How would you model a Bird class? What issues arise when you try to add a Penguin or Ostrich? (LSP violation)
Avoiding Incorrect Inheritance with Capability-Based Modeling
A common modeling mistake is to define Bird with a Fly() method and then derive Penguin and Ostrich from Bird. Penguins and ostriches are birds biologically but cannot satisfy a behavioral contract that requires flying. This can violate the Liskov Substitution Principle because code expecting any Bird to fly would fail for those subclasses. I would separate common bird identity from capabilities such as flying.
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The problem is not that Penguin is not a Bird; it is that Bird has been given an inappropriate behavioral contract.
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Separate capabilities from identity when capabilities are not universal.
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Interfaces such as IFlyingBird can model optional capabilities.
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LSP requires derived types to honor the behavioral expectations of their base abstraction.
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Avoid methods that derived classes must implement by throwing NotSupportedException merely to satisfy inheritance.
Follow-up Questions
- What exactly is violated in the traditional Bird-Fly model?
- How does ISP help solve this problem?
- Can inheritance still be used for Bird?