About Instance Initialization

Transcription

1 Learning Object-Oriented Programming and Design with TDD About Instance Initialization Stéphane Ducasse W5S06

2 W5S06 2 / 26 How to ensure that an instance is well initialized? Possible solution Automatic initialize Lazy initialization Proposing the right interface Providing a default value

3 W5S06 3 / 26 Provider responsibility This is the responsibility of the class to provide well-formed objects A client should not make assumptions or been responsible to send specific sequence of messages to get a working object

4 A First Implementation of Packet W5S06 4 / 26 Object subclass: #Packet instancevariablenames: 'contents addressee originator ' Packet >> printon: astream super printon: astream. astream nextputall: ' addressed to: '; nextputall: self addressee. astream nextputall: ' with contents: '; nextputall: self contents Packet >> addressee ^ addressee Packet >> addressee: asymbol addressee := asymbol

5 Example of instance creation W5S06 5 / 26 Packet new addressee: #mac ; contents: 'hello mac'

6 Fragile Instance Creation W5S06 6 / 26 If we do not specify a contents, it breaks! p p := Packet new addressee: #mac. p printon: astream > error

7 W5S06 7 / 26 Problems Responsibility of the instance creation relies on the clients A client can create packet without contents, without address and the instance variables are not initialized correctly error (for example, printon:) Fragile system (printon: should be robust)

8 W5S06 8 / 26 Fragile Instance Creation Solutions Automatic initialization of instance variables Proposing a solid interface for the creation Lazy initialization Providing a default value

9 Assuring Instance Variable Initialization How to initialize a newly created instance? Define the method initialize Packet >> initialize super initialize. contents := ''. addressee := #noad Makes sure that contents and addressee have always a value W5S06 9 / 26

10 The New/Initialize Couple W5S06 10 / 26 Object >> initialize "do nothing. Called by new my subclasses override me if necessary" ^ self Acts as a constructor in other languages

11 W5S06 11 / 26 Lazy Initialization When some instance variables are: not used all the time consuming space, difficult to initialize because depending on other need a lot of computation Use lazy initialization based on accessors But accessor access should be used consistently!

12 W5S06 12 / 26 Lazy Initialization Example A lazy initialization scheme with default value Packet >> contents contents isnil iftrue: [ contents := 'no contents' ] ^ contents apacket contents or self contents A lazy initialization scheme with computed value Dummy >> ratio ratio isnil iftrue: [ ratio := self heavycomputation ]

13 Better W5S06 13 / 26 Packet >> contents contents isnil iftrue: [contents := 'no contents'] ^ contents is equivalent to Packet >> contents ^ contents ifnil: [ contents := 'no contents ]

14 W5S06 14 / 26 Strengthen instance creation interface Problem: A client can still create apacket without address. Solution: Force the client to use the class interface creation. Providing an interface for creation and avoiding the use of new: Packet send: Hello mac to: #Mac Packet class >> send: astring to: anaddress ^ self new contents: astring ; addressee: anaddress ; yourself

15 W5S06 15 / 26 Examples of instance initialization Step 1. SortedCollection sortblock: [:a :b a name < b name] SortedCollection class>>sortblock: ablock "Answer a new instance of SortedCollection such that its elements are sorted according to the criterion specified in ablock." ^ self new sortblock: ablock Step 2. self new => asortedcollection Step 3. asortedcollection sortblock: ablock

16 Another example W5S06 16 / 26 Step 1. OrderedCollection with: 1 Collection class >> with: anobject "Answer a new instance of a Collection containing anobject." newcollection newcollection := self new. newcollection add: anobject. ^ newcollection

17 Invoking per default the creation interface W5S06 17 / 26 OrderedCollection class >> new "Answer a new empty instance of OrderedCollection." ^ self new: 5

18 W5S06 18 / 26 Forbidding new? Problem: We can still use new to create fragile instances Solution: new should raise an error! Packet class >> new self error: 'Packet should only be created using send:to:'

19 W5S06 19 / 26 Forbidding new implications But we still have to be able to create instance! Packet class >> send: astring to: anaddres ^ self new contents: astring ; addressee: anaddress raises an error Packet class >> send: astring to: anaddress ^ super new contents: astring ; addressee: anaddress

20 W5S06 20 / 26 Forbidding new Solution: use basicnew and basicnew: Packet class >> send: astring to: anaddress ^ self basicnew contents: astring ; addressee: anaddress Conclusion: Do not override basic* methods else you will not be able to invoke them later

21 W5S06 21 / 26 Fluid vs. and default creation interface Often a fixed creation interface gets too large Too many possibilities Too many optional cases

22 Example: Problem with class definition in Pharo W5S06 22 / 26 Object subclass: #Behavior uses: TBehavior instancevariablenames: 'superclass methoddict format layout' classvariablenames: 'ClassProperties ObsoleteSubclasses' package: 'Kernel Classes'

23 Example: Problem with class definition in Pharo W5S06 23 / 26 subclass: s uses: at instancevariablenames: names classvariablenames: cvnames category: cat subclass: s uses: at instancevariablenames: names classvariablenames: cvnames package: cat subclass: s instancevariablenames: names classvariablenames: cvnames pooldictionaries: pools package: cat subclass: s uses: at instancevariablenames: names classvariablenames: cvnames pooldictionaries: pools category: acat......tag:......immediate:......ephemeron:...

24 Example of a fluid interface in Seaside W5S06 24 / 26 ScrapBook>>renderContentOn: html html paragraph: 'A plain text paragraph.'. html paragraph: [ html render: 'A paragraph with plain text followed by a line break.'. html break. html emphasis: 'Emphasized text '. html render: 'followed by a horizontal rule.'. html horizontalrule. html render: 'An image: '. html image url: ' plain.png' ]

25 W5S06 25 / 26 How to ensure that an instance is well initialized? Automatic initialize Lazy initialization Proposing the right interface Providing a default value

26 A course by Stéphane Ducasse Reusing some parts of the Pharo Mooc by Damien Cassou, Stéphane Ducasse, Luc Fabresse Except where otherwise noted, this work is licensed under CC BY-NC-ND 3.0 France