Class diagram is a very important part of UML. It’s a structure diagram and it’s purpose is to display classes in the system with all the relationships between them. In my opinion it’s the most popular type of diagram in software development.
Drawing class diagram of your design really helps to see the problem in broader terms. By writing it down you free space in your head for new ideas :-). It is also easier to understand by others when you want to discuss the problem with someone else. The thing is, I often find myself wondering about the syntax when I read someone else’s diagrams. That’s why I decided to make a little cheat sheet here to remind me.
Class
The UML diagram is a powerful tool which lets visually represent all system's components, the interactions between them and relationships with external user interface. The Rapid UML solution for ConceptDraw DIAGRAM software offers diversity of UML flowchart symbols for drawing all types of UML diagrams. I did all the examples in an open-source diagram editor called Dia. I recommend it by the way. And because it’s such a wonderful editor, here’s a complete cheat sheet (if you’d like to print it).
Kind of a key component in a class diagram. Classes will be shown as nodes and usually as boxes. Here is a example of one. Each class can have methods and attributes defined. The convention is shown on Figure 1.
Inheritance
Class inheritance is in terms of UML a relationship of generalization. It represents “is a” relationship on class level. Figure 2 shows how to portray generalization.
Figure 2: Inheritance
Realization
UML has different relationship for interfaces. When you inherit from an interface you implement it, which is in terms of UML a relationship of realization. It’s visual appearance is similar to inheritance, but the line is dashed. Also the interface class should be marked as abstract (have name written in italic). See on Figure 3.
Association
Another form of relationship in class diagram is association. It’s a object-level relationship (i.e. happens between objects of associated classes). So the whole relationship represents a family of links. There are multiple types of association with stronger policies (composition and aggregation).
Figure 4: Association
Aggregation
Aggregation is a stronger and more specific form of association. It’s “has a” relationship. Graphical representation of aggregation is shown on Figure 5.
Composition
Even stronger form of aggregation is composition. Instead of “has a” it represents “owns a”. It’s suited for relationship when one object can only exist as a part of another. For example if a plane has a wing it’s a composition. What would you do with a wing alone, right? The plane owns it. But when a pond has some ducks in it it’s an aggregation. The ducks will survive without a pond (only probably not that happy). And a pond will still be a pond with or without ducks. Graphical representation of composition is virtually the same as aggregation, only the diamond is filled (see on Figure 6).
Figure 6: Composition
Dependency
Last type of relationship is a dependency. It’s weaker then association and it says, that a class uses another one and therefore is dependant on it. The use of dependency is appropriate for example in cases where an instance of a class is stored as a local variable inside another classes’ method. Or some static methods are used, so the classes are not associated, but one depends on the other.
Cheat Sheet
I did all the examples in an open-source diagram editor called Dia. I recommend it by the way. And because it’s such a wonderful editor, here’s a complete cheat sheet (if you’d like to print it).
UML Class Diagram Cheat Sheet
A UML diagram is a partial graphical representation (view) of a model of a system under design, implementation, or already in existence. UML diagram contains graphical elements (symbols) - UML nodes connected with edges (also known as paths or flows) - that represent elements in the UML model of the designed system. The UML model of the system might also contain other documentation such as use cases written as templated texts.
The kind of the diagram is defined by the primary graphical symbols shown on the diagram. For example, a diagram where the primary symbols in the contents area are classes is class diagram. A diagram which shows use cases and actors is use case diagram. A sequence diagram shows sequence of message exchanges between lifelines.
Uml Component Diagram Cheat Sheet
UML specification does not preclude mixing of different kinds of diagrams, e.g. to combine structural and behavioral elements to show a state machine nested inside a use case. Consequently, the boundaries between the various kinds of diagrams are not strictly enforced. At the same time, some UML Tools do restrict set of available graphical elements which could be used when working on specific type of diagram.
Classification of UML 2.5 Diagrams
UML specification defines two major kinds of UML diagram: structure diagrams and behavior diagrams.
Uml Diagram Template
Structure diagrams show the static structure of the system and its parts on different abstraction and implementation levels and how they are related to each other. The elements in a structure diagram represent the meaningful concepts of a system, and may include abstract, real world and implementation concepts.
Behavior diagrams show the dynamic behavior of the objects in a system, which can be described as a series of changes to the system over time.
UML 2.5 diagrams could be categorized hierarchically as shown below. Note, items shown in blue are not part of official UML 2.5 taxonomy of diagrams.
UML 2.5 Diagrams Overview.
Note, items in blue are not part of official taxonomy of UML 2.5 diagrams.
UML 2.5 Structure Diagrams
Structure diagrams show static structure of the system and its parts on different abstraction and implementation levels and how those parts are related to each other. The elements in a structure diagram represent the meaningful concepts of a system, and may include abstract, real world and implementation concepts.
Structure diagrams are not utilizing time related concepts, do not show the details of dynamic behavior. However, they may show relationships to the behaviors of the classifiers exhibited in the structure diagrams.
