UML has nine kinds of modeling diagrams.They are
- Use case diagrams
- Class diagrams
- Object diagrams
- Sequence diagrams
- Collaboration diagrams
- Statechart diagrams
- Activity diagrams
- Component diagrams
- Deployment diagrams
Use case diagrams
Use case diagrams describe what a system does from the standpoint of an external observer. The emphasis is on what a system does rather than how.
Use case diagrams are closely connected to scenarios. A scenario is an example of what happens when someone interacts with the system. Here is a scenario for a medical clinic.
"A patient calls the clinic to make an appointment for a yearly checkup. The receptionist finds the nearest empty time slot in the appointment book and schedules the appointment for that time slot. "
A use case is a summary of scenarios for a single task or goal. An actor is who or what initiates the events involved in that task. Actors are simply roles that people or objects play. The picture below is a Make Appointment use case for the medical clinic. The actor is a Patient. The connection between actor and use case is a communication association (or communication for short).
Actors are stick figures. Use cases are ovals. Communications are lines that link actors to use cases.A use case diagram is a collection of actors, use cases, and their communications. We've put Make Appointment as part of a diagram with four actors and four use cases. Notice that a single use case can have multiple actors.
Use case diagrams are helpful in three areas.- determining features (requirements). New use cases often generate new requirements as the system is analyzed and the design takes shape.
- communicating with clients. Their notational simplicity makes use case diagrams a good way for developers to communicate with clients.
- generating test cases. The collection of scenarios for a use case may suggest a suite of test cases for those scenarios.
Class diagrams
A Class diagram gives an overview of a system by showing its classes and the relationships among them. Class diagrams are static -- they display what interacts but not what happens when they do interact.
The class diagram below models a customer order from a retail catalog. The central class is the Order. Associated with it are the Customer making the purchase and the Payment. A Payment is one of three kinds: Cash, Check, or Credit. The order contains OrderDetails (line items), each with its associated Item.
UML class notation is a rectangle divided into three parts: class name, attributes, and operations. Names of abstract classes, such as Payment, are in italics. Relationships between classes are the connecting links.
Our class diagram has three kinds of relationships.
Association -- a relationship between instances of the two classes. There is an association between two classes if an instance of one class must know about the other in order to perform its work. In a diagram, an association is a link connecting two classes.
Aggregation -- an association in which one class belongs to a collection. An aggregation has a diamond end pointing to the part containing the whole. In our diagram, Order has a collection of OrderDetails.
Generalization -- an inheritance link indicating one class is a superclass of the other. A generalization has a triangle pointing to the superclass. Payment is a superclass of Cash, Check, and Credit.
An association has two ends. An end may have a role name to clarify the nature of the association. For example, an OrderDetail is a line item of each Order.
A navigability arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item, but not the other way around. The arrow also lets you know who "owns" the association's implementation; in this case, OrderDetail has an Item. Associations with no navigability arrows are bi-directional.
The multiplicity of an association end is the number of possible instances of the class associated with a single instance of the other end. Multiplicities are single numbers or ranges of numbers. In our example, there can be only one Customer for each Order, but a Customer can have any number of Orders.
This table gives the most common multiplicities.
Multiplicities - Meaning
0..1 - zero or one instance. The notation n . . m indicates n to m instances.
0..* or * - no limit on the number of instances (including none).
1 - exactly one instance
1..* - at least one instance
Object diagrams
Object diagrams show instances instead of classes. They are useful for explaining small pieces with complicated relationships, especially recursive relationships.
This small class diagram shows that a university Department can contain lots of other Departments.
The object diagram below instantiates the class diagram, replacing it by a concrete example.
Each rectangle in the object diagram corresponds to a single instance. Instance names are underlined in UML diagrams. Class or instance names may be omitted from object diagrams as long as the diagram meaning is still clear.
Sequence diagrams
Class and object diagrams are static model views. Interaction diagrams are dynamic. They describe how objects collaborate.
A sequence diagram is an interaction diagram that details how operations are carried out -- what messages are sent and when. Sequence diagrams are organized according to time. The time progresses as you go down the page. The objects involved in the operation are listed from left to right according to when they take part in the message sequence.
Below is a sequence diagram for making a hotel reservation. The object initiating the sequence of messages is a Reservation window.
The Reservation window sends a makeReservation() message to a HotelChain. The HotelChain then sends a makeReservation() message to a Hotel. If the Hotel has available rooms, then it makes a Reservation and a Confirmation.
Collaboration diagrams
Collaboration diagrams are also interaction diagrams. They convey the same information as sequence diagrams, but they focus on object roles instead of the times that messages are sent. In a sequence diagram, object roles are the vertices and messages are the connecting links.
The object-role rectangles are labeled with either class or object names (or both). Class names are preceded by colons ( : ).
Each message in a collaboration diagram has a sequence number. The top-level message is numbered 1. Messages at the same level (sent during the same call) have the same decimal prefix but suffixes of 1, 2, etc. according to when they occur.
Objects have behaviors and state. The state of an object depends on its current activity or condition. A statechart diagram shows the possible states of the object and the transitions that cause a change in state.
Our example diagram models the login part of an online banking system. Logging in consists of entering a valid social security number and personal id number, then submitting the information for validation.
Logging in can be factored into four non-overlapping states: Getting SSN, Getting PIN, Validating, and Rejecting. From each state comes a complete set of transitions that determine the subsequent state.
An activity diagram is essentially a fancy flowchart. Activity diagrams and statechart diagrams are related. While a statechart diagram focuses attention on an object undergoing a process (or on a process as an object), an activity diagram focuses on the flow of activities involved in a single process. The activity diagram shows the how those activities depend on one another.
For our example, we used the following process.
"Withdraw money from a bank account through an ATM."
The three involved classes (people, etc.) of the activity are Customer, ATM, and Bank. The process begins at the black start circle at the top and ends at the concentric white/black stop circles at the bottom. The activities are rounded rectangles.
Activity diagrams can be divided into object swimlanes that determine which object is responsible for which activity. A single transition comes out of each activity, connecting it to the next activity.
A transition may branch into two or more mutually exclusive transitions. Guard expressions (inside [ ]) label the transitions coming out of a branch. A branch and its subsequent merge marking the end of the branch appear in the diagram as hollow diamonds.
A transition may fork into two or more parallel activities. The fork and the subsequent join of the threads coming out of the fork appear in the diagram as solid bars.
Component and deployment diagrams
A component is a code module. Component diagrams are physical analogs of class diagram. Deployment diagrams show the physical configurations of software and hardware.
The following deployment diagram shows the relationships among software and hardware components involved in real estate transactions.
The physical hardware is made up of nodes. Each component belongs on a node. Components are shown as rectangles with two tabs at the upper left.
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