Home > Articles

Object Technology: Basic Concepts

OO design pioneer Rebecca Wirfs-Brock provides an introduction to basic concepts for object technology including object machinery, roles, object role stereotypes, responsibilities and collaborations, object contracts, and domain objects.

Wirfs-Brock is the lead author on the book Object Design: Roles, Responsibilities, and Collaborations and Object-Oriented Design. This is one in a series of short articles based on her work.

Like this article? We recommend

Object Machinery

All but the simplest of devices, both hardware and software, are designed from parts. These parts interact according to someone's plan. In a physical machine, these parts touch one another or communicate through a shared medium. Their interactions may give way to force, transfer motion, or conduct heat.

Like all good questions, "What is an object?" raises a number of others. How do objects help us think about a problem? How are object applications different? Once we have found an object solution, can we use it again for other purposes?

Software machinery is similar to physical machinery. A software application is constructed from parts. These parts—software objects—interact by sending messages to request information or action from others. Throughout its lifetime, each object remains responsible for responding to a fixed set of requests. To fulfill these requests, objects encapsulate scripted responses and the information that they base them on (see Figure 1). If an object is designed to remember certain facts, it can use them to respond differently to future requests.

Figure 1. An object encapsulates scripts and information.

So how do we invent these software machines?

At the heart of object-oriented software development there is a violation of real-world physics. We have a license to reinvent the world, because modeling the real world in our machinery is not our goal.

Building an object-oriented application means inventing appropriate machinery. We represent real-world information, processes, interactions, relationships, even errors, by inventing objects that don't exist in the real world. We give life and intelligence to inanimate things. We take difficult-to-comprehend real-world objects and split them into simpler, more manageable software ones. We invent new objects. Each has a specific role to play in the application. Our measure of success lies in how clearly we invent a software reality that satisfies our application's requirements—and not in how closely it resembles the real world.

For example, filling out and filing a form seems simple. But to perform that task in software, behind the simple forms, the application is validating the data against business rules, reading and refreshing the persistent data, guaranteeing the consistency of the information, and managing simultaneous access by dozens of users. Software objects display information, coordinate activities, compute, or connect to services. The bulk of this machine is our invention! We follow a real-world metaphor—forms and files—but our object model includes a much richer set of concepts that are realized as objects.

Because they have machine-like behaviors and because they can be plugged together to work in concert, objects can be used to build very complex machines. To manage this complexity, we divvy the system's behaviors into objects that play well-defined roles. If we keep our focus on the behavior, we can design the application using several complementary perspectives:

An application = a set of interacting objects
An object = an implementation of one or more roles
A role = a set of related responsibilities
A responsibility = an obligation to perform a task or know information
A collaboration = an interaction of objects or roles (or both)
A contract = an agreement outlining the terms of a collaboration

"We take a handful of sand from the endless landscape of awareness around us and call that handful of sand the world. Once we have the handful of sand, the world of which we are conscious, a process of discrimination goes to work on it. This is the knife. We divide the sand into parts. This and that. Here and there. Black and white. Now and then. The discrimination is the division of the conscious universe into parts."
—Robert Pirsig


No object exists in isolation. It is always part of a bigger machine. To fit in, an object has a specific purpose—a role it plays within a given context. Objects that play the same role can be interchanged. For example, there are several providers that can deliver letters and packages: DHL, FedEx, UPS, Post, Airborne. They all have the same purpose, if not the same way of carrying out their business. You choose from among them according to the requirements that you have for delivery. Is it one-day, book rate, valuable, heavy, flammable? You pick among the mail carriers that meet your requirements.

A role is a set of responsibilities that can be used interchangeably.

It is useful to think about an object, asking, "What role does it play?" This helps us concentrate on what it should be and what it should do. We have been speaking of objects and roles loosely. What is the real difference? When a role is always played by the same kind of object, the two are equivalent. But if more than one kind of object can fulfill the same responsibilities within the community, a role becomes a set of responsibilities that can be fulfilled in different ways. A role is a slot in the software machinery to be filled with an appropriate object as the program runs.

Object Role Stereotypes

A well-defined object supports a clearly defined role. We use purposeful oversimplifications, or role stereotypes, to help focus an object's responsibilities. Stereotypes are characterizations of the roles needed by an application. Because our goal is to build consistent and easy-to-use objects, it is advantageous to stereotype objects, ignoring specifics of their behaviors and thinking about them at a higher level. By oversimplifying and characterizing it, we can ponder the nature of an object's role more easily. We find these stereotypes to be useful:

  • Information holder—knows and provides information
  • Structurer—maintains relationships between objects and information about those relationships
  • Service provider—performs work and, in general, offers computing services
  • Coordinator—reacts to events by delegating tasks to others
  • Controller—makes decisions and closely directs others' actions
  • Interfacer—transforms information and requests between distinct parts of our system

Just as an actor tries to play a believable part in a play, an object takes on a character in an application by assuming responsibilities that define a meaningful role.

Software machinery is made of computation of information, maintenance of relationships, control of external programs and devices, formatting of information for display, responding to external events and inputs, error handling, and decision making.

Once we assign and characterize an object's role, its attendant responsibilities will follow. An object may fit into more than one stereotype.

But is it playing one or two roles? Often we find that a service provider holds information that it needs to provide its service. In doing so, it assumes two stereotypes—information holder and service provider—but only one role because the responsibilities are all wrapped up together for the same customers to use. If its information is being used solely to support its service, it assumes two stereotypes but only one role. But if it is perceived as serving two different types of clients for different purposes, it is likely playing two roles.

Some objects are hard to stereotype because they seem to fit into more than one category. They're fuzzy. How can you choose? You must decide what you want to emphasize. A transmission is a service provider if you emphasize the multiplication of power by the gears. It is an interfacer if you emphasize its connections to the engine and wheels. Can objects have more than one stereotype? If you want to emphasize more than one aspect, that's OK. There are blends of stereotypes, just as there are blends of emphasis.

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