Welcome!

Java IoT Authors: Elizabeth White, Scott Allen, Liz McMillan, Roger Strukhoff, Kevin Benedict

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
Why do your mobile transformations need to happen today? Mobile is the strategy that enterprise transformation centers on to drive customer engagement. In his general session at @ThingsExpo, Roger Woods, Director, Mobile Product & Strategy – Adobe Marketing Cloud, covered key IoT and mobile trends that are forcing mobile transformation, key components of a solid mobile strategy and explored how brands are effectively driving mobile change throughout the enterprise.
SYS-CON Events announced today that ReadyTalk, a leading provider of online conferencing and webinar services, has been named Vendor Presentation Sponsor at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. ReadyTalk delivers audio and web conferencing services that inspire collaboration and enable the Future of Work for today’s increasingly digital and mobile workforce. By combining intuitive, innovative tec...
If you’re responsible for an application that depends on the data or functionality of various IoT endpoints – either sensors or devices – your brand reputation depends on the security, reliability, and compliance of its many integrated parts. If your application fails to deliver the expected business results, your customers and partners won't care if that failure stems from the code you developed or from a component that you integrated. What can you do to ensure that the endpoints work as expect...
WebRTC adoption has generated a wave of creative uses of communications and collaboration through websites, sales apps, customer care and business applications. As WebRTC has become more mainstream it has evolved to use cases beyond the original peer-to-peer case, which has led to a repeating requirement for interoperability with existing infrastructures. In his session at @ThingsExpo, Graham Holt, Executive Vice President of Daitan Group, will cover implementation examples that have enabled ea...
There is growing need for data-driven applications and the need for digital platforms to build these apps. In his session at 19th Cloud Expo, Muddu Sudhakar, VP and GM of Security & IoT at Splunk, will cover different PaaS solutions and Big Data platforms that are available to build applications. In addition, AI and machine learning are creating new requirements that developers need in the building of next-gen apps. The next-generation digital platforms have some of the past platform needs a...
So, you bought into the current machine learning craze and went on to collect millions/billions of records from this promising new data source. Now, what do you do with them? Too often, the abundance of data quickly turns into an abundance of problems. How do you extract that "magic essence" from your data without falling into the common pitfalls? In her session at @ThingsExpo, Natalia Ponomareva, Software Engineer at Google, provided tips on how to be successful in large scale machine learning...
In his session at @ThingsExpo, Kausik Sridharabalan, founder and CTO of Pulzze Systems, Inc., will focus on key challenges in building an Internet of Things solution infrastructure. He will shed light on efficient ways of defining interactions within IoT solutions, leading to cost and time reduction. He will also introduce ways to handle data and how one can develop IoT solutions that are lean, flexible and configurable, thus making IoT infrastructure agile and scalable.
SYS-CON Events announced today that Numerex Corp, a leading provider of managed enterprise solutions enabling the Internet of Things (IoT), will exhibit at the 19th International Cloud Expo | @ThingsExpo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Numerex Corp. (NASDAQ:NMRX) is a leading provider of managed enterprise solutions enabling the Internet of Things (IoT). The Company's solutions produce new revenue streams or create operating...
In his general session at 18th Cloud Expo, Lee Atchison, Principal Cloud Architect and Advocate at New Relic, discussed cloud as a ‘better data center’ and how it adds new capacity (faster) and improves application availability (redundancy). The cloud is a ‘Dynamic Tool for Dynamic Apps’ and resource allocation is an integral part of your application architecture, so use only the resources you need and allocate /de-allocate resources on the fly.
IoT is fundamentally transforming the auto industry, turning the vehicle into a hub for connected services, including safety, infotainment and usage-based insurance. Auto manufacturers – and businesses across all verticals – have built an entire ecosystem around the Connected Car, creating new customer touch points and revenue streams. In his session at @ThingsExpo, Macario Namie, Head of IoT Strategy at Cisco Jasper, will share real-world examples of how IoT transforms the car from a static p...
Fifty billion connected devices and still no winning protocols standards. HTTP, WebSockets, MQTT, and CoAP seem to be leading in the IoT protocol race at the moment but many more protocols are getting introduced on a regular basis. Each protocol has its pros and cons depending on the nature of the communications. Does there really need to be only one protocol to rule them all? Of course not. In his session at @ThingsExpo, Chris Matthieu, co-founder and CTO of Octoblu, walk you through how Oct...
Web Real-Time Communication APIs have quickly revolutionized what browsers are capable of. In addition to video and audio streams, we can now bi-directionally send arbitrary data over WebRTC's PeerConnection Data Channels. With the advent of Progressive Web Apps and new hardware APIs such as WebBluetooh and WebUSB, we can finally enable users to stitch together the Internet of Things directly from their browsers while communicating privately and securely in a decentralized way.
"My role is working with customers, helping them go through this digital transformation. I spend a lot of time talking to banks, big industries, manufacturers working through how they are integrating and transforming their IT platforms and moving them forward," explained William Morrish, General Manager Product Sales at Interoute, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
The Internet of Things can drive efficiency for airlines and airports. In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect with GE, and Sudip Majumder, senior director of development at Oracle, will discuss the technical details of the connected airline baggage and related social media solutions. These IoT applications will enhance travelers' journey experience and drive efficiency for the airlines and the airports. The session will include a working demo and a technical d...
Developing software for the Internet of Things (IoT) comes with its own set of challenges. Security, privacy, and unified standards are a few key issues. In addition, each IoT product is comprised of (at least) three separate application components: the software embedded in the device, the back-end service, and the mobile application for the end user’s controls. Each component is developed by a different team, using different technologies and practices, and deployed to a different stack/target –...
Identity is in everything and customers are looking to their providers to ensure the security of their identities, transactions and data. With the increased reliance on cloud-based services, service providers must build security and trust into their offerings, adding value to customers and improving the user experience. Making identity, security and privacy easy for customers provides a unique advantage over the competition.
SYS-CON Events announced today that Commvault, a global leader in enterprise data protection and information management, has been named “Bronze Sponsor” of SYS-CON's 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Commvault is a leading provider of data protection and information management solutions, helping companies worldwide activate their data to drive more value and business insight and to transform moder...
SYS-CON Events has announced today that Roger Strukhoff has been named conference chair of Cloud Expo and @ThingsExpo 2016 Silicon Valley. The 19th Cloud Expo and 6th @ThingsExpo will take place on November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. "The Internet of Things brings trillions of dollars of opportunity to developers and enterprise IT, no matter how you measure it," stated Roger Strukhoff. "More importantly, it leverages the power of devices and the Interne...
Personalization has long been the holy grail of marketing. Simply stated, communicate the most relevant offer to the right person and you will increase sales. To achieve this, you must understand the individual. Consequently, digital marketers developed many ways to gather and leverage customer information to deliver targeted experiences. In his session at @ThingsExpo, Lou Casal, Founder and Principal Consultant at Practicala, discussed how the Internet of Things (IoT) has accelerated our abil...
Digital innovation is the next big wave of business transformation based on digital technologies of which IoT and Big Data are key components, For example: Business boundary innovation is a challenge to excavate third-party business value using IoT and BigData, like Nest Business structure innovation may propose re-building business structure from scratch, as Uber does in the taxicab industry The social model innovation is also a big challenge to the new social architecture with the design fr...