From UML 1.x to UML 2.0

UML 2 represents the biggest change that's happened yet to the UML. All sorts of things have changed with this revision, and many changes have affected UML Distilled.

Within the UML, there have been deep changes to the UML meta-model. Although these changes don't affect the discussion in UML Distilled, they are very important to some groups.

One of the most obvious changes is the introduction of new diagram types. Object diagrams and package diagrams were widely drawn before but weren't official diagram types; now they are. UML 2 changed the name of collaboration diagrams to communication diagrams. UML 2 has also introduced new diagram types: interaction overview diagrams, timing diagrams, and composite structure diagrams.

A lot of changes haven't touched UML Distilled. I've left out discussion of such constructs as state machine extensions, gates in interactions diagrams, and power types in class diagrams.

So for this section, I'm discussing only changes that make it into UML Distilled. These are either changes to things I discussed in previous editions or new things I've started to discuss with this edition. Because the changes are so widespread, I've organized them according to the chapters in this book.

Class Diagrams: The Essentials (Chapter 3)

Attributes and unidirectional associations are now primarily simply different notations for the same underlying concept of property. Discontinuous multiplicities, such as [2, 4], have been dropped. The frozen property has been dropped. I've added a list of common dependency keywords, several of which are new to UML 2. The «parameter», and «local» keywords have been dropped.

Sequence Diagrams (Chapter 4)

The big change here is the interaction frame notation for sequence diagrams to handle iterative, conditional, and various other controls of behavior. This now allows you to express algorithms pretty completely in sequence diagrams, although I'm not convinced that these are any clearer than code. The old iteration markers and guards on messages have been dropped from sequence diagrams. The heads of the lifelines are no longer instances; I use the term participant to refer to them. The collaboration diagrams of UML 1 were renamed to communication diagrams for UML 2.

Class Diagrams: Concepts (Chapter 5)

Stereotypes are now more tightly defined. As a result, I now refer to words in guillemets as keywords, only some of which are stereotypes. Instances on object diagrams are now instance specifications. Classes can now require interfaces as well as provide them. Multiple classification uses generalization sets to group generalizations into groups. Components are no longer drawn with their special symbol. Active objects have double vertical lines instead of thick lines.

State Machine Diagrams (Chapter 10)

UML 1 separated short-lived actions from long-lived activities. UML 2 calls both activities and uses the term do-activity for the long-lived activities.

Activity Diagrams (Chapter 11)

UML 1 treated activity diagrams as a special case of state diagram. UML 2 broke that link and as a result removed the rules of matching forks and joins that UML 2 activity diagrams had to keep to. As a result, they are best understood by token flow rather than by state transition. A whole bunch of new notation thus appeared, including time and accept signals, parameters, join specifications, pins, flow transformations, subdiagram rakes, expansion regions, and flow finals.

A simple but awkward change is that UML 1 treated multiple incoming flows to an activity as an implicit merge, while UML 2 treats them as an implicit join. For this reason, I advise using an explicit merge or join when doing activity diagrams.

Swim lanes can now be multidimensional and are generally called partitions.