public static void main(String[] arge)
    {
        LabelMe: { int assignMe = 100; }
    }
}

[Steve Luke]

Note that putting a label on an variable declaration doesn't make sense anyway. The scope of the label is only the contained statement. So by the time the label could be used it would be out of scope.

[Steven Squeers]

Thanks for the replies.
I understand that braces make a block; what I can't fathom is how a System.out.println() qualifies as a block; perhaps though it doesn't need to be a block. Perhaps a method call and a block (formed with braces) are part of a larger set of syntax elements, and, an assignment is not part of that set. Hmm.

[Steve Luke]

Steven Squeers wrote:Thanks for the replies.
I understand that braces make a block; what I can't fathom is how a System.out.println() qualifies as a block;

System.out.println() is a method, not a block. A method call doesn't have to be immediately enclosed in a block. Nor does a label need to contain a block (it can contain a block, but that is because a block is a statement and a label statement contains a statement). A variable declaration does have to be immediately enclosed in a block.

perhaps though it doesn't need to be a block. Perhaps a method call and a block (formed with braces) are part of a larger set of syntax elements, and, an assignment is not part of that set. Hmm.

It isn't about what the label can do - it is the variable declaration that has special requirements.

[Henry Wong]

Steven Squeers wrote:Thanks for the replies.
I understand that braces make a block; what I can't fathom is how a System.out.println() qualifies as a block; perhaps though it doesn't need to be a block. Perhaps a method call and a block (formed with braces) are part of a larger set of syntax elements, and, an assignment is not part of that set. Hmm.

To follow the specification, System.out.println() is a MethodInvocation, which is a type of StatementExpression, which is part of ExpressionStatement (with the semicolon as the other part), which is a type of StatementWithoutTrailingSubstatement, which is a type of Statement, which is allowed to follow a label Indentifier (as defined by LabelStatement).

On the other hand, "int assignMe = 100" is a LocalVariableDeclaration, which is part of LocalVariableDeclarationStatement (with the semicolon as the other part), which is a BlockStatement, which is part of a BlockStatements (with other BlockStatement as the other parts). And that is it -- none of the items on this list is allowed to follow a label Indentifier (as defined by LabelStatement).

However, if curly braces were added then BlockStatements is allowed within it forming a Block, which is allowed to follow a label Indentifier (as defined by LabelStatement).


The Java Language Specification is as clear as mud right ?? ...

Henry

[Steven Squeers]

I think it's easier to just remember that an assignment can't follow a label, but a method invocation can.
Thanks!

[Steve Luke]

Steven Squeers wrote:I think it's easier to just remember that an assignment can't follow a label, but a method invocation can.
Thanks!

But neither is real useful in a label statement, since there really isn't a chance to use the label for control transfer. Better is to use it with a loop, or a block which contains lots of statements.

[Paul Clapham]

Steven Squeers wrote:I think it's easier to just remember that an assignment can't follow a label, but a method invocation can.

I don't think that's right. It's a declaration that can't follow a label; an assignment should be perfectly fine there. Try it out and see.

[Steven Squeers]

Paul Clapham wrote:

Steven Squeers wrote:I think it's easier to just remember that an assignment can't follow a label, but a method invocation can.

I don't think that's right. It's a declaration that can't follow a label; an assignment should be perfectly fine there. Try it out and see.

Ah well spotted. So, an assignment is ok but a declaration is not. I can't see what the language designers have in mind here but it is what it is.

Thanks.