Are Components better Objects?
Component Technology
Responsibilities
- to specify behavior
what rather than how
Client
- makes request to perform a service
Server
- provides service upon request
Client/Server Model
Contracts
- a set of services
Behavioral refinement
- improving contracts
Contracts
Conformance
if B refines A then B may be used wherever A is allowed
Subtype -- signature and behavior
- preservation of behavioral properties
Behavioral properties
- invariant properties -- true of all states
- history properties -- all execution sequences
Types as Behavior
Example -- IntSet \not<= FatSet
- FatSet -- insert, select, size
- IntSet -- insert, delete, select, size
History -- not satisfied by IntSet
Objects as Behavioral Types
From Metaphor to Calculus
- a strong object is a weak agent
Agents for the Web
Case Study: Verifiable (Component) Software
- e.g. CMG Test Frame -- UML Use cases -> Test Action Words