Java Web Hosting for Developers

Double Dispatching No article on the Visitor pattern is complete without mentioning that you are really dispatching a method twice for the Visitor to work. The Visitor calls the polymorphic accept method of a given object, and the accept method calls the polymorphic visit method of the Visitor. It this bidirectional calling that allows you to add more operations on any class that has an accept method, since each new Visitor class we write can carry out whatever operations we might think of using the data available in these classes. Traversing a Series of Classes The calling program that passes the class instances to the Visitor must know about all the existing instances of classes to be visited and mus keep them in a simple structure such as an array or Vector. Another possibility would be to create an Enumeration of these classes and pass it to the Visitor. Finally, the Visitor itself could keep the list of objects that it is to visit. In our simple example program, we used an array of objects, but any of the other methods would work equally well. Consequence of the Visitor Pattern The Visitor pattern is useful when you want to encapsulate fetching data from a number of instances of several classes. Design Patterns suggests that the Visitor can provide additional functionality to a class without changing it. We prefer to say that a Visitor can add functionality to a collection of classes and encapsulate the methods it uses. The Visitor is not magic, however, and cannot obtain private data from classes: it is limited to the data available from public methods. This might force you to provide public methods that you would otherwise not have provided. However, it can obtain data from a disparate collection of unrelated classes and utilize it to present the results of a global calculation to the user program. It is easy to add new operations to a program using Visitors, since the Visitor contains the code instead of each of the individual classes. Further, Visitors can gather related operations into a single class rather than forcing you to change or derive classes to add these operations. This can make the program simpler to write and maintain. Visitors are less helpful during a program s growth stage, since each time you add new classes which must be visited, you have to add an abstract

Note: If you are looking for good and high quality web space to host and run your application check Lunarwebhost Java Web Hosting services