A Formal Model for Object-Oriented Software Reuse

1. A Formal Model forObject-Oriented Software Reuse Kim Mens Programming Technology Lab Vrije Universiteit Brussel FNRS Meeting May 6th, 1997

2. Contents • Object-Oriented Programming • OO Software Reuse and Evolution • Our Approach: Reuse Contracts • Software Engineering Tools based on Reuse Contracts

3. Object-Oriented Programming • Objects contain both data (instance variables) and operations (methods). • Operations can be invoked and variables retrieved by means of message sending. • Classes are “templates” for objects. • Objects of the same class (instances) have the same implementation, but different internal state. • The implementation of existing classes can be reused (specialised) by means of inheritance.

4. class DesktopFolder(name:String) instance variables xpos, ypos : Number contents : Set(DesktopItem) methods move(dx,dy:Number) = xpos.plus(dx); ypos.plus(dy) add(item:DesktopItem) = contents.include(item) addMany(group:Set(DesktopItem)) foreach item in group doself.add(item) DesktopFolder(HardDisk) xpos:600, ypos: 10, contents: ... DesktopFolder(TrashCan) xpos:580, ypos: 640, contents: ... message send self send Classes and Objects instances

5. parent class (super) class DesktopFolder(name:String) instance variables xpos, ypos : Number contents : Set(DesktopItem) methods move(dx,dy:Number) = ... add(item:DesktopItem) = ... addMany(group:Set(DesktopItem)) inheritance class SizedFolderinherits from DesktopFolder instance variables size : Number methods add(item:DesktopItem) = super.add(item); size.plus(item.getsize()) child class super send Inheritance

6. Software Reuse and Evolution Software Reuse = Reusing existing software parts in new systems. Software Evolution = Updating system parts by more recent versions. Both evolution and reuse give rise to similar problems.

7. What can go wrong ... • Ariane 4 • well-tested software • proven correctly • Ariane 5 • reuse of pieces of Ariane 4 software • result: trivial error (floating point error), large consequences • What happened ? • changed flight specifications

8. OO Software Reuse and Evolution • Current notations/methodologies/tools insufficiently support software reuse • support code reuse rather than design reuse • insufficiently document important dependencies • are too informal • Need for contract betweensoftware providers and reusers. • how can a software component be reused? • how is it actually reused?

9. class DesktopFolder(name:String) instance variables xpos, ypos : Number contents : Set(DesktopItem) methods move(dx,dy:Number) = xpos.plus(dx); ypos.plus(dy) add(item:DesktopItem) = contents.include(item) addMany(group:Set(DesktopItem)) foreach item in group doself.add(item) DesktopFolder xpos, ypos contents move add addMany class SizedFolder inherits from DesktopFolder instance variables size : Number methods add(item:DesktopItem) = super.add; size.plus(item.getsize()) SizedFolder size add Object Modelling Technique (OMT)

10. Our Approach: Reuse Contracts Software Provider Software Reuser • declares how a com-ponent can be reused; • declares what assumptions can be made about the software; • formally, non-coercively • declares how a com-ponent is reused • declares what assumptions are made about the component • formally, disciplined • rules for change propagation (conflict detection)

11. Case Study: Reuse of Classes through Inheritance Class A Class A’ parent class exchange ? SubClass B SubClass B’ SubClass C SubClass C’

12. DesktopFolder xpos, ypos contents move add addMany DesktopFolder xpos, ypos contents move add addMany {add} invokes add Step 1: Documenting the Design documenting (part of) the specialisation interface Specialisation interface The specialisation interface declares theassumptionsthat can be made about a class and how it can be reused.

13. DesktopFolder xpos, ypos, contents move add addMany {add} • Refinement • add{+theSize} SizedFolder SizedFolder size size add {size} add {size} Step 2: Declaring How a Class is Reused DesktopFolder documenting different kinds of reuse xpos, ypos, contents move add addMany {add} The reuse operators declare theassumptionsthat are made about a class and how it is reused.

14. Different Kinds of Reuse • Design “specialising” • Concretisation: replacing dummy or default method implementations by more specific ones • Extension: adding new behaviour • Refinement: refining the overall design by adding method dependencies • Design “generalising” • Abstraction: opposite of concretisation • Cancellation: removing existing behaviour • Coarsening: removing method dependencies

15. OMT notation DesktopFolder DesktopFolder xpos, ypos contents move add addMany xpos, ypos contents move add addMany size + item.getSize() size + item.getSize() SizedFolder SizedFolder size size add add Inconsistent Methods evolution size is not updated when adding many items

16. RC notation SizedFolder SizedFolder size size add {size} add {size} Evolution Conflict Detection • Coarsening • addMany {-add} DesktopFolder DesktopFolder xpos, ypos, contents move add addMany {add} xpos, ypos, contents move add addMany {add} • Refinement • add{+size} • Refinement • add{+size} Inconsistent Methods addMany needs to be overridden too

17. Step 3: Formal Model & Rules for Conflict Detection • Formal model of … reuse contracts, reuse operators, and applicability • Formal rules for ... detecting conflicts on parent class exchange automated conflict checking possible !

18. Reuse contracts for inheritance Software Provider Software Reuser DesktopFolder xpos, ypos, contents move add addMany {add} • Refinement • add{+size} • Reuse contract between provider and reuser • declares how a component can be reused and is reused • formal rules for change propagation

19. Software Engineering Tools based on Reuse Contracts • Impact Analysis & Conflict Detection • Documenting Structure & Design of Software • Extracting Reuse Contracts (from source code) • Reverse Engineering • Verifying Compliance (of code) to Reuse Contracts • Enforcing Design • Generating Code • Quality Assessment of Software Design

20. More Information http://progwww.vub.ac.be/prog/pools/rcs/ my email: kimmens@vub.ac.be