| Diagram | Purpose | Elements |
|---|---|---|
| Class diagram | Shows structure of the designed system, subsystem or component as related classes and interfaces, with their features, constraints and relationships - associations, generalizations, dependencies, etc. | class, interface, feature, constraint, association, generalization, dependency. |
| Object diagram | Instance level class diagram which shows instance specifications of classes and interfaces (objects), slots with value specifications, and links (instances of association). Object diagram was defined in now obsolete UML 1.4.2 Specification as 'a graph of instances, including objects and data values. A static object diagram is an instance of a class diagram; it shows a snapshot of the detailed state of a system at a point in time.' It also stated that object diagram is 'a class diagram with objects and no classes.' UML 2.5 specification simply provides no definition of object diagram. | instance specification, object, slot, link. |
| Package diagram | Shows packages and relationships between the packages. | package, packageable element, dependency, element import, package import, package merge. |
| Model diagram | UML auxiliary structure diagram which shows some abstraction or specific view of a system, to describe architectural, logical or behavioral aspects of the system. It could show, for example, architecture of a multi-layered (aka multi-tiered) application - see multi-layered application model. | model, package, packageable element, dependency. |
| Composite structure diagram | Diagram could be used to show: | |
| Internal structure diagram | Shows internal structure of a classifier - a decomposition of the classifier into its properties, parts and relationships. | structured class, part, port, connector, usage. |
| Collaboration use diagram | Shows objects in a system cooperating with each other to produce some behavior of the system. | collaboration, connector, part, dependency. |
| Component diagram | Shows components and dependencies between them. This type of diagrams is used for Component-Based Development (CBD), to describe systems with Service-Oriented Architecture (SOA). | component, interface, provided interface, required interface, class, port, connector, artifact, component realization, usage. |
| Manifestation diagram | While component diagrams show components and relationships between components and classifiers, and deployment diagrams - deployments of artifacts to deployment targets, some missing intermediate diagram is manifestation diagram to be used to show manifestation (implementation) of components by artifacts and internal structure of artifacts. Because manifestation diagrams are not defined by UML 2.5 specification, manifestation of components by artifacts could be shown using either component diagrams or deployment diagrams. | manifestation, component, artifact. |
| Deployment diagram | Shows architecture of the system as deployment (distribution) of software artifacts to deployment targets. Note, that components were directly deployed to nodes in UML 1.x deployment diagrams. In UML 2.x artifacts are deployed to nodes, and artifacts could manifest (implement) components. Components are deployed to nodes indirectly through artifacts. Specification level deployment diagram (also called type level) shows some overview of deployment of artifacts to deployment targets, without referencing specific instances of artifacts or nodes. Instance level deployment diagram shows deployment of instances of artifacts to specific instances of deployment targets. It could be used for example to show differences in deployments to development, staging or production environments with the names/ids of specific build or deployment servers or devices. | deployment, artifact, deployment target, node, device, execution environment, communication path, deployment specification, |
| Network architecture diagram | Deployment diagrams could be used to show logical or physical network architecture of the system. This kind of deployment diagrams - not formally defined in UML 2.5 - could be called network architecture diagrams. | node, switch, router, load balancer, firewall, communication path, network segment, backbone. |
| Profile diagram | Auxiliary UML diagram which allows to define custom stereotypes, tagged values, and constraints as a lightweight extension mechanism to the UML standard. Profiles allow to adapt the UML metamodel for different
| profile, metaclass, stereotype, extension, reference, profile application. |
UML 2.5 Behavior Diagrams
Behavior diagrams show the dynamic behavior of the objects in a system, which can be described as a series of changes to the system over time.
| Diagram | Purpose | Elements |
|---|---|---|
| Use case diagram | Describes a set of actions (use cases) that some system or systems (subject) should or can perform in collaboration with one or more external users of the system (actors) to provide some observable and valuable results to the actors or other stakeholders of the system(s). Note, that UML 2.4.1 specification (see '16.4 Diagrams') stated that Use Case Diagrams are a specialization of Class Diagrams such that the classifiers shown are restricted to being either Actors or Use Cases.Class diagrams are structure diagrams. | use case, actor, subject, extend, include, association. |
| Information flow diagram | Shows exchange of information between system entities at some high levels of abstraction. Information flows may be useful to describe circulation of information through a system by representing aspects of models not yet fully specified or with less details. | information flow, information item, actor, class. |
| Activity diagram | Shows sequence and conditions for coordinating lower-level behaviors, rather than which classifiers own those behaviors. These are commonly called control flow and object flow models. | activity, partition, action, object, control, activity edge. |
| State machine diagram | Used for modeling discrete behavior through finite state transitions. In addition to expressing the behavior of a part of the system, state machines can also be used to express the usage protocol of part of a system. These two kinds of state machines are referred to as behavioral state machines and protocol state machines. | |
| Behavioral state machine diagram | Shows discrete behavior of a part of designed system through finite state transitions. | behavioral state, behavioral transition, pseudostate. |
| Protocol state machine diagram | Shows usage protocol or a lifecycle of some classifier, e.g. which operations of the classifier may be called in each state of the classifier, under which specific conditions, and satisfying some optional postconditions after the classifier transitions to a target state. | protocol state, protocol transition, pseudostate. |
| Interaction diagram | Interaction diagrams include several different types of diagrams:
| |
| Sequence diagram | Most common kind of interaction diagrams which focuses on the message interchange between lifelines (objects). | lifeline, execution specification, message, combined fragment, interaction use, state invariant, destruction occurrence. |
| Communication diagram (a.k.a. Collaboration diagram in UML 1.x) | Focuses on the interaction between lifelines where the architecture of the internal structure and how this corresponds with the message passing is central. The sequencing of messages is given through a sequence numbering scheme. | lifeline, message. |
| Timing diagram | Shows interactions when a primary purpose of the diagram is to reason about time. Timing diagrams focus on conditions changing within and among lifelines along a linear time axis. | lifeline, state or condition timeline, destruction event, duration constraint, time constraint. |
| Interaction overview diagram | Defines interactions through a variant of activity diagrams in a way that promotes overview of the control flow. Interaction overview diagrams focus on the overview of the flow of control where the nodes are interactions or interaction uses. The lifelines and the messages do not appear at this overview level. | initial node, flow final node, activity final node, decision node, merge node, fork node, join node, interaction, interaction use, duration constraint, time constraint. |
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