http://www.java.devsitehosting.com Developing Applications with Java Wed, 29 Aug 2007 16:32:38 +0000 http://backend.userland.com/rss092 en visit operation to the abstract Visitor class, and you must add an implementation for that class to each concrete Visitor you have written. Visitors can be powerful additions when the program reaches the point where many new classes are unlikely. Visitors can be used very ... http://www.java.devsitehosting.com/2007/08/29/visit-operation-to-the-abstract-visitor-class-and/ 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 ... http://www.java.devsitehosting.com/2007/08/29/double-dispatching-no-article-on-the-visitor-pattern/ total_days += emp.getVacDays(); } } Note that while in this case Boss is derived from Employee, it need not be related at all as long as it has an accept method for the Visitor class. It is quite important, ... http://www.java.devsitehosting.com/2007/08/28/total_days-empgetvacdays-note-that-while/ Visiting Several Classes The Visitor becomes more useful, when there are a number of different classes with different interfaces and we want to encapsulate how we get data from these classes. Let s extend our vacation days model by introducing a new Employee type called ... http://www.java.devsitehosting.com/2007/08/28/visiting-several-classes-the-visitor-becomes-more-useful/ Notice that there is no indication what the Visitor does with teach class in either the client classes or the abstract Visitor class. We can in fact write a whole lot of visitors that do different things to the classes in our program. The Visitor we are ... http://www.java.devsitehosting.com/2007/08/27/notice-that-there-is-no-indication-what-the/ different interfaces. Visitors are also valuable if you have to perform a number of unrelated operations on these classes. On the other hand, as we will see below, Visitors are a good choice only when you do not expect many new classes to be added to ... http://www.java.devsitehosting.com/2007/08/27/different-interfaces-visitors-are-also-valuable-if-you/ drawObject Rectangle Circle Triangle draw The question that most people who first review this pattern ask is what does visiting mean? There is only one way that an outside class can gain access to another class, and that ... http://www.java.devsitehosting.com/2007/08/26/drawobject-rectangle-circle-triangle-draw-the-question-that/ THE VISITOR PATTERN The Visitor pattern turns the tables on our object-oriented model and creates an external class to act on data in other classes. This is useful if there are a fair number of instances of a small number of classes and ... http://www.java.devsitehosting.com/2007/08/26/the-visitor-pattern-the-visitor-pattern-turns-the/ other class in between. Now, when you call the draw method, the derived class moves up the inheritance tree until it finds an implementation of draw. Likewise, for each method called from within draw, the derived class starts at the currently class and moves up the tree ... http://www.java.devsitehosting.com/2007/08/25/other-class-in-between-now-when-you-call/ public void paint(Graphics g) { for (int i = 0; i < triangles.size(); i++) { Triangle tngl = (Triangle)triangles.elementAt(i); tngl.draw(g); } } } An example of two standard triangles is shown below in the left window, ... http://www.java.devsitehosting.com/2007/08/25/public-void-paintgraphics-g-for-int-i/