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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.

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