Re: When and where to use Visitor Pattern?

Sam Hwang wrote: 
Hello,
I am confused about Visiotr Pattern. Observe this example from
<snip> 
Why make it so complicated? Can't we use this one instead?
class IntExp1 extends Expression
{
   int value;

   void visit()
   {
      System.out.println(value);
   }
}

class AddExp1 extends Expression
{
  Expression e1, e2;

  void visit()
  {
     e1.visit();
    System.out.print(" + ");
    e2.visit();
  }
}
I dont know in what cirumstances the pattern is used sensablely. I hope
someone can address my confusion. Thanks!


Okay, suppose you instead wanted to find
- the largest integer in your expression.
- The smallest integer
- count the number of additions
- count the number of integers
- find the leftmost integer
- find the rightmost integer
- write to a file, not System.out

You can either write a "visit" method for each of those tasks in the above list, thereby inserting lots of junk into your IntExp and AddExp classes, or you can use a visitor. Visitor moves methods into classes, which can structure the problem better.

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