Theory

Object Oriented Design

• Design helps in specifying the structure of how a software system will be written and function, without writing the complete implementation.
• It is a transition from "what" the system must do, to "how" the system will do it,
• Designing Classes:
  • class identification from project spec / requirements
    • nouns are potential classes, objects, fields
    • verbs are potential methods or responsibilities of a class
  • Use Unified Modelling Language(UML) to represent the classes and objects
    • UML is an open standard; lots of companies use it
    • UML is a descriptive language: rigid formal syntax (like programming)
    • UML is a prescriptive language: shaped by usage and convention
    • it's okay to omit things from UML diagrams if they aren't needed by team/supervisor/instructor
• Beginning of software system design is depicted in the following picture:

Time Sequence

• It describes the flow of messages, events, actions between objects.
• It shows concurrent processes and activations.
• It shows time sequences that are not easily depicted in other diagrams.
• It is typically used during analysis and design to document and understand the logical flow of your system

Sequence diagram emphasized on time ordering!
          

• Sequence Diagram Key Parts:

  • participant: an object or entity that acts in the sequence diagram.

  • message: communication between participant objects

    • Messages
      • The name of the service requested by the calling object.
      • Copies of the information required to execute the service and the name of a holder for the result of the service.
    • In practice, messages are often implemented by procedure calls.
      • Name = procedure name.
      • Information = parameter list.

  • the axes in a sequence diagram:

    • horizontal: which object/participant is acting.

      • X-axis is objects.
        • Object that initiates interaction is left most
        • Object to the right are increasingly more subordinate
          

    • vertical: time (down implies forward in time)

      • Y-axis is time
        • Messages sent and received are ordered by time.
          
          
        • Activation (double line) is the execution of the procedure
          
        • Object lifelines represent the existence over a period of time

• Steps for Building a Sequence Diagram

  • Identify which objects and actors will participate.
    • Set the lifeline for each object/actor.
  • Identify the messages from the use case.
    • They are the verb phrases in the use case.
  • Lay out the messages from the top to the bottom of the diagram based on the order in which they are sent.
  • Add the focus of control for each objects or actor’s lifeline.