Home > Articles > Software Development & Management > UML

Analysis: Defining Object Behavior

Behavior binds the structure of objects to their attributes and relationships so that objects can meet their responsibilities. Ultimately, an object's operations implement its behavior. In this chapter Bruce Powel Douglass tells us how we can define and refine operations and behaviors. There are a number of means for specifying overall object behavior, the most important of these being modeling the object as a finite state machine.
This chapter is from the book

Chapter 6 showed how to define the system structure by identifying the fundamental objects and classes and their relationships. In this chapter, we define and refine operations and behaviors. There are a number of means for specifying overall object behavior, the most important of these being modeling the object as a finite state machine. Scenario modeling helps you test your behavioral models to ensure that the objects can collaborate to achieve the system responsibilities. The state and scenario models lead to the definitions of class operations required to process the incoming messages and events.

Notation and Concepts Discussed

Simple Behavior

Or-state

Action

Continuous Behavior

And-state

Pseudostate

State Behavior

Event

Statechart

State

Transition

Operation

7.1 Object Behavior

Chapter 6 presented the analysis and decomposition of systems into their object structure and relationships. The other pillar of object-oriented analysis is the specification of dynamic behavior. Behavior binds the structure of objects to their attributes and relationships so that objects can meet their responsibilities. Ultimately, an object's operations implement its behavior. There are means for constraining and controlling these primitive operations into permissible sequences. The most important of these is the finite state machine. The behavioral notations and semantics of the UML were introduced in Chapter 3; this chapter focuses on how to effectively apply these notations and semantics to solve our real-world problems.

7.1.1 Simple Behavior

We define three types of behavior: simple, state, and continuous. The object with simple behavior performs services on request and keeps no memory of previous services. A simple object always responds to a given input in exactly the same way regardless of its history. Some examples of simple behaviors are

  • Simple mathematical functions, such as cosine or square root

  • A search operation of a static data structure that always starts from the same point, such as the search of a static binary tree

  • Sort operations

  • A knob that returns the number of clicks for a given user action

For example,

07equ01.gif

returns the same value regardless of what value can call the COS() function with previously. In other cases, the distinction is not as clear. Is the search of a binary tree simple or state-driven? If the behavior changes due to previous input, then it cannot by definition be simple. If the binary tree provides methods like next() and previous(), then it must maintain an internal state between calls. If the tree only provides calls such as find(), then at least to its clients it exhibits stateless behavior.

Activity diagrams provide token-flow semantics. That is, an activity is decomposed into subactivities, until at the bottom we find actions. Actions execute when they receive a control token from each of their predecessor activities. In the case of simple sequential actions within an activity, this is just a statement that an activity (or action) begins to execute when the activity (or action) that comes before it completes. In the case of concurrency, an activity (or action) can have multiple predecessors, and so must receive a control token from every one of its predecessors before it is allowed to execute.

Figure 7-1 shows an activity diagram for computing net worth. The forks and joins show activities that may be allowed to execute in parallel. Token flow semantics are simple to explain in the figure:

  • For a sequential flow (simple arrow between lines) a control token is passed when the previous activity completes; the subsequent activity begins once it receives the control token.

  • For a fork, a control token is passed to every subsequent activity, meaning that they are all free to execute, and the order of execution of its concurrent peer activities is not defined.

  • For a join, a control token must be passed by every predecessor activity before the subsequent activity is allowed to run. Thus, all of the following activities must complete before the "print Assets" activity is allowed to run:

    • Assets += Get Property Assets

    • Assets += Get Bank Assets

    • Assets += Get Investment Assets

  • For branches (the diamond), only a single control token is passed. The active branch is selected on the basis of guard conditions; if a guard is TRUE then that branch may be passed the control token when the predecessor activity completes. If multiple guards evaluate to TRUE, then any of the true branches may be selected, but only one will receive the control token (however, it is impossible to say which one will be selected).

07fig01.gifFigure 7-1. Token-Flow Semantics

7.1.2 State Behavior

The second type of object behavior is called state, state-driven, or reactive. Our definition of a state is as follows:

A state is an ontological condition that persists for a significant period of time that is distinguishable from other such conditions and is disjoint from them. A distinguishable state means that it differs from other states in the events it accepts, the transitions it takes as a result of accepting those events, or the actions it performs. A transition is a response to an event that causes a change in state.

Modeling an object as a finite state machine (FSM) attempts to reduce the behavioral complexity by making some simplifying assumption. Specifically, it assumes the following:

  • The system being modeled can assume only a finite number of existence conditions, called states.

  • The system behavior within a given state is essentially identical and is defined by

    • The messages and events accepted

    • The actions associated entering or leaving the state

    • The activities performed while in the state

    • The reachability graph of subsequent states

    • The complete set of transition-target state pairs

  • The system resides in states for significant periods of time.

  • The system may change these conditions only in a finite number of well-defined ways, called transitions.

  • Transitions run to completion, including the action execution, whether they are associated directly with the transition itself, or entry or exit actions for the state entered or exited.

The set of all possible behaviors of an object is bounded by the set of operations defined on the object. An FSM adds constraints regarding when and under what conditions those operations will execute.

7.1.3 Continuous Behavior

The third kind of object behavior is called continuous. Many objects show continuous behavior, including digital filters and PID1 control loops. All that is required is that the current output depend on the previous history in a smooth way. An object with continuous behavior is one with an infinite, or at least unbounded, set of existence conditions. PID control systems, fuzzy sets, and neural networks all display continuous behavior.

The UML is very expressly a discrete modeling language, and provides no direct means for modeling continuous system behavior.2 However, many kinds of continuous behavior can be defined using token flow semantics. This is normally done by expressing the algorithm as difference equations (derived from the differential equations). Thus, activity diagrams can be used to express behavior that is continuous in value but discrete in execution. A PID control loop, for example, implemented with a difference equation, is executed periodically, and the object uses attributes to remember the previous value(s) necessary for the computations. However, activity diagrams cannot, without extension, model behavior that is continuous in time, such as the continuously executing mixing of gases, or combustion of fuel. It is often sufficient to use a difference equation approach for the behavioral modeling, but not always.

It is even sometimes appropriate to mix state and continuous behavior. For example, different sets of trajectory differential equations may be used, depending on whether a spacecraft is undergoing launch, achieving orbit, in orbit, or cruising. The sets of equations used in this case depend on the state (in this case, phase of flight) of the trajectory object. This is accomplished by modeling the overall behavior of the object as a state machine; the set of difference or differential equations is modeled as a "do activity" within the states. As the object enters a new state, a different set of differential equations governs the execution of the activity within the state.

Since the UML relies so heavily on finite state machines to represent discrete behavior, let's now explore what than means in more detail.

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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