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Object vs Class composition

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.

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Object vs Class composition

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  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 => methods arranged in layers • Cycles => 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 don’t 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

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