CS535: Doc 15, Class Invariants

	CS 535 Object-Oriented Programming Fall Semester, 2003 Class Invariants
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Reading Assignment
Object-Oriented Design Heuristics, Chapter three


Doc 15, Class Invariants Slide # 2

Class invariants
“Class invariants are predicates of (statements about) a class that should always be true”

John Farrell, http://c2.com/cgi/wiki?CodeClassInvariants

Examples are:

An instance variable is not nil
An instance variable is an ordered collection
An integer value has to be in a certain range


Doc 15, Class Invariants Slide # 3

Example
Stack

Instance variables: elements, top

elements – Array containing the element of the stack
top – An integer pointing to element that is currently the top of the stack

Stack>>isEmpty
   ^top = 0

Stack>>isFull
   ^top = elements size

Stack>>pop
   self isEmpty ifTrue: [invoke your empty stack policy].
   topElement := elements at: top.
   top := top – 1.
   ^topElement

Stack>>push: anObject
   self isFull ifTrue: [invoke your full stack policy].
   elements at: (top := top + 1) put: anObject.


Doc 15, Class Invariants Slide # 4

Class Invariants
Are to be true

Once the object is constructed
Before and after each message send to an object

In the middle of a message send to an object the class invariant may be false


Doc 15, Class Invariants Slide # 5

WordStream
Parent class: ReadStream

Inherited instance variables:

collection <SequenceableCollection> elements to read

position <Integer> pointer to the current access position

readLimit <Integer> size of the collection

writeLimit <Integer> farthest that has been written into the collection

policy <StreamPolicy> policy for choosing the print format for various entities, such as Dates, Times, currencies, or other context-sensitive information

WordStream>>on: aCollection
   ^super on: (aCollection runsFailing: [:each | each isWordSeparator])

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