Home > Articles > Programming > Java

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

The Details

There are a vast number of details and adornments that can be added to UML class diagrams. Most of the time these details and adornments should not be added. But there are times when they can be helpful.

Class stereotypes

Class stereotypes appear between guillemet3 characters, usually above the name of the class. We have seen them before. The «interface» denotation in Figure 3-8 is a class stereotype. «interface» is one of two standard stereotypes that can be used by Java programmers. The other is «utility».


All the methods of classes marked with this stereotype are abstract. None of the methods can be implemented. Moreover, «interface» classes can have no instance variables. The only variables they can have are static variables. This corresponds exactly to Java interfaces. See Figure 3-9.

03fig09.gifFigure 3.9 «interface» class stereotype.

I draw interfaces so often that spelling the whole stereotype out at the white board can be pretty inconvenient. So I often use the shorthand in the lower part of Figure 3-9 to make the drawing easier. It's not standard UML, but it's much more convenient.


All the methods and variables of a «utility» class are static. Booch4 used to call these class utilities. See Figure 3-10.

03fig10.gifFigure 3.10 «utility» class stereotype.

You can make your own stereotypes if you like. I often use stereotypes like «persistent», «C-API», «struct», or «function». You just have to make sure that the people who are reading your diagrams know what your stereotype means.

Abstract classes

In UML there are two ways to denote that a class or a method is abstract. You can write the name in italics, or you can use the {abstract} property. Both options are shown in Figure 3-11.

03fig11.gifFigure 3.11 Abstract classes.

It's a little difficult to write italics at a white board, and the {abstract} property is wordy. So at the white board, if I need to denote a class or method as abstract, I use the convention shown in Figure 3-12. Again, this isn't standard UML, but at the white board it is a lot more convenient.5

03fig12.gifFigure 3.12 Unofficial denotation of abstract classes.


Properties, like {abstract} can be added to any class. They represent extra information that's not usually part of a class. You can create your own properties at any time.

Properties are written in a comma separated list of name–value pairs, like this:

{author=Martin, date=20020429, file=shape.java, private}

The properties in the preceding example are not part of UML. The {abstract} property is the only defined property of UML that Java programmers would find useful.

If a property does not have a value, it is assumed to take the boolean value true. Thus, {abstract} and {abstract = true} are synonyms.

Properties are written below and to the right of the name of the class, as shown in Figure 3-13.

03fig13.gif Figure 3.13 Properties.

Other than the {abstract} property, I don't know when you'd this useful. Personally, in the many years that I've been writing UML diagrams, I've never had occasion to use class properties for anything.


Aggregation is a special form of association that connotes a "whole/part" relationship. Figure 3-14 shows how it is drawn and implemented. Notice that the implementation shown in Figure 3-14 is indistinguishable from association. That's a hint.

03fig14.gif Figure 3.14 Aggregation.

Unfortunately, UML does not provide a strong definition for this relationship. This leads to confusion because various programmers and analysts adopt their own pet definitions for the relationship. For that reason I don't use the relationship at all, and I recommend that you avoid it as well. In fact, this relationship has been dropped from UML 2.0.

The one hard rule that UML gives us regarding aggregations is simply this: A whole cannot be its own part. Therefore instances cannot form cycles of aggregations. A single object cannot be an aggregate of itself, two objects cannot be aggregates of each other, three objects cannot form a ring of aggregation, and so on. See Figure 3-15

03fig15.gifFigure 3.15 Illegal cycles of aggregation between instances.

I don't find this to be a particularly useful definition. How often am I concerned about making sure that instances form a directed acyclic graph? Not very often. Therefore I find this relationship useless in the kinds of diagrams I draw.


Composition is a special form of aggregation, as shown in Figure 3-16. Again, notice that the implementation is indistinguishable from association. However, this time the reason is not due to a lack of definition; this time it's because the relationship does not have a lot of use in a Java program. C++ programmers, on the other hand, find a lot of use for it.

03fig16.gif Figure 3.16 Composition.

The same rule applies to composition that applied to aggregation. There can be no cycles of instances. An owner cannot be its own ward. However, UML provides quite a bit more definition.

  • An instance of a ward cannot be owned simultaneously by two owners. The object diagram in Figure 3-17 is illegal. Note, however, that the corresponding class diagram is not illegal. An owner can transfer ownership of a ward to another owner.

    03fig17.gifFigure 3.17 Illegal composition.

