State Transition Diagram
A graphical notation which shows the states of a system and the transitions among those states. See state-based testing
State diagrams are used to describe the
behavior of a system. State diagrams describe all of the possible states
of an object as events occur. Each diagram usually represents objects of
a single class and track the different states of its objects through the
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
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
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.
The diagram below shows a super-state.
Both the Checking and Dispatching states can transition into the Canceled
state, so a transition is shown from a super-state named Active to the
state Cancel. By contrast, the state Dispatching can only transition to
the Delivered state, so we show an arrow only from the Dispatching state to
the Delivered state.