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  Sequence Diagrams

Introduction to Sequence Diagrams

Sequence  diagrams  and collaboration diagrams are basically  interaction diagrams. So  first we should  understand  what are  these  interaction diagrams.

Interaction diagrams  shown an   interaction  consisting of  a set of  objects and  their  relationship including the  messages that may  be dispatched among them.

So sequence diagrams  is an interaction  diagram that emphasize the structural  organization of the  objects. They send receive  messages. They model  the dynamic  aspects of a system. The  context of the   sequence  diagrams ( or  interacting  diagrams ) can  be  understand  by  a simple  analogy.

The analogy of sequence Diagrams

Consider  a system  which is in running  state. For gaining insight to  the system, a section of money  has to  be observed for  the change that take  place over  the time. With a bit  of more  focus  several  objects of interest  may be revealed over a period  of time.  The creation of objects changes in their  attributes and then  the destruction  of some  of the  objects may  be seen.

Concept of sequence Diagrams

Sequence diagram  emphasizes basically the time  ordering  of message. It  can be   represented by a table  with,

X - axis : represents objects

Y - axis: to have  messages arranged in  increasing time  sequence diagram to contain objects  links  and messages

It  sequence  diagram  objects  that initiate  the  interaction are placed at the  left  and more subordinate objects are placed increasingly  to the right. A sequence  diagram  for  elevator  system  is shown  in fig 8.24,

The message that  the objects send  and receive are  placed  along  the y axis  in the  order of increasing  times  from   top to bottom.

This  ordering  of objects  and message  gives  a clear  view  of control  over to time.

In fig  8.25,  a sequence  diagram  is shown  for  the flow  from  the database to  browser.

Consider one more  example  of sequence  diagram. In a simple  two party  phone  call system we can  assume followings things.

1. There  are two  objects  of class caller  that  stands for  sender and r  that  for  receiver. There  is an  objects d of the switch  an lastly there  is an object c  class conversation. It is  shown in fig 8.26.

2. Every  objects  has a lifeline shown  in fig 8.27, by a vertical  dashed  line. This line  represents  existence  of an  objects over a period  of time. An objects  may be  either present from the very  beginning  or it may possible  that an  objects has been  created during  an interaction.  So an objects  can be  either be created or  can be  destroyed and the life  of the  objects  is thus  represented  by a  lifeline.

3. Another  important  part of a sequence diagram  is focus of control. It  is shown  by a thin  rectangle that  is drawn  over the lifeline. The  focus  of control  shows  the period during  which  an objects  is performing  an action.

4. Returning to the  example  of two  party call  system.  The sequence  begins  with the  action  taken  by the  caller  by picking  up the  receiver.  Thus a message lift receiver goes from  the objects s to the objects d. Now  in return  the objects  d calls  a method set dial tone of class caller.  Then again  the objects s sends message dial digit d to  d. The objects s now  creates  an objects  c of class conversation. The objects c created  by  s now  calls a method  ring of class caller. And when  the person  at the   other  end  hears  the ring and picks  up the  receiver the objects r  sends messages to the  objects c, the it  calls a method  connect of class switch  to connect the two  parties.

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