Liskov Substitution Principle

LSP presentation.md

"If S is a subtype of T, then objects of type S should be able to replace objects of type T without altering desireable properties of the program"

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Goals of SOLID:

• Address problems of rotting design
  • Rigidity
    • small changes take a long time
    • affects management
      • code freezes
      • feature freezes

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• Fragility
  • changes create bugs
  • bug fixes create more bugs

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• Immobility
  • little to no code re-use
    • re-writing similar tools throughout the system
    • dependencies of old tools make them too difficult to extract

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• Viscocity
  • 2 types:
    • (software) design
      • sticking to the design costs more than adding hacks
    • environment
      • hour long compile times, build times, deploy times

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• SOLID(er)
  • "Dependencies (and dependency changes) are the cause of design rot"
  • set up design firewalls
    • don't allow dependency changes to propagate across the system

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Liskov Substitution Principle

• "If S is a subtype of T, then objects of type S should be able to replace objects of type T without altering desireable properties of the program"

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• Stated another way

  • "Preconditions are no stronger than the base class' method"

  • "Postconditions are no weaker than a base class' method"

  • "Expect no more, provide no less"

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• Seems a bit obvious?
  • Violations of LSP become OCP violations

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• Example
  • Circle-ellipse problem
    • "Circle is an ellipse"
    • class Circle < Ellipse
      • violates "preconditions are no stronger"
      • code that tries to set major/minor axes independently will have unexpected results

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• Person-prisoner inheritance [1]
  • person method has move_north(meters)
  • violates "preconditions are no stronger"
  • a prisoner doesn't have the same freedom to move

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• Honorable mention
  • Does Liskov substitution apply to interfaces? [2]
  • Prefer composition to inheritance [3]

References

[1] http://en.wikipedia.org/wiki/Circle-ellipse_problem#Challenge_the_premise_of_the_problem

[2] http://stackoverflow.com/questions/12252363/does-liskov-substitution-principle-also-apply-to-classes-implementing-interfaces

[3] http://en.wikipedia.org/wiki/Composition_over_inheritance