Frogs design

Revision as of 01:52, 23 July 2008

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

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