State Diagrams

Behavioral Modeling Diagrams

State Diagrams

State diagrams are used to graphically represent finite state machines. State transition tables are another possible representation.

There are many forms of state diagrams, which differ slightly and have different semantics.

When to Use: State Diagrams?

Use state diagrams to demonstrate the behavior of an object through many use cases of the system. Only use state diagrams for classes where it is necessary to understand the behavior of the object through the entire system. Not all classes will require a state diagram and state diagrams are not useful for describing the collaboration of all objects in a use case. State diagrams are other combined with other diagrams such as interaction diagrams and activity diagrams.

State Diagram

The state diagram shows the change of an object through time. Based upon events that occur, the state diagram shows how the object changes from start to finish.

States are represented as a rounded rectangle with the name of the state shown. Optionally you can include an activity that represents a longer running task during that state.

Connecting states together are transitions. These represent the events that cause the object to change from one state to another. The guard clause of the label is again mutually exclusive and must resolve itself to be either true or false. Actions represent tasks that run causing the transitions.

Actions are different from activities in that actions cannot be interrupted, while an activity can be interrupted by an incoming event. Both ultimately represent an operation on the object being studied. For example, an operation that sets an attribute would be considered an action, while a long calculation might be an activity. The specific separation between the two depends on the object and the system being studied. Like activity diagrams, state diagrams have one start and one end from at which the state transitions start and end respectively.

How to Draw: State Diagrams

State diagrams have very few elements. The basic elements are rounded boxes representing the state of the object and arrows indicting the transition to the next state. The activity section of the state symbol depicts what activities the object will be doing while it is in that state.

All state diagrams being with an initial state of the object. This is the state of the object when it is created. After the initial state the object begins changing states. Conditions based on the activities can determine what the next state the object transitions to.

Below is an example of a state diagram might look like for an Order object. When the object enters the Checking state it performs the activity "check items". After the activity is completed the object transitions to the next state based on the conditions [all items available] or [an item is not available]. If an item is not available the order is canceled. If all items are available then the order is dispatched. When the object transitions to the Dispatching state the activity "initiate delivery" is performed. After this activity is complete the object transitions again to the Delivered state.

State diagrams can also show a super-state for the object. A super-state is used when many transitions lead to the a certain state. Instead of showing all of the transitions from each state to the redundant state a super-state can be used to show that all of the states inside of the super-state can transition to the redundant state. This helps make the state diagram easier to read.