A ``cute'' fact about finally

Normally, we believe the following two pieces of code to be equivalent:

Iteration using for

   for (i = 0; i < n; i++){         
     statement 1;                   
     statement 2;                   
     ...                            
     statement k;                   
   }

Iteration using while

  i = 0;                
  while (i < n){        
    statement 1;        
    statement 2;        
    ...                 
    statement k;        
    i++;                                                    
    }

However, observe what happens if one of the internal statements is continue (or, more precisely in Java, continue l where l is a loop label).

   l:                      
   for (i = 0; i < n; i++){
     statement 1;          
     statement 2;          
     ...                   
     continue l;           
     statement k;          
   }

   l:            
   i = 0;        
   while (i < n){
     statement 1;
     statement 2;
     ...         
     continue l  
     statement k;
     i++;        
   }

The statement continue l causes the loop l to start its next iteration without executing any of the statements that follow it in the loop. In the for loop, this still results in the increment i++ being executed since it is part of the loop definition and not a statement ``inside'' the loop. In the while loop, on the other hand, the increment i++ is skipped along with statement k so that the loop restarts with the same value of iq.

To fix this, we should use the try-finally construction (without a catch clause) as follows:

   l:
   i = 0;
   while (i < n){
     try{
       statement 1;
       statement 2;
       ...
       continue l
       statement k;
     }
     finally{
       i++;
     }
   }

Now, the continue l causes the try block to terminate prematurely, but the finally block is still executed before reentering the loop, so this version does preserve the semantics of the for.