Labeled statements

[Fahad Muhammad]

Hi,

why this code doesn't compile

public class Testing {

    public static void main (String[] args) {
    	outer :
    		int[] array = {1,2,3};
    		System.out.print(array.length);
    	}
    
}

this will compile

public class Testing {

    public static void main (String[] args) {
    	outer :{
    		int[] array = {1,2,3};
    		System.out.print(array.length);
    	}
    		
    }
    
}

also this will compile

public class Testing {

    public static void main (String[] args) {
    	outer :
    		for(int i=0; i<10; i++){
    			int[] array = {1,2,3};
    			System.out.print(array.length);
    		}
    		
    	
    		
    }
    
}

The difference I can see is using blocks { } and for block.
Are there any rules regarding labeling statements?

Thanks in advance.

[Mala Gupta]

As the name implies, a labeled statement is supposed to define a set of statements, which can be referred using a label. A set of statements can be defined as a block by either grouping all the statements using {} or by using any of the loops - while/ do-while/ for. When you issue the command break label or continue label, JVM should know the block of statements it is supposed to exit/ start over again.

cheers
Mala

[Fahad Muhammad]

Thank you Mala.

I also noticed that declaring variables inside a labeled statement causes a compilation error.
So, Is it safe to say that declaring variable is NOT a statement?

[Mala Gupta]

Fahad Muhammad wrote:
I also noticed that declaring variables inside a labeled statement causes a compilation error.
So, Is it safe to say that declaring variable is NOT a statement?

Fahad,

I modified your code and added a line of variable declaration within the labeled statement and it compiles fine:

public class Testing {
	public static void main (String[] args) {
		outer :
			for(int i=0; i<10; i++){
				int aVar = 10;			// COMPILES OK!
				int[] array = {1,2,3};
				System.out.print(array.length);
			}
	}
}

Or did you mean something else?

cheers
Mala

[Fahad Muhammad]

I mean this which will not compile

    public class Testing {  
      
        public static void main (String[] args) {  
            outer :  
                int[] array = {1,2,3};  // variable declaration
                System.out.print(array.length);  
            }  
          
    }  

Versus this wich will compile

    public class Testing{

    public static void main (String[] args) {
    	int[] array;
    	outer :
  		array = new int[]{1,2,3};
    		System.out.print("");	
    }
    
}

[Mala Gupta]

Fahad,

Labeled statements do not work with declarations. The following is 'labeled' declaration, which won't work:

outer :    
	int[] array = {1,2,3};  // variable declaration- fails compilation  

Labels can be used with a looping statements (for, while, do), if constructs (if statement), expression, assignment, return, try, throws, and with a block. It is interesting to note that the above declaration can be defined within a block statement as follows:

outer : {   			    // opening and closing braces - define a block
            int[] array = {1,2,3};  // variable declaration - compiles OK.
        }

A little, funny & weird story may help you to remember this Java Rule:

Boss (Java) was announcing bonus (privilege of using labels) for all its employees (language constructs) that were present in the office. 'declaration' wasn't awarded this privilege because it was away, partying, and hence missed on it.

I know its very weird and funny, but it will help you to retain this info!

cheers
Mala

[Mala Gupta]

A little funny image may help as well:



cheers
Mala

[Fahad Muhammad]

Thank you very much Mala

[rohit sahai]

Hi Mala

can you please tell me why the class does not comile ,if assignment is allowed


public class Breaker {
static String o = "";
public static void main(String[] args) {
z:
o = o + 2;
for(int x = 3; x < 8; x++) {
if(x==4) break;
if(x==6) break z;
o = o + x;
}
}
}

[Cyril Sadasivan]

rohit sahai wrote:Hi Mala

can you please tell me why the class does not comile ,if assignment is allowed


public class Breaker {
static String o = "";
public static void main(String[] args) {
z:
o = o + 2;
for(int x = 3; x < 8; x++) {
if(x==4) break;
if(x==6) break z;
o = o + x;
}
}
}

Rohit, the problem is not with the assignment statement. The label 'z' is applicable only for the statement "o=o+2;"

z:o=o+2;

is same as

z:{
o=o+2;
}

The "break z;" statement is within the for loop which is an unlabeled block.
For labeled break statements to work, the "break label" should appear within a block labeled with the particular label.
The code should be rewritten as follows:

public class Breaker {
	static String o = "";

	public static void main(String[] args) {
		z: {
			o = o + 2;
			for (int x = 3; x < 8; x++) {
				if (x == 4)
					break;
				if (x == 6)
					break z;
				o = o + x;
			}
		}
	}
}