Common Mechanisms of UML

Statechart Diagrams

✱A Statechart Diagrams illustrates a state machine, highlighting the flow of control from one state to another state.

Contents 

• Statechart diagram contains 

➤Simple states and composite states 

➤Transitions, including events and actions 

• It also contains notes and constraints

Uses 

To model reactive objects 

• Reactive object is idle till it receives an event 
• When an event is received the response depends on previous events 
• After responding, the object becomes idle again

Common Modeling Techniques 

Modeling Reactive Objects To model a reactive object:

• A class, a use case or the system as a whole can be modeled as a state machine 
• Select the initial and final states for the object 
• Indicate the pre and the post conditions of the initial and the final states that define what the modeled component (class, use case, etc.) should do 
• Establish the stable states of the object by considering the conditions in which the object may be present for some significant amount of time 
• Starting with the high level states of the object, identify their substates 
• Decide on the significant partial ordering of stable states over the lifetime of the object 
• Decide on the events that trigger the transitions from one state to another state 
• Model these events as triggers to different transitions that shift from one legal ordering of states to another 
• Add actions to these transitions/states. Find out ways to simplify the machine by the use of branches, joins, forks, history states, and substates 
• Make sure that all the states are reachable with some sequence of events and their corresponding transitions 
• Verify that no state is a dead end from which no sequence of events will transition the object out of that state 
• Trace through the state machine, either manually or by using the tools so as to check it against important sequences of events and their responses 
• The image shows the statechart diagram for determining a simple context-free language message : '<' string '>' string ';' 
• The first string indicates a tag; the second string represents the body of the message