Welcome!

Java IoT Authors: Elizabeth White, Pat Romanski, Yeshim Deniz, Liz McMillan, Stackify Blog

Related Topics: Java IoT

Java IoT: Article

Deriving the Visitor Pattern: A Review and Discussion

Why is the visitor pattern the way it is?

Like most other self-respecting developers I had also read the GoF book, including the section on the visitor pattern. However, when a colleague came over to me with a question, I could not initially justify the complexity of the example code I saw in the book. What follows is a discussion of why the visitor pattern is the way it is.

Brief Review of the Pattern
The definitive description of the pattern is in the GoF book Design Patterns, Chapter 5 (pp 331-344)(see References section). The Wikipedia has a concise and good description, which formed the basis for my brief review here. The visitor pattern is classified as a Behavioral pattern, so the thing to notice is the way in which the classes and objects interact and distribute responsibility. A typical application of this pattern occurs in the following scenario: we have a number of elements in an object structure (common structures include trees & lists) and we want to perform a bunch of disparate operations (e.g. printing or cloning each element) on the elements of the structure.

The visitor pattern is a way of separating the operation from the object structure and a way of collecting together the different implementations of an operation for different kinds of elements in the object structure. A Visitor class is created which knows how to perform a particular operation on the different kinds of elements in the object structure. Each type of element in the structure defines an accept() method that can accept any kind of Visitor. The visitor is passed to each element in the structure in turn, by calling its accept() method and the Visitor then performs the operation on the visited element. One important consequence of this separation of object structure and operation is that we can later add a new operation (a new kind of Visitor) without having to modify the element classes of the object structure.

Each type of Visitor defines several visit()methods, one for each kind of element. The basic insight is that the precise set of instructions to execute (i.e. the method or function to call) depends on the run-time types of both the Visitor & the visited element. Java only lets us call different methods based on the run-time type of one object (via virtual functions), so the pattern advocates a clever solution: The second dependency on the type of element visited is first resolved by polymorphically calling the accept() method of the visited element. accept() then resolves the first dependency by turning around and polymorphically calling the visit()method for its class.

An Example
Before this description gets too confusing, let us study the pattern in the context of a concrete problem: Let us say we need to traverse a list collecting node-specific information. The list has two kinds of nodes, say, Red and Black, which needed to be processed differently. It seems like an ideal application for the visitor pattern. Listing 1 shows the code. (All code samples in this article use a J2SE 5.0 compatible compiler.)

To me and my colleague, this initially seemed like an overly complex solution for a simple problem. NodeVisitor.doVisit() calls into the Node's accept methods, which simply delegates back into NodeVisitor. Furthermore, the accept() methods of RedNode and BlackNode are almost identical. Finally, notice that if we now add a GreenNode class, we need to add a new visitGreen() method to the NodeVisitor class and re-compile it (not to speak of the almost redundant implementation of accept() in the GreenNode class). Ugh! This does not seem kosher by any OO standard.

The Need for the accept() Methods
Novice armchair Java developers might ask why we can't do something simpler, like Listing 2, for example, without touching the Node interface, or the classes RedNode and BlackNode which implement it.

Listing 2 has two significant differences from the previous. First, there is no redundant method (namely accept()) for each node type to implement. Second, we use function name overloading for the visit() implementations, thus enabling the "clever" foreach loop, which iterates over each node and calls the appropriate overloaded version of visit() depending on the type of the current element. With this, we hope to contain all the visiting logic within NodeVisitor.

Alas, real developers have a more difficult job than arm-chair developers! If you are using a language like Java or C++, an overloaded function name like visit() has to get resolved at compile time. Thus line 6.iii will not compile because none of the visit() methods provided in NodeVisitor know how to accept a generic "Node" as argument.

For line 6.iii to work the way we want it to, the decision on what operation needs to be performed has to be delayed until we can determine at runtime the type of the node n being examined in the current iteration of the for-each loop.

Traditional OO languages (Java, C++ etc) provide us with one standard tool for delaying function resolution until run-time: virtual functions. Thus, in Listing 1, 6.iii is modified to a virtual function call n.accept(nv). So the actual function that gets called is decided at run-time. The version called then delegates work by invoking the right version of NodeVisitor.visit().

