The Visitor pattern is useful in designing an operation (method or algorithm) across a collection of objects or a class hierarchy. The Visitor pattern allows the operation to be defined without changing the classes of the collection. To accomplish this, the Visitor pattern suggests defining the operation in a separate class referred to as a visitor class. For every new operation to be defined, a new visitor class is created.
Usually visitor pattern is implemented this way:
1- Every visitor class that operates on objects of the same set of classes can be designed to implement a corresponding VisitorInterface interface. A typical VisitorInterface declares a set of visit(ObjectType) methods, one for each object type from the object collection: visit(Type1 t1)
2- Classes from the object collection need to define a method: accept(VisitorInterface vi) to be visitable.
3- Whenever a new operation is to be defined across the object collection, a new visitor class needs to be created with implementation for the new operation. The visitor class needs to implement all of the visit(ObjectType) methods declared in the VisitorInterface interface to process different types of objects. With this design, defining a new operation does not require any changes to the classes of the object collection.
4- Whenever a new type of object is to be added to the object collection and is to be visitable, the class of the object must provide a method similar to accept(Visitor) and as part of this method implementation it should invoke the visit(ObjectType) method on the visitor object passing itself as an argument to it. Also a corresponding visit(ObjectType) method needs to be added to the VisitorInterface interface and needs to be implemented by all the concrete visitor classes.
See this more real example:
