Object vs Class composition

By Marine Ruhamanya

Disciplined Inheritance

Problems with implementation inheritance: Encapsulation Fragile Base Class problem

However it is a powerful mechanism.

Several attempts to discipline the mechanism

The specialization interface

Special interface between a class and its subclasses

C++, Java, C# support the notion of protected features of a class

Typing the specialization interface

Overriding methods needs to be done carefully

Proposal (Lamping,1993): Type system to improve control

Declare method dependencies Acyclic graph => methods arranged in layers Cycles => all methods form a group

Subclass has to override methods group by group

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

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.

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

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.

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 don’t introduce new ones. Very restrictive, but interesting formal

proof

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.

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’

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

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

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

End