Boolean in for loop??

[Cathy Song]

Hi,
I would have thought that the following program would not compile..but it does..any thoughts on how the for loop works?

  
    public class BooleanInFor {
       static boolean b(char c) {
         System.out.print(c);
         return true;
      }
      
       public static void main(String[] args) {
         int i =0;
         for ( b('X'); b('Y')&&(i<2); b('Z')){  //-->very interetsing
            i++ ;
            b('A');
         }   
      }
   }

Output:
XYAZYAZY

[Ernest Friedman-Hill]

The first and third parts of the "for" loop control line can be more or less any expressions, including none. The second one, though, has to be a boolean. There's no restriction that there has to be an integer loop counter.
I personally write this code all the time:

Collection c = ...
for (Iterator it = c.iterator(); it.hasNext() :wink:  {
  processObject(it.next());
}

[Vinod Venkatasubramanian]

Here's what the Java Language Specification says about the for loop:
"
The for statement executes some initialization code, then executes an Expression, a Statement, and some update code repeatedly until the value of the Expression is false.
ForStatement:
for ( ForInit ; Expression ; ForUpdate )
Statement
ForStatementNoShortIf:
for ( ForInitopt ; Expressionopt ; ForUpdateopt )
StatementNoShortIf
ForInit:
StatementExpressionList
LocalVariableDeclaration
ForUpdate:
StatementExpressionList
StatementExpressionList:
StatementExpression
StatementExpressionList , StatementExpression
The Expression must have type boolean, or a compile-time error occurs.
"
So the only real constraint is that the "Expression" part must return a boolean while the "ForInit" and "ForUpdate" can be valid Java statement(s) separated by commas... IN fact even the following works though you would not want to use it :-)
for (; ;) {
System.out.println("Never ending....");
}
[ October 20, 2003: Message edited by: Vinod Venkatasubramanian ]

[Andres Gonzalez]

Cathy, why do you think the program would not compile?

[Cathy Song]

Because I thought the first part of the for loop (if present) was initialization and third part (if present) was some expression.

[Andres Gonzalez]

Originally posted by Cathy Song:
Because I thought the first part of the for loop (if present) was initialization and third part (if present) was some expression.

the only thing to remember about the first part is that it is going to be executed *only once*. the third part is going to be executed at the end of every iteration of the loop.

[Harwinder Bhatia]

Hey Cathy ...
And one more point:
In the first part of the 'for' loop, comma-separated declaration statement(s) cannot be mixed with expression(s).
The following code fragment, would not compile:

for (int i = 0, System.out.print ("Hello Cathy !");  ; ) 
{
    System.out.println ("This will not work");
}

whereas, the following would work just fine:

int i;
for (i = 0, System.out.println ("Hello again Cathy !");  ; ) 
{
    System.out.println ("Please kill me");
}

Cheers
Harwinder
[ October 20, 2003: Message edited by: Harwinder Bhatia ]

[Cathy Song]

Hi All,
Thanks for your inputs. Harwinder, that an interesting piece of code.