Home > Articles > Programming > Ruby

Practical Object-Oriented Design in Ruby: Managing Dependencies

Because well designed objects have a single responsibility, their very nature requires that they collaborate to accomplish complex tasks. This collaboration is powerful and perilous. To collaborate, an object must know something know about others. Knowing creates a dependency. If not managed carefully, these dependencies will strangle your application.
This chapter is from the book

Object-oriented programming languages contend that they are efficient and effective because of the way they model reality. Objects reflect qualities of a real-world problem and the interactions between those objects provide solutions. These interactions are inescapable. A single object cannot know everything, so inevitably it will have to talk to another object.

If you could peer into a busy application and watch the messages as they pass, the traffic might seem overwhelming. There’s a lot going on. However, if you stand back and take a global view, a pattern becomes obvious. Each message is initiated by an object to invoke some bit of behavior. All of the behavior is dispersed among the objects. Therefore, for any desired behavior, an object either knows it personally, inherits it, or knows another object who knows it.

The previous chapter concerned itself with the first of these, that is, behaviors that a class should personally implement. The second, inheriting behavior, will be covered in Chapter 6, Acquiring Behavior Through Inheritance. This chapter is about the third, getting access to behavior when that behavior is implemented in other objects.

Because well designed objects have a single responsibility, their very nature requires that they collaborate to accomplish complex tasks. This collaboration is powerful and perilous. To collaborate, an object must know something know about others. Knowing creates a dependency. If not managed carefully, these dependencies will strangle your application.

Understanding Dependencies

An object depends on another object if, when one object changes, the other might be forced to change in turn.

Here’s a modified version of the Gear class, where Gear is initialized with four familiar arguments. The gear_inches method uses two of them, rim and tire, to create a new instance of Wheel. Wheel has not changed since you last you saw it in Chapter 2, Designing Classes with a Single Responsibility.

1 class Gear
2  attr_reader :chainring, :cog, :rim, :tire
3  def initialize(chainring, cog, rim, tire)
4    @chainring = chainring
5    @cog      = cog
6    @rim      = rim
7    @tire     = tire
8  end
10 def gear_inches
11   ratio * Wheel.new(rim, tire).diameter
12 end
14 def ratio
15    chainring / cog.to_f
16 end
17# ...
20class Wheel
21  attr_reader :rim, :tire
22  def initialize(rim, tire)
23    @rim      = rim
24    @tire     = tire
25  end
27  def diameter
28    rim + (tire * 2)
29  end
30 # ...
31 end
33 Gear.new(52, 11, 26, 1.5).gear_inches

Examine the code above and make a list of the situations in which Gear would be forced to change because of a change to Wheel. This code seems innocent but it’s sneakily complex. Gear has at least four dependencies on Wheel, enumerated as follows. Most of the dependencies are unnecessary; they are a side effect of the coding style. Gear does not need them to do its job. Their very existence weakens Gear and makes it harder to change.

Recognizing Dependencies

An object has a dependency when it knows

  • The name of another class. Gear expects a class named Wheel to exist.
  • The name of a message that it intends to send to someone other than self. Gear expects a Wheel instance to respond to diameter.
  • The arguments that a message requires. Gear knows that Wheel.new requires a rim and a tire.
  • The order of those arguments. Gear knows the first argument to Wheel.new should be rim, the second, tire.

Each of these dependencies creates a chance that Gear will be forced to change because of a change to Wheel. Some degree of dependency between these two classes is inevitable, after all, they must collaborate, but most of the dependencies listed above are unnecessary. These unnecessary dependencies make the code less reasonable. Because they increase the chance that Gear will be forced to change, these dependencies turn minor code tweaks into major undertakings where small changes cascade through the application, forcing many changes.

Your design challenge is to manage dependencies so that each class has the fewest possible; a class should know just enough to do its job and not one thing more.

Coupling Between Objects (CBO)

These dependencies couple Gear to Wheel. Alternatively, you could say that each coupling creates a dependency. The more Gear knows about Wheel, the more tightly coupled they are. The more tightly coupled two objects are, the more they behave like a single entity.

If you make a change to Wheel you may find it necessary to make a change to Gear. If you want to reuse Gear, Wheel comes along for the ride. When you test Gear, you’ll be testing Wheel too.

Figure 3.1 illustrates the problem. In this case, Gear depends on Wheel and four other objects, coupling Gear to five different things. When the underlying code was first written everything worked fine. The problem lies dormant until you attempt to use Gear in another context or to change one of the classes upon which Gear depends. When that day comes the cold hard truth is revealed; despite appearances, Gear is not an independent entity. Each of its dependencies is a place where another object is stuck to it. The dependencies cause these objects to act like a single thing. They move in lockstep; they change together.

Figure 3.1

Figure 3.1. Dependencies entangle objects with one another.

When two (or three or more) objects are so tightly coupled that they behave as a unit, it’s impossible to reuse just one. Changes to one object force changes to all. Left unchecked, unmanaged dependencies cause an entire application to become an entangled mess. A day will come when it’s easier to rewrite everything than to change anything.

Other Dependencies

The remainder of this chapter examines the four kinds of dependencies listed above and suggests techniques for avoiding the problems they create. However, before going forward it’s worth mentioning a few other common dependency related issues that will be covered in other chapters.

One especially destructive kind of dependency occurs where an object knows another who knows another who knows something; that is, where many messages are chained together to reach behavior that lives in a distant object. This is the “knowing the name of a message you plan to send to someone other than self” dependency, only magnified. Message chaining creates a dependency between the original object and every object and message along the way to its ultimate target. These additional couplings greatly increase the chance that the first object will be forced to change because a change to any of the intermediate objects might affect it.

This case, a Law of Demeter violation, gets its own special treatment in Chapter 4, Creating Flexible Interfaces.

Another entire class of dependencies is that of tests on code. In the world outside of this book, tests come first. They drive design. However, they refer to code and thus depend on code. The natural tendency of “new-to-testing” programmers is to write tests that are too tightly coupled to code. This tight coupling leads to incredible frustration; the tests break every time the code is refactored, even when the fundamental behavior of the code does not change. Tests begin to seem costly relative to their value. Test-to-code over-coupling has the same consequence as code-to-code over-coupling. These couplings are dependencies that cause changes to the code to cascade into the tests, forcing them to change in turn.

The design of tests is examined in Chapter 9, Designing Cost-Effective Tests.

Despite these cautionary words, your application is not doomed to drown in unnecessary dependencies. As long as you recognize them, avoidance is quite simple. The first step to this brighter future is to understand dependencies in more detail; therefore, it’s time to look at some code.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020