  • The owner is responsible for the lifetime of the ward. If the owner is destroyed, the ward must be destroyed with it. If the owner is copied, the ward must be copied with it.

In Java destruction happens behind the scenes by the garbage collector, so there is seldom a need to manage the lifetime of an object. Deep copies are not unheard of, but the need to show deep copy semantics on a diagram is rare. So, though I have used composition relationships to describe some Java programs, such use is infrequent.

Figure 3-18 shows how composition is used to denote deep copy. We have a class named Address that holds many Strings. Each string holds one line of the address. Clearly, when you make a copy of the Address, you want the copy to change independently of the original. Thus, we need to make a deep copy. The composition relationship between the Address and the Strings indicates that copies need to be deep.6

03fig18.gifFigure 3.18 Deep copy is implied by composition.


Objects can hold arrays or vectors of other objects, or they can hold many of the same kind of objects in separate instance variables. In UML this can be shown by placing a multiplicity expression on the far end of the association. Multiplicity expressions can be simple numbers, ranges, or a combination of both. For example, Figure 3-19 shows a BinaryTreeNode, using a multiplicity of 2.

03fig19.gifFigure 3.19 Simple multiplicity.

Here are the allowable forms:

• Digit.

The exact number of elements.

• * or 0..*

Zero to many.

• 0..1

Zero or one. In Java this is often implemented with a reference that can be null.

• 1..*

One to many.

• 3..5

Three to five.

• 0, 2..5, 9..*

Silly, but legal.

Association stereotypes

Associations can be labeled with stereotypes that change their meaning. Figure 3-20 shows the ones that I use most often.

03fig20.gifFigure 3.20 Association stereotypes.

The «create» stereotype indicates that the target of the association is created by the source. The implication is that the source creates the target and then passes it around to other parts of the system. In the example I've shown a typical factory.

The «local» stereotype is used when the source class creates an instance of the target and holds it in a local variable. The implication is that the created instance does not survive the member function that creates it. Thus, it is not held by any instance variable nor passed around the system in any way.

The «parameter» stereotype shows that the source class gains access to the target instance though the parameter of one of its member functions. Again, the implication is that the source forgets all about this object once the member function returns. The target is not saved in an instance variable.

Using dashed dependency arrows, as the diagram shows, is a common and convenient idiom for denoting parameters. I usually prefer it to using the «parameter» stereotype.

The «delegate» stereotype is used when the source class forwards a member function invocation to the target. There are a number of design patterns where this technique is applied, such as PROXY, DECORATOR, and COMPOSITE7 . Since I use these patterns a lot, I find the notation helpful.

Inner classes

Inner (nested) classes are represented in UML with an association adorned with a crossed circle, as shown in Figure 3-21.

03fig21.gifFigure 3.21 Inner class.

Anonymous inner classes

One of Java's more interesting features is anonymous inner classes. While UML does not have an official stance on these, I find the notation in Figure 3-22 works well for me. It is concise and descriptive. The anonymous inner class is shown as a nested class that is given the «anonymous» stereotype, and is also given the name of the interface it implements.

03fig22.gifFigure 3.22 Anonymous inner class.

Association classes

Associations with multiplicity tell us that the source is connected to many instances of the target, but the diagram doesn't tell us what kind of container class is used. This can be depicted by using an association class, as shown in Figure 3-23.

03fig23.gifFigure 3.23 Association class.

Association classes show how a particular association is implemented. On the diagram they appear as a normal class connected to the association with a dashed line. As Java programmers we interpret this to mean that the source class really contains a reference to the association class, which in turn contains references to the target.

Association classes can also be used to indicate special forms of references, such as weak, soft, or phantom references. See Figure 3-24. On the other hand, this notation is a bit cumbersome and is probably better done with stereotypes as in Figure 3-25.

03fig24.gifFigure 3.24 Association class denoting a weak reference.

03fig25.gifFigure 3.25 Stereotype denoting a weak reference.

Association qualifiers

Association qualifiers are used when the association is implemented through some kind of key or token, instead of with a normal Java reference. The example in Figure 3-26 shows a LoginServlet associated with an Employee. The association is mediated by a member variable named empid, which contains the database key for the Employee.

03fig26.gifFigure 3.26 Association qualifier.

I find this notation useful in rare situations. Sometimes it's convenient to show that an object is associated to another through a database or dictionary key. It is important, however, that all the parties reading the diagram know how the qualifier is used to access the actual object. This is not something that's immediately evident from the notation.

  • + Share This
  • 🔖 Save To Your Account

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