====== Week 9 ======

~~NOTOC~~


===== Type Hierarchy =====

//**Liskov Substitution Principle: **//
//objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.// 

The //substitution principle// requires that the subtype specification support reasoning based on the supertype specification. Three properties must be supported:
  - //Signature Rule//. The subtype objects must have all the methods of the supertype, and the signatures of the subtype methods must be //compatible// with the signatures of the corresponding supertype methods.
  - //Methods Rule//. Calls of these subtype methods must "behave like" calls to the corresponding supertype methods.
  - // Properties Rule//. The subtype must preserve all properties that can be proved about supertype objects.

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  * Liskov Ch. 7
  * Lecture notes: {{courses:cs203-201301:ch7-type-hierarchy.pdf|PDF}}
  * Wikipedia article: http://en.wikipedia.org/wiki/Liskov_substitution_principle
  
===== The Codeless Code =====

  * The Backwards Monk: http://thecodelesscode.com/case/44
  * What does this mean to you?