Purpose

An activity diagram (AD) describes processes that involve more than one object. They are useful for showing workflow and parallel processing.

Compared with statechart diagram

Statechart and activity diagrams both describe state transitions and share many of the same elements. A statechart diagram is useful for describing the behavior of a single object across several use cases.

Example activity diagram

A sample activity diagram.

Control panel

The control panel of the activity diagram.

States

Action state

An action state represents an activity. If necessary, you can use another activity diagram to provide details about the activity; that is, you can use activity diagrams to decompose action states.

An action state corresponds to an STDs state with internal actions when the state with internal actions has at least one outgoing transition that occurs when the internal action completes.

Label

activity [(parameter[,parameter...])]

Start state

The start state begins the sequence of activities. It is the same as the initial state in an STD.

Label

name

Final state

The final state ends the sequence of activities. It is the same as the final state in an STD.

Label

name[(parameter[,parameter...])]

Transitions

Transition

Transitions (solid arrows) order the sequence of activities. Completing an activity fires the outgoing transitions of the action state representing the activity.

An action state must have at least one outgoing transition. If an action state has more than one outgoing transition, each must have a guard condition.

Label

See Events and Transitions.

Complex transition

A complex transition (a thick line) synchronizes activities. The AD shown previously (see Example activity diagram) includes a number of complex transitions:

• After the Choose Dinner activity completes, two activities occur: Find Recipe and Set Table. The complex transition indicates that these two activities can occur in any order.
• The next complex transition in the AD indicates that the Eat activity does not begin until after both the Cook and Set Table activities are complete.

Label

Same as for a Transition.

Decision

You can use a decision diamond to express nested decisions. In the AD shown previously (see Example activity diagram), the two outgoing transitions of the Choose Dinner activity reflect a decision about whether to eat in. The [do not eat in] transition leads to a second decision, represented by the decision diamond, about whether to eat out.

Label

No label.

Objects, Object Flows, and Signals

Object in state

An object in state represents instances of a specified object in a specified state. You must specify the object name. The state name is optional; if specified, the UML suite automatically encloses it in square brackets. Folding an object hides its state name.

Label

object-name [state-name]

Object flow

You use object flows (dashed arrows) to show how objects in state are used by action states and transitions. If an action state generates an object in state that is used by the subsequent action state, you can omit the transition (solid arrow) between the two states.

Label

No label.

Example of objects in state and object flows

In this example, completing the Take Order activity, creates an Order in the [entered] state. The Fill Order activity requires an Order in the [entered] state and creates an Order in the [filled] state.

Signal sending

A signal sending symbol shows a transition sending a signal. Do not label transitions connected to a signal sending symbol. Optionally, use an object flow to indicate the object that receives the signal.

Label

[(parameter[,parameter...])]

Signal receipt

A signal receipt symbol shows a transition receiving a signal. Do not label transitions connected to a signal receipt symbol. Optionally, use an object flow to indicate the object that sends the signal.

Label

[(parameter[,parameter...])]

Example of signal sending and receipt

In this example, the transition from Take Order to Fill Order sends an enterOrder signal to the Order object. The transition from Fill Order to Deliver Order receives an orderFilled signal from the Order object.

Partitions and Swimlanes

Partitions

You use partitions to group the states of an activity model. Typically, each partition corresponds to an organizational unit in a business model. You can use partitions to allocate characteristics or resources among the states of an activity model.

Label

No label.

Swimlane separator

You use swimlane separators to divide a partition into horizontal or vertical zones called swimlanes. A partition with no swimlane separators contains one swimlane.

Label

You label each swimlane with the name of the class or department responsible for the activity states in the swimlane.

Example

To create the two swimlanes, use a partition and a horizontal or vertical swimlane separator.