More On Visitors

Things to remember about the Visitor design pattern:

• The host structure and the Visitor interface are invariant.
• The concrete visitors are variants.
• The invariant and variant parts of a system cannot run without each other.
  • You only get a complete system with complete behaviors when the invariant portion is combined with a variant portion.
  • You get different total behaviors when you combine different variant portions with the invariant portion.
  • The host provides the behaviors that are intrinsic and invariant -- the "atomic" behaviors of the system. The host is a "framework" that provides the base-level "services" to the system. In general, one strives for a complete and minimal set of host behaviors.
  • The visitors provide variant behaviors composed from the invariant behaviors provided by the host -- the "molecular" behaviors of the system. The visitors are the "components" that provide the high-level operations of the system.
• The visitor patterns separates the data structure (the host) from the algorithms on it. (the visitors).
  • See Dung and Stephen's SIGCSE '99 paper on this.
  • The data and the algorithms thus become completely decoupled.
• The visitor pattern guarantees that each host always matched with the behavior tha is appropriate for it.
  • Checking the type of the host is never needed.
  • The behaviors for each host are cleanly separated.
  • Every visitor has a method for each possible host type.
  • The system does not need to know either the host type or the visitor type.

Example

Here is an example that illustrates how the visitor will always deliver correct behavior for the given host type.

See the Documentation

Download the code:: MaleFemale.java

The host is an APerson. There are two types of concrete hosts: Male and Female.

The visitors are ITasks. There are 3 kinds of concrete visitors: EatTask, GetDressedTask and WeekendTask.

The Tasker object randomly selects either a Male or a Female host to execute the supplied ITask (visitor) with the supplied parameter. The correct behavior for the selected host will always be run.

Here is a typical DrJava Interactions pane session:

Welcome to DrJava.
> Tasker tasker = new Tasker();
> ITask getDressedTask = new GetDressedTask();
> ITask eatTask = new EatTask();
> ITask weekendTask = new WeekendTask();
> tasker.doTask(eatTask, "steak")
Male host selected:

"You got a beer to go with that steak?"
> tasker.doTask(eatTask, "steak")
Male host selected:

"You got a beer to go with that steak?"
> tasker.doTask(eatTask, "steak")
Female host selected:

"No thanks, may I have a salad with dressing on the side, please?"
> tasker.doTask(getDressedTask, null)
Female host selected:

"I'll be ready in a minute!"
> tasker.doTask(getDressedTask, null)
Male host selected:

"Does "black tie" mean I can't wear shorts?"
> tasker.doTask(getDressedTask, null)
Male host selected:

"Does "black tie" mean I can't wear shorts?"
> tasker.doTask(weekendTask, "9")
Female host selected:

"Only have 9 hours, so let's clean the house, wash the clothes and weed the garden!"
> tasker.doTask(weekendTask, "9")
Female host selected:

"Only have 9 hours, so let's clean the house, wash the clothes and weed the garden!"
> tasker.doTask(weekendTask, "9")
Male host selected:

"Where's the remote?"
>