Catch-All Operations Using Visitors in Java

Drawer PDF417 in Java Catch-All Operations Using Visitors
Catch-All Operations Using Visitors
Generating PDF 417 In Java
Using Barcode creation for Java Control to generate, create PDF-417 2d barcode image in Java applications.
In all of the previous cases, the Visitor class had a visit method for each visiting class, such as the following:
Print Bar Code In Java
Using Barcode printer for Java Control to generate, create bar code image in Java applications.
public void visit(Employee emp) {} public void visit(Boss emp) {}
Reading Bar Code In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
However, if you start subclassing your Visitor classes and adding new classes that might visit, you should recognize that some visit methods might not be satisfied by the methods in the derived class These might instead "fall through" to methods in one of the parent classes in which that object type is recognized This provides a way of specifying default Visitor behavior Note that every class must override accept(v) with its own implementation so that the return call vvisit(this) returns an object this of the correct type and not of the superclass's type Let's suppose that we introduce another layer of management into the company, discussed earlier in the chapter: the Manager Managers are subclasses of Employees, and now they have the privileges formerly reserved for Bosses, that is, extra vacation days And Bosses now have an additional reward: stock options If we run the same program to compute vacation days but do not revise out Visitor to look for Managers, it will recognize Managers as mere Employees and count only their regular vacation days and not their extra vacation days However, the catch-all parent class is good to have if subclasses might be added to the application from time to time and we want the Visitor operations to continue to run without modification
Encode PDF-417 2d Barcode In C#.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create PDF 417 image in VS .NET applications.
There are three ways to integrate the new Manager class into the visitor system In one, we could define a ManagerVisitor In the second, we could use the BossVisitor to handle both However, there could be conditions when it is not desirable to modify the Visitor structure continually In that third case, you could simply test for this in the EmployeeVisitor class
PDF417 Creation In .NET
Using Barcode creator for ASP.NET Control to generate, create PDF 417 image in ASP.NET applications.
public void visit (Employee emp) { total_days += empgetVacDays(); //special test in catch-all parent class if(emp instanceof Manager) total_days += ((Manager) emp)getBonusDays(); }
PDF417 Creation In .NET Framework
Using Barcode generation for Visual Studio .NET Control to generate, create PDF417 image in VS .NET applications.
While this seems "unclean" at first compared to defining classes properly, it can provide a method of catching special cases in derived classes without writing whole new Visitor program hierarchies This catch-all approach is discussed in some detail in the book Pattern Hatching [Vlissides, 1998]
PDF417 Creator In Visual Basic .NET
Using Barcode generator for .NET framework Control to generate, create PDF417 image in Visual Studio .NET applications.
Double Dispatching
Paint Bar Code In Java
Using Barcode creation for Java Control to generate, create barcode image in Java applications.
No discussion of the Visitor pattern is complete without mentioning that you are really dispatching a method twice in order 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 is this bidirectional calling that allows you to add more operations on any class that has an accept method, since each new visitor class can carry out whatever operations you might think of using the data available in these classes
Code128 Drawer In Java
Using Barcode creation for Java Control to generate, create Code128 image in Java applications.
Traversing a Series of Classes
EAN / UCC - 13 Printer In Java
Using Barcode printer for Java Control to generate, create GTIN - 13 image in Java applications.
In order to visit a series of classes, the calling program that passes the class instances to the Visitor must know about all of the existing instances of classes to be visited and must 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 the earlier example program, we used an array of objects, but any of the other methods would work equally well
UPC-A Drawer In Java
Using Barcode drawer for Java Control to generate, create UPC-A image in Java applications.
Consequences of the Visitor Pattern
Encoding USS Code 39 In Java
Using Barcode creator for Java Control to generate, create Code 3 of 9 image in Java applications.
The Visitor pattern is useful when you want to encapsulate fetching data from a number of instances of several classes Design Patterns suggest that the Visitor can provide additional functionality to a class without changing it However, it's preferable to say that a Visitor can add functionality to a collection of classes and encapsulate the methods that 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 provide However, the Visitor 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
International Standard Serial Number Encoder In Java
Using Barcode drawer for Java Control to generate, create International Standard Serial Number image in Java applications.
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 This is because each time that you add new classes that must be visited, you must add an abstract visit operation to the abstract Visitor class and an implementation for that class to each concrete Visitor that you've written Visitors can be powerful additions when the program reaches the point at which many new classes are unlikely They also can be used very effectively in Composite systems For example, the boss-employee system just illustrated could well be a Composite like the one used in 11, The Composite Pattern
Create Data Matrix ECC200 In VB.NET
Using Barcode drawer for .NET framework Control to generate, create ECC200 image in Visual Studio .NET applications.
Code 128 Code Set B Creation In .NET Framework
Using Barcode generator for VS .NET Control to generate, create Code 128A image in .NET applications.
Scan UPC Symbol In Visual Studio .NET
Using Barcode reader for .NET Control to read, scan read, scan image in .NET applications.
Code 39 Extended Generation In Visual Studio .NET
Using Barcode printer for ASP.NET Control to generate, create Code 39 image in ASP.NET applications.