Lecture of 17 January 1995

• is-a
  Gives the hierarchy of object structures in the system
• has-a (or part-of)
  Gives the hierarchy of class structures in the system

Limitations of Humans in Dealing with Complexity

• One is obliged to reference Miller, G. The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information. The Psychological Review, vol. 63(2), p.86. People don't deal well with large systems containing many different objects. They need hierarchies to simplify the problem.

Bringing Order to Chaos

Role of Decomposition

• We must employ the method of divide et impera (divide and rule, often glossed as divide and conquer).

Algorithmic Decomposition

• This is the classical top-down structured design approach.
• In this approach, the system is decomposed using a state-machine approach. We decompose the large mapping from initial to final state by subdividing it into subcomponent state mappings, then further refining these state mappings, until we have gotten down to primitive instructions (which provide the finest granularity of state mapping specifiable on our computer).

Object-Oriented Decomposition

• In this alternative approach, we think of the system in terms of objects.
• We consider the whole system a single object, and decompose it into its relevant object parts. These objects have specific responsibilities with respect to the system as a whole, respond to specific stimuli, and interact in well-defined ways.

Algorithmic vs. Object-Oriented Decomposition

• Algorithmic view emphasizes the sequencing of events
• Object-oriented view emphasizes the agents who are partaking in these events
• Neither view is the one true way. We must employ multiple views if we want to solve complex problems with elegant, simple solutions.
• Experience indicates it is better to take the object-oriented view first when designing

Role of Abstraction

• We remove complexity by clustering information together.

Role of Hierarchy

• Hierarchies give us a way to reveal or obscure the elements from which an abstraction is constructed.

Designing Complex Systems

Engineering as a Science and an Art

• We must make software work, yet we must also apply an artistic aesthetic when constructing software

The Meaning of Design

• Functional Specifications
• Limitations of the target media
• Performance requirements and resource limitations
• Constraints on form
• Restrictions on the design process

Model Building

• Models appeal to the principles of decomposition, abstraction, and hierarchy.
• Different models can be employed to describe different aspects of a system.
• Successful system design processes involve multiple models.

Elements of Software Design Methods

• Notation
  A language in which to express the models
• Process
  Activities that lead to construction of the system models
• Tools
  Specific artifacts that are employed to provide support for the creation of models

Models of Object-Oriented Development

• Two Orthogonal Considerations:
  • Static vs. Dynamic nature of a model
  • Logical vs. Physical nature of a model
• When we discuss the physical nature of a model, we mean this in a metaphorical sense. Madsen, Moeller-Pederson, and Nygaard describe this metaphor as follows:

  In computerized physical models, objects are the material used for representing physical phenomena from the application domain. Objects are thus considered as similar to cardboard, sheets, Lego bricks, etc., in the sense that objects are physical material that may be used for representing (or modeling) physical phenomena. Objects are computerized material.

The Object Model

My guess is that object-oriented programming will be in the 1980s what structured programming was in the 1970s. Everyone will be in favor of it. Every manufacturer will promote his products as supporting it. Every manager will pay lip service to it. Every programmer will practice it (differently). And no one will know just what it is. (T. Rentsch)

Evolution of the Object Model

Trends in Software Engineering

Programming Language Generations

• From early to later programming languages, we have seen a shift of increasing support for abstraction mechanisms and access control (or encapsulation).
• These trends have led to the development of object-oriented and Object-Based languages such as Smalltalk, Object Pascal, C++, CLOS, Ada, and Eiffel.

Foundations of the Object Model

• The Object Model represents an evolutionary development in programming languages, not a revolutionary one.

OOP, OOD, and OOA

• Objects have state and identity. They can change state, behave, be manipulated, or stand in relation to other objects in self-consistent ways. That is, objects have integrity. If a programming construct does no have this kind of integrity, it does not correspond to an object.

OOP

• Object-oriented programming involves organizing programs as cooperative collections of objects.
• It is of value to use and object-oriented language to write object-oriented programs

OOD

• Object-oriented design uses object-oriented decomposition (as opposed to algorithmic decomposition) and depicts the Logical/Physical and Static/Dynamic models of a system

OOA

• Object-oriented analysis is the method of examining requirements from the perspective of the classes and objects found in the vocabulary of the problem domain.