面向对象分析与设计（第二版）

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"《对象导向分析与设计》(第二版)是Grady Booch的一本经典著作，由Addison-Wesley出版。本书探讨了面向对象的分析与设计方法，并结合实际应用进行了深入讲解。书中的内容涵盖了各种编程语言和技术的商标信息，如Adobe Illustrator、Apple的Macintosh和MacApp、Grady Booch的Booch Components、Eiffel、Motif、Objective-C、Objectworks、Smalltalk-80、OS/2、Pure Software、Rational和Rational Rose、Simula 67以及UNIX等。" 在面向对象分析与设计（OOD）中，几个核心概念和方法包括： 1. **面向对象编程**：这是软件开发的一种范式，强调将数据和操作数据的方法封装在一起，形成对象。对象是类的实例，类定义了对象的属性和行为。 2. **分析**：在OOD中，分析阶段主要是理解问题域，识别关键实体和它们之间的关系，形成对象模型。这通常通过创建用例图、类图和对象图来完成。 3. **设计**：设计阶段涉及将分析结果转化为实现结构，包括选择合适的设计模式、架构和类结构。这包括接口设计、继承、多态性和组合等原则的应用。 4. **Booch方法**：Grady Booch提出了一套面向对象分析和设计的方法，包括对象模型、动态模型和功能模型，用于系统建模。 5. **Eiffel**：这是一种强类型、静态类型的面向对象编程语言，以其设计时质量保证特性而闻名，支持“设计by-contract”编程。 6. **Motif**：这是一个用户界面工具包，用于构建X Window System下的图形用户界面，提供了标准的外观和行为。 7. **Objective-C**：它是C语言的超集，添加了Smalltalk的面向对象特性，常用于Apple的Mac OS X和iOS操作系统开发。 8. **Rational Rose**：Rational公司的这款工具是UML（统一建模语言）的可视化建模工具，广泛用于软件开发的建模和需求管理。 9. **Simula 67**：这是一种早期的面向对象编程语言，对后来的OOP语言产生了深远影响，首次引入了类、对象和子程序的概念。 10. **UNIX**：这是一个多用户、多任务的操作系统，其设计哲学和工具链对现代操作系统和软件工程有着深远的影响。这本书通过对这些技术和概念的详细讲解，帮助读者理解和应用面向对象分析与设计的原理，以提高软件开发的质量和效率。同时，它也反映了软件行业的多样性和不同工具之间的相互关联。

Chapter 1: Complexity 7

of behavior would be unlikely, but in discrete systems all external events can affect any part

of the system's internal state. Certainly, this is the primary motivation for vigorous testing of

our systems, but for all except the most trivial systems, exhaustive testing is impossible. Since

we have neither the mathematical tools nor the intellectual capacity to model the complete

behavior of large discrete systems, we must be content with acceptable levels of confidence

regarding their correctness.

The Consequences of Unrestrained Complexity

"The more complex the system, the more open it is to total breakdown" [5]. Rarely would a

builder think about adding a new sub-basement to an existing 100-story building; to do so

would be very costly and would undoubtedly invite failure. Amazingly, users of software

systems rarely think twice about asking for equivalent changes. Besides, they argue, it is only

a simple matter of programming.

Our failure to master the complexity of software results in projects that are late, over budget,

and deficient in their stated requirements. We often call this condition the software crisis, but

frankly, a malady that has carried on this long must be called normal. Sadly, this crisis

translates into the squandering of human resources - a most precious commodity - as well as

a considerable loss of opportunities. There are simply not enough good developers around to

create all the new software that users need. Furthermore, a significant number of the

developmental personnel in any given organization must often be dedicated to the

maintenance or preservation of geriatric software. Given the indirect as well as the direct

contribution of software to the economic base of most industrialized countries, and

considering the ways in which software can amplify the powers of the individual, it is

unacceptable to allow this situation to continue.

How can we change this dismal picture? Since the underlying problem springs from the

inherent complexity of software, our suggestion is to first study how complex systems in

other disciplines are organized. Indeed, if we open our eyes to the world about us, we will

observe successful systems of significant complexity. Some of these systems are the works of

humanity, such as the Space Shuttle, the England/France tunnel, and large business

organizations such as Microsoft or General Electric. Many even more complex systems

appear in nature, such as the human circulatory system or the structure of a plant.

