Feeds

To iterate is human

Iterator versus the Enumeration Method

Combat fraud and increase customer satisfaction

Enumeration Method is a general design pattern, not just a language-specific idiom. In the right situation, it offers a number of significant benefits. For example, if you have a relatively stable set of actions you want to perform during an iteration, the fact that a language does not support closures becomes less of an issue. Passing blocks as objects is particularly effective when iteration is ad hoc but, when the kinds of loop action you have are well characterised and bounded, common actions can be wrapped up and provided as predefined Command classes.

Alternatively, when traversing aggregate data structures holding different kinds of elements, such as objects that represent the syntax structure of source code or a document, Enumeration Method with a Visitor interface offers a far simpler programming model than working multiple nested loops, multiple Iterator types and explicit runtime type checking. It is also much easier to write unit tests against such structures by passing Mock Objects to the Enumeration Method.

But what of the original observation that kicked off this article? If you recall:

There are many ways to offer traversal, but if the caller needs to be able to know the position of elements in some way there are essentially only three general designs that keep the collection's internal representation hidden from the caller.

Whenever you mention magic numbers — three in this case — you implicitly invite others to check your working (in a discipline where we don't do nearly as much reviewing and checking as we should, this is no bad thing). The claim is that I was off by one and that, including Enumeration Method (or Internal Iterator), there are four. Given that I seem to be quite a fan of Enumeration Method, have written about it going back many years, and even mention it in the very same article, is this correction correct? Not quite, and it is worth understanding why.

Had the problem to be solved just been how to provide some mechanism for traversal that kept the collection's internal representation hidden from the caller, three would have been right out. In fact, so would four. Here's a summary of the three approaches from the previous article:

First, it is possible to use an index into the collection. [...] The second approach is to internalise a cursor within the collection. [...] And the third option is to introduce an Iterator object.

And then add Enumeration Method. And then add Batch Method. And then you can choose whether or not to count separately variations of these or combinations that draw on more than one approach, such as Batch Iterator (also known as Chunky Iterator).

However, the phrasing of the design issue was more constrained:

... if the caller needs to be able to know the position of elements in some way...

This constraint is important because not only does it subset the available solutions, it also highlights a significant distinction between Iterator and Enumeration Method. While many choices are driven by idiom and context, keep in mind that the detail of the problem being solved casts an important vote (or veto) in choosing an appropriate solution.

In this particular case, one of Enumeration Method's strengths — that of fully encapsulating any concept of position and iteration mechanism from the caller — is a mismatch for the problem we want to solve. What is needed are solutions that have the property of both traversal and persistent position, hence the inclusion of Iterator and the exclusion of Enumeration Method.

So, to summarise, Iterator supports traversal by encapsulating position and offering traversal control whereas Enumeration Method supports traversal by encapsulating the whole loop and all of its associated mechanisms. Specific requirements and general context help to determine which is the more appropriate solution in a given situation. ®

Combat fraud and increase customer satisfaction

More from The Register

next story
Ubuntu 14.04 LTS: Great changes, but sssh don't mention the...
Why HELLO Amazon! You weren't here last time
This time it's 'Personal': new Office 365 sub covers just two devices
Redmond also brings Office into Google's back yard
Next Windows obsolescence panic is 450 days from … NOW!
The clock is ticking louder for Windows Server 2003 R2 users
Half of Twitter's 'active users' are SILENT STALKERS
Nearly 50% have NEVER tweeted a word
OpenBSD founder wants to bin buggy OpenSSL library, launches fork
One Heartbleed vuln was too many for Theo de Raadt
Got Windows 8.1 Update yet? Get ready for YET ANOTHER ONE – rumor
Leaker claims big release due this fall as Microsoft herds us into the CLOUD
Batten down the hatches, Ubuntu 14.04 LTS due in TWO DAYS
Admins dab straining server brows in advance of Trusty Tahr's long-term support landing
Red Hat to ship RHEL 7 release candidate with a taste of container tech
Grab 'near-final' version of next Enterprise Linux next week
Apple inaugurates free OS X beta program for world+dog
Prerelease software now open to anyone, not just developers – as long as you keep quiet
prev story

Whitepapers

Mobile application security study
Download this report to see the alarming realities regarding the sheer number of applications vulnerable to attack, as well as the most common and easily addressable vulnerability errors.
3 Big data security analytics techniques
Applying these Big Data security analytics techniques can help you make your business safer by detecting attacks early, before significant damage is done.
The benefits of software based PBX
Why you should break free from your proprietary PBX and how to leverage your existing server hardware.
Securing web applications made simple and scalable
In this whitepaper learn how automated security testing can provide a simple and scalable way to protect your web applications.
Combat fraud and increase customer satisfaction
Based on their experience using HP ArcSight Enterprise Security Manager for IT security operations, Finansbank moved to HP ArcSight ESM for fraud management.