Grady Booch

In object-oriented analysis, we seek to model the world by identifying the classes and objects that form the vocabulary of the problem domain, and in object-oriented design, we invent the abstractions and mechanisms that provide the behavior that this model requires.

[A class category is] a collection of classes, some of which are visible to other class categories, and others of which are hidden.

Ultimately, discovery and invention are both problems of classification, and classification is fundamentally a problem of finding sameness. When we classify, we seek to group things that have a common structure or exhibit a common behavior.

Today, we're at the beginning stages of the next level. Executable UML is the next logical, and perhaps inevitable, evolutionary step in the ever-rising level of abstraction at which programmers express software solutions. Rather than elaborate an analysis product into a design product and then write code, application developers of the future will use tools to translate abstract application constructs into executable entities. Someday soon, the idea of writing an application in Java or C++ will seem as absurd as writing an application in assembler does today. And the code generated from an Executable UML model will be as uninteresting and typically unexamined as the assembler pass of a third generation language compile is today. This shift is made possible by the confluence of four factors:

The entire history of software engineering is that of the rise in levels of abstraction.

Every software system needs to have a simple yet powerful organizational philosophy (think of it as the software equivalent of a sound bite that describes the system's architecture)... [A] step in [the] development process is to articulate this architectural framework, so that we might have a stable foundation upon which to evolve the system's function points.

As Cox points out, "Without inheritance, every class would be a free-standing unit, each developed from the ground up. Different classes would bear no relationship with one another, since the developer of each provides methods in whatever manner he chooses. Any consistency across classes is the result of discipline on the part of the programmers. Inheritance makes it possible to define new software in the same way we introduce any concept to a newcomer, by comparing it with something that is already familiar."

Structured design does not scale up well for extremely complex systems, and this method is largely inappropriate for use with object-based and object-oriented programming languages.

#The development of the Model Driven Architecture (MDA) standards by the Object Management Group (OMG)

If an abstraction is a kind of some other abstraction, or if it is exactly equal to the sum of its components, then inheritance is a better approach

A physician, a civil engineer, and a computer scientist were arguing about what was the oldest profession in the world. The physician remarked, "Well, in the Bible, it says that God created Eve from a rib taken out of Adam. This clearly required surgery, and so I can rightly claim that mine is the oldest profession in the world." The civil engineer interrupted, and said, "But even earlier in the book of Genesis, it states that God created the order of the heavens and the earth from out of the chaos. This was the first and certainly the most spectacular application of civil engineering. Therefore, fair doctor, you are wrong: mine is the oldest profession in the world." The computer scientist leaned back in her chair, smiled, and then said confidently, "Ah, but who do you think created the chaos?"

Philosophy and cognitive science have contributed to the advancement of the object model. The idea that the world could be viewed either in terms of objects or processes was a Greek innovation, and in the seventeenth century, we find Descartes observing that humans naturally apply an object-oriented view of the world. In the twentieth century, Rand expanded upon these themes in her philosophy of objectivist epistemology. More recently, Minsky has proposed a model of human intelligence in which he considers the mind to be organized as a society of otherwise mindless agents. Minsky argues that only through the cooperative behavior of these agents do we find what we call intelligence.

In a quality object-oriented software system, you will find many classes that speak the language of the domain expert

The function of good software is to make the complex appear to be simple.

The object-oriented paradigm is useful when building software systems where there is a hierarchy defined as a ranking or ordering of abstractions.