|By Nishanth Sastry||
|October 23, 2005 11:00 AM EDT||
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.
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.
|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",
In his session at @ThingsExpo, Chris Klein, CEO and Co-founder of Rachio, will discuss next generation communities that are using IoT to create more sustainable, intelligent communities. One example is Sterling Ranch, a 10,000 home development that – with the help of Siemens – will integrate IoT technology into the community to provide residents with energy and water savings as well as intelligent security. Everything from stop lights to sprinkler systems to building infrastructures will run ef...
May. 6, 2016 04:00 AM EDT Reads: 1,335
We’ve worked with dozens of early adopters across numerous industries and will debunk common misperceptions, which starts with understanding that many of the connected products we’ll use over the next 5 years are already products, they’re just not yet connected. With an IoT product, time-in-market provides much more essential feedback than ever before. Innovation comes from what you do with the data that the connected product provides in order to enhance the customer experience and optimize busi...
May. 6, 2016 02:00 AM EDT Reads: 1,408
Manufacturers are embracing the Industrial Internet the same way consumers are leveraging Fitbits – to improve overall health and wellness. Both can provide consistent measurement, visibility, and suggest performance improvements customized to help reach goals. Fitbit users can view real-time data and make adjustments to increase their activity. In his session at @ThingsExpo, Mark Bernardo Professional Services Leader, Americas, at GE Digital, will discuss how leveraging the Industrial Interne...
May. 6, 2016 01:45 AM EDT Reads: 1,434
The increasing popularity of the Internet of Things necessitates that our physical and cognitive relationship with wearable technology will change rapidly in the near future. This advent means logging has become a thing of the past. Before, it was on us to track our own data, but now that data is automatically available. What does this mean for mHealth and the "connected" body? In her session at @ThingsExpo, Lisa Calkins, CEO and co-founder of Amadeus Consulting, will discuss the impact of wea...
May. 6, 2016 01:00 AM EDT Reads: 1,259
Increasing IoT connectivity is forcing enterprises to find elegant solutions to organize and visualize all incoming data from these connected devices with re-configurable dashboard widgets to effectively allow rapid decision-making for everything from immediate actions in tactical situations to strategic analysis and reporting. In his session at 18th Cloud Expo, Shikhir Singh, Senior Developer Relations Manager at Sencha, will discuss how to create HTML5 dashboards that interact with IoT devic...
May. 6, 2016 12:00 AM EDT Reads: 1,487
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...
May. 5, 2016 11:30 PM EDT Reads: 1,358
Artificial Intelligence has the potential to massively disrupt IoT. In his session at 18th Cloud Expo, AJ Abdallat, CEO of Beyond AI, will discuss what the five main drivers are in Artificial Intelligence that could shape the future of the Internet of Things. AJ Abdallat is CEO of Beyond AI. He has over 20 years of management experience in the fields of artificial intelligence, sensors, instruments, devices and software for telecommunications, life sciences, environmental monitoring, process...
May. 5, 2016 07:45 PM EDT Reads: 1,511
The IoTs will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, will demonstrate how to move beyond today's coding paradigm and share the must-have mindsets for removing complexity from the development proc...
May. 5, 2016 06:00 PM EDT Reads: 1,058
A critical component of any IoT project is the back-end systems that capture data from remote IoT devices and structure it in a way to answer useful questions. Traditional data warehouse and analytical systems are mature technologies that can be used to handle large data sets, but they are not well suited to many IoT-scale products and the need for real-time insights. At Fuze, we have developed a backend platform as part of our mobility-oriented cloud service that uses Big Data-based approache...
May. 5, 2016 04:00 PM EDT Reads: 791
trust and privacy in their ecosystem. Assurance and protection of device identity, secure data encryption and authentication are the key security challenges organizations are trying to address when integrating IoT devices. This holds true for IoT applications in a wide range of industries, for example, healthcare, consumer devices, and manufacturing. In his session at @ThingsExpo, Lancen LaChance, vice president of product management, IoT solutions at GlobalSign, will teach IoT developers how t...
