CSC/ECE 517 Fall 2012/ch2a 2w15 rr
Introduction
Object-oriented design is a programming model that began in the late 60's as software programs became more complex. The idea behind the approach was to build software systems by modeling them based on the real-world objects that they were trying to represent. For example, banking systems would contain customer objects, account objects and so on. Today, object-oriented design has been widely adopted <ref> Introduction to Object Oriented Design</ref>. When done properly, this approach leads to simple, robust, flexible and modular software. When something goes wrong, the results could be bad. Object oriented design can be seen from different perspectives. In the language-centric perspective, objects are containers of data and methods. The model-centric perspective views the objects as model elements reflecting the real world objects and Responsibility-centric perspective views objects as interacting elements each playing a role in object community. Each of these perspectives are outlined in the following sections.
Background
The history of object oriented design has many branches, and many people have contributed to this domain. The 1960s and 1970s saw the emergence of object oriented programming languages, such as Simula and Smalltalk, with key contributors such as Kristen Nygaard and Alan Kay, the visionary computer scientist who founded Smalltalk.<ref>History of Object Oriented Design</ref> But object oriented design was informal through that period, and it was only form 1982 that it became popular. Contributors in this domain include Grady Booch, Kent Beck, Peter Coad, Don Firesmith, Ivar Jacobson (a UML founder), Steve Mellor, Bertrand Meyer, Jim Rumbaugh (a UML founder) and Rebecca Wirfs-Brock among others. Each of these perspectives are outlined in the following sections.
Object Oriented Design Perspectives
Language-centric perspective
The focus of the language-centric perspective is classes and objects as building blocks for developing software. The emphasis is on the internal structure which are the fields and methods. Thus it is inherently a compile-time or static view which means that the definition speaks in terms of what it looks like in the following code:
public class foo
{
private int x;
public static double y;
public int double (int x)
{
return (3*x);
}
}
The implicit aspect of this view is that, an object is an entity in its own right and the definition is closed. Any object can be understood in isolation where the fields and methods are just enumerated. The advantage of this perspective is of that it is very concrete, closely related to the programming language level and therefore it is easy to understand. The disadvantage is that it does not help to structure the collaboration between objects, the dynamics, and this is typically where the hard challenges of design lie. Thus the perspective offers little guidance in the process of designing any realistic system.<ref>Henrik Baerbak Christensen, "Implications of Perspective in Teaching Objects First and Object Design", Proceedings of the 10th annual SIGCSE conference on Innovation and technology in computer science education, Department of Computer Science, University of Aarhus, 2005.</ref>
Model-centric perspective
In this perspective, the objects(classes, fields) and their relationships like inheritance, association, aggregation are designed by using some model. Here the program execution is a simulation of some part of the world, and objects are perceived as the parts of the model. The perspective is often explained through analogies to other models like toy railways, traffic simulations, computer games, etc. This perspective stresses objects as entities in a larger context (they are parts of a model) as opposed to the self-contained language-centric definition. This naturally leads to a strong focus on what the relations are between the parts: association, generalization, composition. Dynamics is an inherent part of the concept simulation and the explicit guideline for designing object interaction is to imitate real world or model interactions.
Responsibility-centric perspective
This perspective stresses on the behavior of software systems and the concepts used to do design include behavior, roles, responsibilities and protocol. Here, the primary focus is the dynamics of the program. Roles and responsibilities are the core concepts in this perspective and hence it has more coupling to behavior when compared to static elements like objects or model parts. There is a difference between behavior and responsibility, where behavior is the concrete actions taken by objects where as responsibility is more abstract "obligation to fulfill request" through any number of concrete behaviors.<ref>Henrik Baerbak Christensen, "Implications of Perspective in Teaching Objects First and Object Design", Proceedings of the 10th annual SIGCSE conference on Innovation and technology in computer science education, Department of Computer Science, University of Aarhus, 2005.</ref> One can look at responsibility-centric perspective as an extension of the model-centric perspective with a shift of focus (from a focus on model parts to a focus on the simulation or dynamical aspects).
References
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