Visitor Pattern
Motivation
Collections are data types widely used in object oriented programming. Often collections contain objects of different types and in those cases some operations have to be performed on all the collection elements without knowing the type.
A possible approach to apply a specific operation on objects of different types in a collection would be the use if blocks in conjunction with 'instanceof' for each element. This approach is not a nice one, not flexible and not object oriented at all. At this point we should think to the Open Close principle and we should remember from there that we can replace if blocks with an abstract class and each concrete class will implement its own operation.