Home > Articles > Programming > Java

UML Class Diagrams for Java Programmers

UML class diagrams allow us to denote the static contents of — and the relationships between — classes. In this chapter, Robert Martin explains the basics of UML class diagrams in a practical way.

Read UML for Java Programmers and more than 24,000 other books and videos on Safari Books Online. Start a free trial today.



This chapter is from the book

This chapter is from the book

UML class diagrams allow us to denote the static contents of — and the relationships between — classes. In a class diagram we can show the member variables, and member functions of a class. We can also show whether one class inherits from another, or whether it holds a reference to another. In short, we can depict all the source code dependencies between classes.

This can be valuable. It can be much easier to evaluate the dependency structure of a system from a diagram than from source code. Diagrams make certain dependency structures visible. We can see dependency cycles, and determine how best to break them. We can see when abstract classes depend upon concrete classes, and determine a strategy for rerouting such dependencies.

The Basics

Classes

Figure 3-1 shows the simplest form of class diagram. The class named Dialer is represented as a simple rectangle. This diagram represents nothing more than the code shown to its right.

03fig01.gifFigure 3.1 Class icon.

This is the most common way you will represent a class. The classes on most diagrams don't need any more than their name to make clear what is going on.

A class icon can be subdivided into compartments. The top compartment is for the name of the class, the second is for the variables of the class, and the third is for the methods of the class. Figure 3-2 shows these compartments and how they translate into code.

03fig02.gifFigure 3.2 Class icon compartments with corresponding code.

Notice the character in front of the variables and functions in the class icon. A dash (–) denotes private, a hash (#) denotes protected, and a plus (+) denotes public.

The type of a variable, or a function argument is shown after the colon following the variable or argument name. Similarly, the return value of a function is shown after the colon following the function.

This kind of detail is sometimes useful, but should not be used very often. UML diagrams are not the place to declare variables and functions. Such declarations are better done in source code. Use these adornments only when they are essential to the purpose of the diagram.

Association

Associations between classes most often represent instance variables that hold references to other objects. For example, in Figure 3-3 we see an association between Phone and Button. The direction of the arrow tells us that Phone holds a reference to Button. The name near the arrowhead is the name of the instance variable. The number near the arrowhead tells us how many references are held.

03fig03.gif Figure 3.3 Association.

In Figure 3-3 we saw that 15 Button objects were connected to the Phone object. In Figure 3-4, we see what happens when there is no limit. A Phonebook is connected to many PhoneNumber objects. The star means many. In Java this is most commonly implemented with a Vector, a List, or some other container type.

03fig04.gifFigure 3.4

You may have noticed that I avoided using the word "has". I could have said: "A Phonebook has many PhoneNumbers." This was intentional. The common OO verbs HASA and ISA have lead to a number of unfortunate misunderstandings. We'll explore some of them later in Chapter 6. For now, don't expect me to use the common terms. Rather, I'll use terms that are descriptive of what actually happens in software, such as: "is connected to."

Inheritance

You have to be very careful with your arrowheads in UML. Figure 3-5 shows why. The little arrowhead pointing at Employee denotes inheritance1. If you draw your arrowheads carelessly, it may be hard to tell whether you mean inheritance or association. To make it clearer, I often make inheritance relationships vertical and associations horizontal.

03fig05.gif Figure 3.5 Inheritance.

In UML all arrowheads point in the direction of source code dependency. Since it is the SalariedEmployee class that mentions the name of Employee, the arrowhead points at Employee. So, in UML, inheritance arrows point at the base class.

UML has a special notation for the kind of inheritance used between a Java class and a Java interface. It is shown, in Figure 3-6, as a dashed inheritance arrow2. In the diagrams to come, you'll probably catch me forgetting to dash the arrows that point to interfaces. I suggest you forget to dash the arrows that you draw on white boards too. Life's too short to be dashing arrows.

03fig06.gifFigure 3.6 Realizes relationship.

Figure 3-7 shows another way to convey the same information. Interfaces can be drawn as little lollipops on the classes that implement them. We often see this kind of notation in COM designs.

03fig07.gifFigure 3.7 Lollipop interface Indicator.

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