Design Tips

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Design Tips
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Becoming Good OO Developers

• Developing good OO Software is hard
• Takes a lot of time to take advantage of all OO
  features
• Lots of Experience needed to become good
  designers
• Many systems are not leveraging OO advantages
• To master OO software design, study experienced
  developers design
• Reuse proven software design
• gt Patterns can help

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Design Tips Hints (High level Patterns)

• Layering
• Improves stability, understandability,
  flexibility
• Reduces impact to changes
• Restrict dependencies/coupling between
  packages/subsystems
• GUI (Boundary Classes) separated in one
  layer/package
• Allow the User Interface to change without
  impacting the rest of the system
• Package Functionally related Classes
• Changes in one Class impact the other Class
• Rich Interaction between Classes
• Package should be cohesive
• Reuse proven software design
• gt Patterns can help

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Design
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Design

• Refine the Class Diagram
• Structure system
• Subsystems, Interfaces, Classes
• Define subsystems dependencies
• Capture major interfaces between subsystems
• Assign responsibilities to new design classes
• Describe realization of Use Cases
• Use Sequence and Collaboration Diagrams
• Assign visibility to class attributes
• Define Methods signature
• Develop State diagram for relevant design classes
• Use Interaction Diagram to distribute behavior
  among classes
• Use Design Patterns for parts of the system

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Design Problems

• Rigidity Every change causes a cascade of
  subsequent changes in dependent modules
• Fragility Tendency to break in many places every
  time the software is changed
• Immobility Inability to reuse software from
  other projects or parts of same projects
• Viscosity Easier to change the software by
  breaking the architecture rules than by
  respecting them.

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Design Principles

• Open-Closed Concept
• Software Entities (Classes, Modules, Functions)
  should be Open for Extension, but Closed for
  Modification
• (Change the what modules do without modifying
  their code)
• Design by Contract
• Use Assertion, Pre-condition, Post-condition, to
  ensure robustness and corrrectness.
• Assertion property of some of the values of the
  program entities.
• Pre-condition expresses the properties that must
  hold whenever the method is called
• Post-condition expresses the properties that the
  method guarantees when it is called

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Design Principles (cont.)

• Dependency Inversion
• Primary mechanism / strategy
• Depend upon interfaces or abstract classes rather
  than upon concrete methods and classes.
• Interface segregation
• When a class has multiple clients, create
  specific interfaces for each client.
• Packaging principles
• Common Closure Classes that change together,
  belong together.
• Dependencies between packages should not form
  cycles
• Depend in the direction of stability

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Other Processes

• OPEN
• Object-oriented Process, Environment, and
  Notation
• Third-generation, public domain, full lifecycle,
  process-focused, methodological approach
• Work Products components that are developed by
  the project.
• Languages components used to document most work
  products.
• Producers components that develop work products.
  
• Work Units components that model the operations
  performed by producers when developing work
  products.
• Stages time intervals that provide a macro
  organization to the work units.

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Other Processes (cont.)

• Extreme Programming

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UML Use

• OPEN
• Web Applications
• J2EE Architecture
• XML applications