Common Mechanisms of UML

Introduction 

State Machines 

State machines indicate the sequences of states an object undergoes in response to events throughout its lifetime 

State machines are used to model 

• the behavior of an individual object 
• the dynamic aspects of a system 

State machines can visualize by,

• highlighting the flow control from activity to activity 
• highlighting the potential states of objects and the transition among those states

Statechart Diagram 

• Shows a state machine, emphasizing the flow of control from state to state 
• Statechart diagram is used to model 

dynamic aspects of a system 

the behavior of reactive objects 

• Important for constructing executable systems through forward and reverse engineering

A state machine refers to a behavior that recognizes the sequence of states an object undergoes in response to events throughout its lifetime.

A state refers to a situation of an object, where it satisfies some conditions, carries out some activity, or waits for some event. 

Name 

• A textual string 

Entry/Exit actions 

• Actions executed on entering and exiting the state 

Internal transitions 

• Transitions that are handled without causing a change in state 

Substates 

• The nested structure of a state 

Deferred events 

• A list of events that are not handled in that state, but are postponed and queued for handling by the object in another state 

Initial and final states 

• Initial state shows the beginning of the state machine and final state shows the ending of the state machine

Transitions 

• A transition is a relationship between two states 
• An object performs certain actions in the first state. It then enters the second state when specified events happen and specified conditions are satisfied 

The parts of transition are: 

Source state: State affected by transition 

Event trigger:  Event whose reception by the object in the source state makes the transition eligible to fire

Guard condition: A Boolean expression that is evaluated when the transition is triggered by the reception of event trigger

Effect: It is a behavior which executes when a transition fires

Action: State that is active after the completion of transition

Target state: An executable atomic computation