visitor

Visitor

Type

Behavioral Pattern

Goal

Say there are existing elements that we don't want to modify (too much), but we want to introduce new behaviors to them. For instance, there are classes representing different types of buildings. We want each of them to do something without introducing the behavior directly into the classes.

For that we introduce the Visitor interface. Classes implementing this interface will operate on the building objects. Each Visitor adds one type of behavior. Because we have different classes of buildings, the Visitor logic will be different, e.g. visitor.doForHouse(), visitor.doForApartment(), visitor.doForSkyscraper(). If the language supports overloading (like Java), the method name can be the same, but accepting different parameter types.

To know which behavior implementation to use, the building classes will have accept(Visitor visitor) methods, where they "accept" a Visitor.

Structure

Main actors:

• Visitor Interface
• Concrete Visitor
• Element Interface
• Concrete Element
• Client

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