1.2 The Structure of Complex Systems

Examples of Complex Systems

The Structure of a Personal Computer A personal computer is a device of moderate

complexity. Most of them are composed of the same major elements: a central processing unit

(CPU), a monitor, a keyboard, and some sort of secondary storage device, usually either a

floppy disk or a hard disk drive. We may take any one of these parts and further decompose

Chapter 1: Complexity 8

it. For example, a CPU typically encompasses primary memory, an arithmetic/logic unit

(ALU), and a bus to which peripheral devices are attached. Each of these parts may in turn be

further decomposed: an ALU may be divided into registers and random control logic, which

themselves are constructed from even more primitive elements, such as NAND gates,

inverters, and so on.

Here we see the hierarchic nature of a complex system. A personal computer functions

properly only because of the collaborative activity of each of its major parts. Together, these

separate parts logically form a whole. Indeed, we can reason about how a computer works

only because we can decompose it into parts that we can study separately. Thus, we may

study the operation of a monitor independently of the operation of the hard disk drive.

Similarly, we may study the ALU without regard for the primary memory subsystem.

Not only are complex systems hierarchic, but the levels of this hierarchy represent different

levels of abstraction, each built upon the other, and each understandable by itself. At each

level of abstraction, we find a collection of devices that collaborate to provide services to

higher layers. We choose a given level of abstraction to suit our particular needs. For instance,

if we were trying to track down a timing problem in the primary memory, we might properly

look at the gate-level architecture of the computer, but this level of abstraction would be

inappropriate if we were trying to find the source of a problem in a spreadsheet application.

The Structure of Plants and Animals In botany, scientists seek to understand the

similarities and differences among plants through a study of their morphology, that is, their

form and structure. Plants are complex multicellular organisms, and from the cooperative

activity of various plant organ systems arise such complex behaviors as photosynthesis and

transpiration.

Plants consist of three major structures (roots, stems, and leaves), and each of these has its

own structure. For example, roots encompass branch roots, root hairs, the root apex, and the

root cap. Similarly, a cross-section of a leaf reveals its epidermis, mesophyll, and vascular

tissue. Each of these structures is further composed of a collection of cells, and inside each cell

we find yet another level of complexity, encompassing such elements as chloroplasts, a

nucleus, and so on. As with the structure of a computer, the parts of a plant form a hierarchy,

and each level of this hierarchy embodies its own complexity.

All parts at the same level of abstraction interact in well-defined ways. For example, at the

highest level of abstraction, roots are responsible for absorbing water and minerals from the

soil. Roots interact with stems, which transport these raw materials up to the leaves. The

leaves in turn use the water and minerals provided by the stems to produce food through

photosynthesis.

There are always clear boundaries between the outside and the inside of a given level. For

example, we can state that the parts of a leaf work together to provide the functionality of the

leaf as a whole, and yet have little or no direct interaction with the elementary parts of the

Chapter 1: Complexity 9

roots. In simpler terms, there is a clear separation of concerns among the parts at different

levels of abstraction.

In a computer, we find NAND gates used in the design of the CPU as well as in the hard disk

drive. Likewise, a considerable amount of commonality cuts across all parts of the structural

hierarchy of a plant. This is God's way of achieving an economy of expression. For example,

cells serve as the basic building blocks in all structures of a plant; ultimately, the roots, stems,

and leaves of a plant are all composed of cells. Yet, although each of these primitive elements

is indeed a cell, there are many different kinds of cells. For example, there are cells with and

without chloroplasts, cells with walls that are impervious to water and cells with walls that

are permeable, and even living cells and dead cells.

In studying the morphology of a plant, we do not find individual parts that are each

responsible for only one small step in a single larger process, such as photosynthesis. In fact,

there are no centralized parts that directly coordinate the activities of lower level ones.

Instead, we find separate parts that act as independent agents, each of which exhibits some

fairly complex behavior, and each of which contributes to many higher-level functions. Only

through the mutual cooperation of meaningful collections of these agents do we see the

higher-level functionality of a plant. The science of complexity calls this emergent behavior: The

behavior of the whole is greater than the sum of its parts [6].

Turning briefly to the field of zoology, we note that multicellular animals exhibit a

hierarchical structure similar to that of plants: collections of cells form tissues, tissues work

together as organs, clusters of organs define systems (such as the digestive system), and so

on. We cannot help but again notice God's awesome economy of expression: the fundamental

building block of all animal matter is the cell, just as the cell is the elementary structure of all

plant life. Granted, there are differences between these two. For example, plant cells are

enclosed by rigid cellulose walls, but animal cells are not. Notwithstanding these differences,

however, both of these structures are undeniably cells. This is an example of commonality

that crosses domains.

A number of mechanisms above the cellular level are also shared by plant and animal fife. For

example, both use some sort of vascular system to transport nutrients within the organism,

and both exhibit differentiation by sex among members of the same species.

