Visitor Pattern

Let's say we have some classes that are of same base type. Now we want to add some extra functionality to them.
Eg We have CricketBall, CricketBat, VolleyBall class. All are shoppingElements.
New functionality is that we want to add a discount in this season. And the developer of this module don't want us to do many changes. All the changes should be safe.
Here comes Visitor Pattern.
For every new algo/change we are going to add there's going to be a new visitor.
We call visit method on the objects and provide the visitor. Rest the object is going to handle the invocation and the logic of algo resides with the visitor.

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