Statechart diagrams

Statechart diagrams

Statechart diagrams shows how the object’s state changes as a result of events handled by the object

State

• A condition or situation in the life time of an object, during which, it satisfies some condition or performs some activity
• The state of an object is determined by :  
• values of the attributes. 
• relationships to other objects.

Event

• A significant occurrence in space and time
• Stimulus which can cause a state transition
• On the diagram, an event can be drawn using an operation name or by simply using an English phrase.
• Events can have arguments

Transition

• Relationship between two states, indicating that an object in the first state will enter the second state when an event occurs
• On the diagram, each transition is drawn as an arrow from the originating state to the succeeding state.
• Transitions can also be reflexive. Something may happen that causes an object to transition back to the state it is currently in.

Activity

• Ongoing non-atomic operation

Action

• Executable atomic operation
• An Entry Action is a behavior that occurs while the object is transitioning into the state.
• An Exit Action occurs as part of the transition out of a state

Guard condition

• Controls when a transition can or cannot occur.
• A guard condition is drawn along the transition line, after the event name, and enclosed in square brackets.
• Guard conditions are optional

A state diagram looks like a collection of vertices and arcs.

These diagrams serve to document the dynamic behavior of a class so that developers and analysts will have a clear understanding of its behavior.

Example

Exercise

A bank account object can be open, closed and overdrawn

Open – If balance is greater than zero

Closed – If customer has requested closure

Overdrawn – If balance is less than zero

On closure, remaining balance will be credited to customer•On overdraft, an e-mail notice will be sent to customer.