Labeled Statement block in Java?

Question

I was browsing through some of the base Java objects when I found a section of code surrounded by a scan: {} block. The following code is from the toLowerCase() method inside the String class.

scan: {
            for (firstUpper = 0 ; firstUpper < len; ) {
                char c = value[firstUpper];
                if ((c >= Character.MIN_HIGH_SURROGATE)
                        && (c <= Character.MAX_HIGH_SURROGATE)) {
                    int supplChar = codePointAt(firstUpper);
                    if (supplChar != Character.toLowerCase(supplChar)) {
                        break scan;
                    }
                    firstUpper += Character.charCount(supplChar);
                } else {
                    if (c != Character.toLowerCase(c)) {
                        break scan;
                    }
                    firstUpper++;
                }
            }
            return this;
        }

Could someone please explain what the scan:{} block is used for and where this syntax comes from? I've yet to see a colon after a word like this in Java unless used in a ternary operator.

Thanks!

Edit: Updated title to correctly match answered question.

score 18 · Accepted Answer · answered Jan 03 '13 at 21:50

18

Here, scan: is simply a label. The break <label> syntax allows one to break out of outer loops, and to simulate some forms of the goto statement. The syntax is documented in the JLS:

A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a switch, while, do, or for statement.

answered Jan 03 '13 at 21:50

NPE

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Is this practice as reviled as goto's? – Daniel Jan 03 '13 at 21:55
2

@Daniel: I don't want to start any religious debates. ;) However, since this form can be considered more "structured" than a raw `goto`, I would imagine some anti-`goto` advocates might consider it less objectionable. – NPE Jan 03 '13 at 21:56
1

@Daniel no, because you can't use it to arbitrarily jump around in code. It's useful with `break` in nested loops to specify which loop to break out of. – Alex Jan 03 '13 at 21:56
@NPE: Your link answered my question perfectly and totally explains why I was unable to find a scan block! Thanks again – Falkenfighter Jan 03 '13 at 22:13

score 2 · Answer 2 · answered Jan 03 '13 at 21:50

2

It is a labeled block. where scan: is a label. It is commonly used when breaking/continue in case you have multiple loops. In this case break scan; simply breaks outta the labeled block(scan) when executed.

answered Jan 03 '13 at 21:50

PermGenError

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score 2 · Answer 3 · answered Jan 03 '13 at 21:50

2

You can set a label to break / or continue from within multiple loops deep.

Example

 outer:
 for(int i=...){
   for(int j=..){
     ...
     break outer; // leaves both loops   

   } 

 }

answered Jan 03 '13 at 21:50

MrSmith42

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score 1 · Answer 4 · answered Jan 03 '13 at 21:51

1

It is a label. It is a indicator for flow control.

If you look at your code, you see below

 break scan;

When this happens, the flow exits completely the scan block.

By the way, it can be any identifier, scan is not a keyword at all.

answered Jan 03 '13 at 21:51

SJuan76

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Labeled Statement block in Java?

4 Answers4

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