Object vs Class composition

1
Object vs Class composition

• By Marine Ruhamanya

2
Disciplined Inheritance

• Problems with implementation inheritance
• Encapsulation
• Fragile Base Class problem
• However it is a powerful mechanism.
• Several attempts to discipline the mechanism

3
The specialization interface

• Special interface between a class and its
  subclasses
• C, Java, C support the notion of protected
  features of a class

4
Typing the specialization interface

• Overriding methods needs to be done carefully
• Proposal (Lamping,1993)
• Type system to improve control
• Declare method dependencies
• Acyclic graph gt methods arranged in layers
• Cycles gt all methods form a group
• Subclass has to override methods group by group

5
Behavioral specification

• Semantic issues of implementation inheritance
• mostly related with re-entrance caused by
  self-recursion
• Proposal (Stata and Guttag, 1995)
• View a class as a combined definitions of
  interacting parts objects
• Division of labor or method groups
• No direct access between divisions

6
Behavioral specification(2)

• Use of specification techniques and the notion of
  behavioral subtyping
• Stata-Guttag vs Lampig
• Stata-Guttag split state and behavior into
  groups. Methods groups encapsulate part of the
  state
• Lampig any dependency of methods on state
• Tight semantic control leads to object
  composition.

7
Reuse contracts

• Less restrictive forms of implementation
• Proposal (Stayeart et al., 1996)
• Annotated interface reuse contracts
• Determine how reuse happens between classes and
  its subclasses
• Specify structural aspects of a base class
• Specify only the part of a call stucture
  subclasses can rely on.
• New set of modification operators

8
Representation Invariants Methods Refinements

• Proposal (Stephen, 1999)
• Associate invariants to a class specification
  that refers to protected variables
• Proposal (Stata, 1999)
• Separate the notion of subtyping and subclassing
  to allow partial overriding
• Supercalls are allowed if overriding method is a
  proper refinement of the base class method.

9
Disciplined inheritance to avoid the FBC problem

• Proposal (Mikhajlov-Sekerinski, 1998)
• Set of rules
• Construction of subclasses based on superclass
  specification
• Eliminate the introduction of new cyclic method
  dependencies
• Superclasses instance variables are private and
  subclasses dont introduce new ones.
• Very restrictive, but interesting formal proof

10
Creating subclasses without seeing the superclass
code

• Proposal (Leavens et al., 2000)
• Java Modeling Language defined
• Focus on the inverse of FBC problem
• Provide 3 parts to a class specification
• 1st and 2nd public and protected parts
• 3rd subclassing contract provide information
  accessed variables and methods call dependencies
• Link to the FBC not further explored.

11
From Class to Object Composition

• Object composition - simpler form of composition
• Object send message to other object asking for
  support
• Outer and inner objects, forwading
• Differences to implementation inheritance
• Outer object does not reimplement inner object
  functionality
• Implicit self-recursion or possesion of common
  self

12
From Class to Object Composition

• Implicit self-recursion or possession of
  common self
• There is no common self in object composition
• In implementation inheritance upcalls due to
  method invocation
• Example ( page 135).
• Advantage dynamic

13
Forwarding vs Delegation

• Message passing
• Delegation message-send is classified as either
  regular (forwarding) or self-recursive
  (delegation)
• Example (page 136 137)
• Message resending
• Complexity exploses under delegation
• Interaction diagram similar to that of
  implementation inheritance

14
Delegation Inheritance

• Delegation powerful programming tool
• System based on OC and delegation are highly
  dynamic.
• Languages are called prototype-based languages
  e.g. Self.
• Not yet mainstream
• Delegation is behind in discipline and modularity
  while inheritance is behind in system dynamics
  and late compsition

15
End