The Structure of Matter The study of fields as diverse as astronomy and nuclear physics

provides us with many other examples of incredibly complex systems. Spanning these two

disciplines, we find yet another structural hierarchy. Astronomers study galaxies that are

arranged in clusters, and stars, planets, and various debris are the constituents of galaxies.

Likewise, nuclear physicists are concerned with a structural hierarchy, but one on an entirely

different scale. Atoms are made up of electrons, protons, and neutrons; electrons appear to be

elementary particles, but protons, neutrons, and other particles are formed from more basic

components called quarks.

Chapter 1: Complexity 10

Again we find that a great commonality in the form of shared mechanisms unifies this vast

hierarchy. Specifically, there appear to be only four distinct kinds of forces at work in the

universe: gravity, electromagnetic interaction, the strong force, and the weak force. Many

laws of physics involving these elementary forces, such as the laws of conservation of energy

and of momentum, apply to galaxies as well as quarks.

The Structure of Social Institutions As a final example of complex systems, we turn to the

structure of social institutions. Groups of people join together to accomplish tasks that cannot

be done by individuals. Some organizations are transitory, and some endure beyond many

lifetimes. As organizations grow larger, we see a distinct hierarchy emerge. Multinational

corporations contain companies, which in turn are made up of divisions, which in turn

contain branches, which in turn encompass local offices, and so on. If the organization

endures, the boundaries among these parts may change, and over time, a new, more stable

hierarchy may emerge.

The relationships among the various parts of a large organization are just like those found

among the components of a computer, or a plant, or even a galaxy. Specifically, the degree of

interaction among employees within an individual office is greater than that between

employees of different offices. A mail clerk usually does not interact with the chief executive

officer of a company but does interact frequently with other people in the mail room. Here

too, these different levels are unified by common mechanisms. The clerk and the executive

are both paid by the same financial organization, and both share common facilities, such as

the company's telephone system, to accomplish their tasks.

The Five Attributes of a Complex System

Drawing from this line of study, we conclude that there are five attributes common to all

complex systems. Building upon the work of Simon and Ando, Courtois suggests the

following:

1. "Frequently, complexity takes the form of a hierarchy, whereby a complex system is composed

of interrelated subsystems that have in turn their own subsystems, and so on, until some lowest

level of elementary components is reached" [7].

Simon points out that "the fact that many complex systems have a nearly decomposable,

hierarchic structure is a major facilitating factor enabling us to understand, describe, and even

'see' such systems and their parts" [8]. Indeed, it is likely that we can understand only those

systems that have a hierarchic structure.

It is important to realize that the architecture of a complex system is a function of its

components as well as the hierarchic relationships among these components. As Rechtin

observes, "All systems have subsystems and all systems are parts of larger systems . . . The

valued added by a system must come from the relationships between the parts, not from the

parts per se" [9].

Chapter 1: Complexity 11

Regarding the nature of the primitive components of a complex system, our experience

suggests that

2. The choice of what components in a system are primitive is relatively arbitrary and is largely

up to the discretion of the observer of the system.

What is primitive for one observer may be at a much higher level of abstraction for another.

Simon calls hierarchic systems decomposable, because they can be divided into identifiable

parts; he calls them nearly decomposable, because their parts are not completely

independent. This leads us to another attribute common to all complex systems:

3. “Intracomponent linkages are generally stronger than intercommoning linkages. This fact has

the effect of separating the high-frequency dynamics of the components - involving the internal

structure of the components - from the low-frequency dynamics - involving interaction among

components"[10].

This difference between intra- and intercomponent interactions provides a clear separation of

concerns among the various parts of a system, making it possible to study each part in

relative isolation.

As we have discussed, many complex systems are implemented with an economy of

expression. Simon thus notes that

4. "Hierarchic systems are usually composed of only a few different kinds of subsystems in

various combinations and arrangements " [11].

In other words, complex systems have common patterns. These patterns may involve the

reuse of small components, such as the cells found in both plants and animals, or of larger

structures, such as vascular systems, also found in both plants and animals.

Earlier, we noted that complex systems tend to evolve over time. As Simon suggests,

"complex systems will evolve from simple systems much more rapidly if there are stable

intermediate forms than if there are not” [12]. In more dramatic terms, Gall states that

5. “A complex system that works is invariably found to have evolved from a simple system that

worked.... A complex system designed from scratch never works and cannot be patched up to

make it work. You have to start over, beginning with a working simple system " [13].

As systems evolve, objects that were once considered complex become the primitive objects

upon which more complex systems are built. Furthermore, we can never craft these primitive

objects correctly the first time: we must use them in context first, and then improve them over

time as we learn more about the real behavior of the system.

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