May. 5, 2016 03:45 PM EDT Reads: 767
Digital payments using wearable devices such as smart watches, fitness trackers, and payment wristbands are an increasing area of focus for industry participants, and consumer acceptance from early trials and deployments has encouraged some of the biggest names in technology and banking to continue their push to drive growth in this nascent market. Wearable payment systems may utilize near field communication (NFC), radio frequency identification (RFID), or quick response (QR) codes and barcodes...
May. 5, 2016 02:45 PM EDT Reads: 1,086
SYS-CON Events announced today that Peak 10, Inc., a national IT infrastructure and cloud services provider, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Peak 10 provides reliable, tailored data center and network services, cloud and managed services. Its solutions are designed to scale and adapt to customers’ changing business needs, enabling them to lower costs, improve performance and focus inter...
May. 5, 2016 02:30 PM EDT Reads: 1,496
We're entering the post-smartphone era, where wearable gadgets from watches and fitness bands to glasses and health aids will power the next technological revolution. With mass adoption of wearable devices comes a new data ecosystem that must be protected. Wearables open new pathways that facilitate the tracking, sharing and storing of consumers’ personal health, location and daily activity data. Consumers have some idea of the data these devices capture, but most don’t realize how revealing and...
May. 5, 2016 01:45 PM EDT Reads: 781
The demand for organizations to expand their infrastructure to multiple IT environments like the cloud, on-premise, mobile, bring your own device (BYOD) and the Internet of Things (IoT) continues to grow. As this hybrid infrastructure increases, the challenge to monitor the security of these systems increases in volume and complexity. In his session at 18th Cloud Expo, Stephen Coty, Chief Security Evangelist at Alert Logic, will show how properly configured and managed security architecture can...
May. 5, 2016 01:30 PM EDT Reads: 620
There is an ever-growing explosion of new devices that are connected to the Internet using “cloud” solutions. This rapid growth is creating a massive new demand for efficient access to data. And it’s not just about connecting to that data anymore. This new demand is bringing new issues and challenges and it is important for companies to scale for the coming growth. And with that scaling comes the need for greater security, gathering and data analysis, storage, connectivity and, of course, the...
May. 5, 2016 12:30 PM EDT Reads: 1,341
The IETF draft standard for M2M certificates is a security solution specifically designed for the demanding needs of IoT/M2M applications. In his session at @ThingsExpo, Brian Romansky, VP of Strategic Technology at TrustPoint Innovation, will explain how M2M certificates can efficiently enable confidentiality, integrity, and authenticity on highly constrained devices.
May. 5, 2016 12:30 PM EDT Reads: 1,371
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, will provide tips on how to be successful in large scale machine lear...
May. 5, 2016 10:15 AM EDT Reads: 1,584
You think you know what’s in your data. But do you? Most organizations are now aware of the business intelligence represented by their data. Data science stands to take this to a level you never thought of – literally. The techniques of data science, when used with the capabilities of Big Data technologies, can make connections you had not yet imagined, helping you discover new insights and ask new questions of your data. In his session at @ThingsExpo, Sarbjit Sarkaria, data science team lead ...
May. 5, 2016 09:00 AM EDT Reads: 1,298
SYS-CON Events announced today that Ericsson has been named “Gold Sponsor” of SYS-CON's @ThingsExpo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. Ericsson is a world leader in the rapidly changing environment of communications technology – providing equipment, software and services to enable transformation through mobility. Some 40 percent of global mobile traffic runs through networks we have supplied. More than 1 billion subscribers around the world re...
May. 5, 2016 08:45 AM EDT Reads: 1,411
You deployed your app with the Bluemix PaaS and it's gaining some serious traction, so it's time to make some tweaks. Did you design your application in a way that it can scale in the cloud? Were you even thinking about the cloud when you built the app? If not, chances are your app is going to break. Check out this webcast to learn various techniques for designing applications that will scale successfully in Bluemix, for the confidence you need to take your apps to the next level and beyond.
May. 3, 2016 12:15 PM EDT Reads: 1,665