So Why Not Just Use Plain Vanilla Inheritance?
The explanation I just gave is good, but not good enough. I can almost hear you ask: why doesn't accept() do the work itself? Why does it have to delegate back to NodeVisitor? There are three reasons:

1.  Accumulating state: If you read the problem I presented closely, you will notice that I specified a need to collect node-specific information. Since the doVisit passes the same NodeVisitor instance to each accept(), the visitor can be used to accumulate state across the different Node objects. For example, say you have an Employee HR application where the Red nodes represent employees, the Black nodes represent managers, visitRed() calculates the pay raises for programmers, and visitBlack the pay raises for managers. The NodeVisitor nv could print a report of the total increase in salary expense at the end of the for loop.

2.  Supporting more than one visitor (the need for double dispatch): Say the next version of your Employee HR application needs to add a new HRPolicyVisitor that checks for compliance with some HR policy and the implementation is different for managers and programmers.

To accommodate both the types of Visitors, we introduce an additional layer of indirection - an abstract EmployeeNodeVisitor interface with virtual visitXXX() functions for each type of element to visit, namely visitProgrammer() & visitManager(). The old PayRaiseVisitor and the new HRPolicyVisitor both implement EmployeeNodeVisitor. The decision on which version of visit() gets called now gets determined by a two-step process. The first step is as before. The node type of the visited element n in the foreach loop determines which version of the virtual function accept() gets called. In the second step, the type of the EmployeeVisitor passed in to accept() determines the (virtual function) version of visitXXX() called. The source files that come with this article show the skeleton of this implementation. Figure 1 illustrates the sequence of calls from both doPayHike(), which uses a PayRaiseVisitor to raise the pay of each employee, and doEnforcePolicy() which uses a HRPolicyVisitor to check HR policy compliance.

More Stories By Nishanth Sastry

Nishanth Sastry is a software developer at IBM working on the WebSphere Portal & Workplace family of products. He has a Masters degree in Computer Science from the University of Texas at Austin. He enjoys well-written code & fall in New England, among other things. He lives in Concord, Mass.

Comments (2) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Rishi Sharma 12/27/05 06:01:14 AM EST

good article. Well laid with sequence diagram and code.

Bruce Wallace 10/23/05 06:31:14 PM EDT

In your Oct, 2005 JDJ article "Deriving the Visitor Pattern",
the author correctly describes the need for double dispatch
and hence the traditional approach of using an "accept"
method. However, in my article from 2001, I showed how
accept methods can be done away with via a couple of lines of
reflection code. See the article listed below for the technique
that can make the Visitor pattern more widely applicable since
the visited objects need not cooperate by defining an accept
method ahead of time.

[1] http://www.polyglotinc.com/articles.html
[2] http://www.ftponline.com/Archives/premier/mgznarch/javapro/2001/03mar01/...

