Component%20Software%20Beyond%20Object-Oriented%20Programming

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Component Software Beyond Object-Oriented
Programming

• Clements Szyperski
• Chapter 7 Object versus class composition or
  how to avoid inheritance

Alexandre Alvaro aa2_at_cin.ufpe.br Universidade
Federal de Pernambuco (UFPE)
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Agenda

• Overview
• Various aspects of Inheritance
• Problems
• Fragile base class problems
• Approaches to discipline inheritance
• From class to object composition
• Forwarding x Delegation

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Inheritance the soup of the day ?

• Simula 67 - 1970
• Inheritance of implementation
• Inheritance of interfaces
• Establishment of substitutability
• Smalltalk - 1983
• Inheritance of implementation
• Inheritance of interfaces

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Inheritance the soup of the day ?

• Eiffel
• Possible to undefine inheritance interface
  feature
• Emerald (1987), Java, C
• Interface and implementation inheritance have
  been separated
• COM and OMG IDL
• Interface definition language

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Inheritance the soup of the day ?

• Three facets of inheritance
• Subclassing
• Subtyping
• Promise of substitutability
• How to avoid inheritance ?

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More flavors to the soup

• Multiple Inheritance

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More flavors to the soup

• Mixing implementation fragments

Do both superclasses B1 and B2 get their own copy
of the state defined by the superclass A ?
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More flavors to the soup

• Mixing implementation fragments

About C class ?
Diamond inheritance problem
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More flavors to the soup

• Some approaches to discipline
• CLOS (Common Lisp Object System)
• Linear order of inheritance
• C
• Maintaining the integrity of sub-objects
• Java
• Limited to single implementation inheritance
• OMG IDL and COM
• Not support implementation inheritance at all

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More flavors to the soup

• Mixins

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More flavors to the soup

• Mixins

abstract class X1 implements B void X ()
... // X2.Y ()
Interface B void X () void Y ()
abstract class X2 implements B void Y ()
...
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Back to basic ingredients

• The fragile base class problem

Application
Application
Application
Base Class(es)
SO
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The fragile base class problem

• Syntatic
• Binary compatibility
• Compiled classes with new binary releases of
  superclass
• Not need recompilation after syntatic changes
• Example methods may move up in the class
  hierarchy

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The fragile base class problem
1ª 2ª 3ª
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The fragile base class problem
1ª 2ª 3ª
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The fragile base class problem
1ª 2ª 3ª
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The fragile base class problem
1ª 2ª 3ª
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The fragile base class problem

• Semantic
• How can a subclass remain valid in the presence
  of different version of its superclasses ?
• Parameters
• Methods name
• Return type

Contracts
Versions
Re-entrance
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Inheritance more knots than meet the eye
abstract class Text ... void write (pos, ch)
.... setCaret (pos) void setCaret
(int pos) caret pos ...
class SimpleText extends Text ... void setCaret
(int pos) int old caretPos() if (old
! pos) hideCaret()
super.setCaret(pos) showCaret()
....
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Inheritance more knots than meet the eye
X
abstract class Text ... void write (pos, ch)
.... pos void setCaret (int
pos) caret pos ...
class SimpleText extends Text ... void setCaret
(int pos) int old caretPos() if (old
! pos) hideCaret()
super.setCaret(pos) showCaret()
....
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Approaches to discipline inheritance

• 1) The specialization interface
• C, Java, C
• Protected
• Acessible only to subclasses
• Public
• The client interface
• Private
• Private to a class, not an object

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Approaches to discipline inheritance

• 2) Typing the specialization interface
• What are the legal modifications a subclass can
  apply ?
• Protected interface
• 1993, John Lamping
• Statically
• Acyclic
• Arranged in layers
• Cyclic
• Form a group

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Approaches to discipline inheritance

• 2) Typing the specialization interface
• The developer determines the groups or layers

specialization interface Text state
caretRep state textRep abstract
posToXCoord abstract posToYCorrd concrete
caretPos caretPos concrete setCaret
caretRep concrete write textRep, caretPos,
setCaret concrete delete textRep, caretPos,
setCaret ...
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Approaches to discipline inheritance

• 3) Behavioral specification of the specialization
  interface
• Semantic issues
• 1995, Stata Guttag
• Class as a combined definition of interacting
  parts objects
• Method groups
• Algebraic specification techniques
• Notion of behavioral subtyping

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Approaches to discipline inheritance

• 3) Behavioral specification of the specialization
  interface

Inheritance Independent classes
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Approaches to discipline inheritance

• 3) Behavioral specification of the specialization
  interface

A
B
C
Class Three groups
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Approaches to discipline inheritance

• 4) Reuse and cooperation contracts
• 1996, Steyaert, et. al.
• Returned to the idea of statically verifiable
  annotations
• Reuse contract

reuse contract Text abstract
posToXCoord posToYCorrd concrete
caretPos setCaret write caretPos,
setCaret delete caretPos, setCaret ...
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Approaches to discipline inheritance

• 4) Reuse and cooperation contracts
• Real innovation
• Set of modification operators
• Concretization
• Extension
• Refinement

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Approaches to discipline inheritance

• 5) Representation invariants and methods
  refinements
• 1996, Edwards
• Generalization of the Stata Guttag
• Overriding a method in a method group
• Associating invariants with a class

• Protected
• Public
• Private
• Etc.

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Approaches to discipline inheritance

• 6) Disciplined inheritance to avoid fragile base
  class problems
• 1998, Mikhajlov Sekerinski

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Approaches to discipline inheritance

• 7) Creating correct subclasses without seeing
  superclass code
• 2000, Ruby Leavens
• Inverse problem of the semantic FBC problem

Inverse problem FBC problem
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Approaches to discipline inheritance

• 7) Creating correct subclasses without seeing
  superclass code
• 2000, Ruby Leavens
• Inverse problem of the semantic FBC problem

Fragile subclass problem
Inverse problem FBC problem
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Approaches to discipline inheritance

• 7) Creating correct subclasses without seeing
  superclass code
• Provide 3 parts to a class specification
• Public
• Protected
• Automatic analysis of the initial source code of
  the base class

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From class to object composition
outer object
inner object
ltltmessagegtgt
Object B
Object A
Delegation ?
Difference between Inheritance and Forwarding ?
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Forwarding x Delegation

• Forwarding
• Regular Message
• Delegation
• Self-recursive one
• Strengthened
• Identity is remenbered
• What the diference between Forwarding and
  Delegation ?

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Forwarding x Delegation
Forwarding Delegation
InsertChar InsertChar SetCaret
InsertChar delegate(InsertChar) delegate(SetChar)
resend(SetChar)
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