Nithin Bekal

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Object Oriented Design

SOLID Principles

  • Single responsibility principle:
    • there should never be more than one reason for a class to change
  • Open closed principle:
    • a module should be open for extension and closed for modification
  • Liskov substitution principle:
    • subclasses should be substitutable for their base classes
  • Interface segregation principle:
    • many client specific interfaces are better than one general purpose interface
  • Dependency inversion principle:
    • Depend upon abstractions, not concretions

Questions to ask about a piece of code (mentioned in Sandi Metz’ SOLID Design talk.)

  • Is it DRY?
  • Does it have one responsibility?
  • Does everything in it change at the same rate?
  • Does it depend on things that change less often that it does?

Some ideas on tests from Sandi Metz’ SOLID Design talk:

  • Only mock classes you own
  • Don’t mock/stub object under test
  • If tests are hard to write, it indicates a problem with design
  • TDD will punish you if you don’t understand design

Desirable characteristics of object-oriented code, described in Growing Object Oriented Software, Guided by Tests, a book by Steve Freeman and Nat Pryce:

  • Loosely coupled
  • Highly cohesive
  • Easily composable
  • Context independent

Symptoms of rotting design

(Described in Roebrt Martin’s paper, Design Principles and Design Patterns)

  • Rigidity: tendency for software to be difficult to change, even in simple ways
  • Fragility: tendency of the software to break in many places every time it is changed
  • Immobility: inability to reuse software from other projects or from parts of the same project
  • Viscosity:
    • Viscosity of design: design preserving methods are harder to employ than hacks.
    • Viscosity of environment: dev environment is slow and inefficient. e.g. slow compile times, long CI cycle.

Venkat Subramaniam - Core Software Design Principles (video)

  • Provides vocabulary for communicating intent
  • Help build better quality software