Frogs design

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* [[Beware type switches]] - ''Frog'' phases: ''Egg'', ''Tadpole'', ''AdultFrog''.
 
* [[Beware type switches]] - ''Frog'' phases: ''Egg'', ''Tadpole'', ''AdultFrog''.
 +
* [[Avoid "becomes"]] - ''Frog'' phases of life are modeled by inheritance: ''Egg'', ''Tadpole'', ''AdultFrog''
 
* [[Beware value switches]] - ''Tadpole''
 
* [[Beware value switches]] - ''Tadpole''
 
* [[Avoid no-op overrides]] ("Design by Contract") - the derived class ''Egg'' overrides the base class's ''hop()'' and ''swim()'' methods with methods which does nothing.
 
* [[Avoid no-op overrides]] ("Design by Contract") - the derived class ''Egg'' overrides the base class's ''hop()'' and ''swim()'' methods with methods which does nothing.

Revision as of 01:52, 23 July 2008

An OO model of frogs. This beautiful design appeared in the 2004 427 exam.

Frogs.gif

Contents

Design notes

  • This design models the lifecycles of Frogs.
  • A Frog begins life as an Egg, grows into a Tadpole and then an AdultFrog.
  • A FrogBrain belongs to a Frog and manages its behaviour.
  • There is one Biologist, Igor, who sometimes moves Frogs around. Igor has been known to fry their legs in white wine.
  • Toads don’t have the same lifecycle as Frogs, but they look and act a lot like Frogs.
  • Toads eat AdultFrogs.

Design Critique

This design is perfect. It cannot be faulted.

Broken heuristics

  • Beware type switches - Frog phases: Egg, Tadpole, AdultFrog.
  • Avoid "becomes" - Frog phases of life are modeled by inheritance: Egg, Tadpole, AdultFrog
  • Beware value switches - Tadpole
  • Avoid no-op overrides ("Design by Contract") - the derived class Egg overrides the base class's hop() and swim() methods with methods which does nothing.
  • One key abstraction - the Move interface contains more than one key abstraction. It contains methods to move such as hop() and swim(), but it also contains a display() method which is a separate key abstraction.
This is a good start, but needs more explanation of how the rules are broken here. Also needs a note about this contribution on the discussion page. --Wal

"Feels Bad"

  • Toad is a subclass of AdultFrog.
  • FrogBrain contains an array of Egg
Why do these things feel bad? --Wal
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