@ThingsExpo Stories
No hype cycles or predictions of zillions of things here. IoT is big. You get it. You know your business and have great ideas for a business transformation strategy. What comes next? Time to make it happen. In his session at @ThingsExpo, Jay Mason, Associate Partner at M&S Consulting, presented a step-by-step plan to develop your technology implementation strategy. He discussed the evaluation of communication standards and IoT messaging protocols, data analytics considerations, edge-to-cloud tec...
Announcing Poland #DigitalTransformation Pavilion
Digital Transformation is much more than a buzzword. The radical shift to digital mechanisms for almost every process is evident across all industries and verticals. This is often especially true in financial services, where the legacy environment is many times unable to keep up with the rapidly shifting demands of the consumer. The constant pressure to provide complete, omnichannel delivery of customer-facing solutions to meet both regulatory and customer demands is putting enormous pressure on...
DXWorldEXPO LLC announced today that All in Mobile, a mobile app development company from Poland, will exhibit at the 22nd International CloudEXPO | DXWorldEXPO. All In Mobile is a mobile app development company from Poland. Since 2014, they maintain passion for developing mobile applications for enterprises and startups worldwide.
CloudEXPO | DXWorldEXPO are the world's most influential, independent events where Cloud Computing was coined and where technology buyers and vendors meet to experience and discuss the big picture of Digital Transformation and all of the strategies, tactics, and tools they need to realize their goals. Sponsors of DXWorldEXPO | CloudEXPO benefit from unmatched branding, profile building and lead generation opportunities.
The best way to leverage your CloudEXPO | DXWorldEXPO presence as a sponsor and exhibitor is to plan your news announcements around our events. The press covering CloudEXPO | DXWorldEXPO will have access to these releases and will amplify your news announcements. More than two dozen Cloud companies either set deals at our shows or have announced their mergers and acquisitions at CloudEXPO. Product announcements during our show provide your company with the most reach through our targeted audienc...
@DevOpsSummit at Cloud Expo, taking place November 12-13 in New York City, NY, is co-located with 22nd international CloudEXPO | first international DXWorldEXPO and will feature technical sessions from a rock star conference faculty and the leading industry players in the world.
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution. In his session at @ThingsExpo, Akvelon expert and IoT industry leader Sergey Grebnov provided an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.
DXWorldEXPO | CloudEXPO are the world's most influential, independent events where Cloud Computing was coined and where technology buyers and vendors meet to experience and discuss the big picture of Digital Transformation and all of the strategies, tactics, and tools they need to realize their goals. Sponsors of DXWorldEXPO | CloudEXPO benefit from unmatched branding, profile building and lead generation opportunities.
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
In his keynote at 19th Cloud Expo, Sheng Liang, co-founder and CEO of Rancher Labs, discussed the technological advances and new business opportunities created by the rapid adoption of containers. With the success of Amazon Web Services (AWS) and various open source technologies used to build private clouds, cloud computing has become an essential component of IT strategy. However, users continue to face challenges in implementing clouds, as older technologies evolve and newer ones like Docker c...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
JETRO showcased Japan Digital Transformation Pavilion at SYS-CON's 21st International Cloud Expo® at the Santa Clara Convention Center in Santa Clara, CA. The Japan External Trade Organization (JETRO) is a non-profit organization that provides business support services to companies expanding to Japan. With the support of JETRO's dedicated staff, clients can incorporate their business; receive visa, immigration, and HR support; find dedicated office space; identify local government subsidies; get...
Dion Hinchcliffe is an internationally recognized digital expert, bestselling book author, frequent keynote speaker, analyst, futurist, and transformation expert based in Washington, DC. He is currently Chief Strategy Officer at the industry-leading digital strategy and online community solutions firm, 7Summits.
Bill Schmarzo, author of "Big Data: Understanding How Data Powers Big Business" and "Big Data MBA: Driving Business Strategies with Data Science," is responsible for setting the strategy and defining the Big Data service offerings and capabilities for EMC Global Services Big Data Practice. As the CTO for the Big Data Practice, he is responsible for working with organizations to help them identify where and how to start their big data journeys. He's written several white papers, is an avid blogge...
In past @ThingsExpo presentations, Joseph di Paolantonio has explored how various Internet of Things (IoT) and data management and analytics (DMA) solution spaces will come together as sensor analytics ecosystems. This year, in his session at @ThingsExpo, Joseph di Paolantonio from DataArchon, added the numerous Transportation areas, from autonomous vehicles to “Uber for containers.” While IoT data in any one area of Transportation will have a huge impact in that area, combining sensor analytic...
Bill Schmarzo, author of "Big Data: Understanding How Data Powers Big Business" and "Big Data MBA: Driving Business Strategies with Data Science," is responsible for setting the strategy and defining the Big Data service offerings and capabilities for EMC Global Services Big Data Practice. As the CTO for the Big Data Practice, he is responsible for working with organizations to help them identify where and how to start their big data journeys. He's written several white papers, is an avid blogge...
Charles Araujo is an industry analyst, internationally recognized authority on the Digital Enterprise and author of The Quantum Age of IT: Why Everything You Know About IT is About to Change. As Principal Analyst with Intellyx, he writes, speaks and advises organizations on how to navigate through this time of disruption. He is also the founder of The Institute for Digital Transformation and a sought after keynote speaker. He has been a regular contributor to both InformationWeek and CIO Insight...
Michael Maximilien, better known as max or Dr. Max, is a computer scientist with IBM. At IBM Research Triangle Park, he was a principal engineer for the worldwide industry point-of-sale standard: JavaPOS. At IBM Research, some highlights include pioneering research on semantic Web services, mashups, and cloud computing, and platform-as-a-service. He joined the IBM Cloud Labs in 2014 and works closely with Pivotal Inc., to help make the Cloud Found the best PaaS.
It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...