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download sensor data, ignite starter, fire engine 3, etc.) May change, the basic objects (e.g. cameras,
sensors, starter, engines, operator, etc.) needed to create a system usually are constant. That is, while
there may now be three cameras instead of two, the new Camera-3 is just an instance of the basic object
‘camera’. Or while an infrared camera may now be the type needed, there is still a ‘camera’ and the
differences in power, warm-up time, and data storage may change, all that is kept isolated (hidden) from
affecting the rest of the system.
OOA incorporates the principles of abstraction, information hiding, inheritance, and a method of
organizing the problem space by using the three most “human” means of classification. These combined
principles, if properly applied, establish a more modular, bounded, stable and understandable software
system. These aspects of OOA should make a system created under this method more robust and less
susceptible to changes, properties which help create a safer software system design.
Abstraction refers to concentrating on only certain aspects of a complex problem, system, idea or
situation in order to better comprehend that portion. The perspective of the analyst focuses on similar
characteristics of the system objects that are most important to them. Then, at a later time, the analyst can
address other objects and their desired attributes or examine the details of an object and deal with each in
more depth. Data abstraction is used by OOA to create the primary organization for thinking and
specification in that the objects are first selected from a certain perspective and then each object is defined
in detail. An object is defined by the attributes it has and the functions it performs on those attributes.
An abstraction can be viewed, as per Shaw, as “a simplified description, or specification, of a system that
emphasizes some of the system’s details or properties while suppressing others. A good abstraction is
one that emphasizes details that are significant to the reader or user and suppresses details that are, at least
for the moment, immaterial or diversionary”.
Information hiding also helps manage complexity in that it allows encapsulation of requirements, which
might be subject to change. In addition, it helps to isolate the rest of the system from some object specific
design decisions. Thus, the rest of the s/w system sees only what is absolutely necessary of the inner
workings of any object.
Inheritance “ defines a relationship among classes [objects], wherein one class shares the structure or
behavior defined in one or more classes... Inheritance thus represents a hierarchy of abstractions, in which
a subclass [object] inherits from one or more superclasses [ancestor objects]. Typically, a subclass
augments or redefines the existing structure and behavior of its superclasses”.
Classification theory states that humans normally organize their thinking by: looking at an object and
comparing its attributes to those experienced before (e.g. looking at a cat, humans tend to think of its size,
color, temperament, etc. in relation to past experience with cats) distinguishing between an entire object
and its component parts (e.g., a rose bush versus its roots, flowers, leaves, thorns, stems, etc.)
classification of objects as distinct and separate groups (e.g. trees, grass, cows, cats, politicians).
In OOA, the first organization is to take the problem space and render it into objects and their attributes
(abstraction). The second step of organization is into Assembly Structures, where an object and its parts
are considered. The third form of organization of the problem space is into Classification Structures
during which the problem space is examined for generalized and specialized instances of objects
FAA System Safety Handbook, Appendix D
December 30, 2000
D - 4
(inheritance). That is, if looking at a railway system the objects could be engines (provide power to pull
cars), cars (provide storage for cargo), tracks (provide pathway for trains to follow/ride on), switches
(provide direction changing), stations (places to exchange cargo), etc. Then you would look at the
Assembly Structure of cars and determine what was important about their pieces parts, their wheels, floor
construction, coupling mechanism, siding, etc. Finally, Classification Structure of cars could be into
cattle, passenger, grain, refrigerated, and volatile liquid cars.
The purpose of all this classification is to provide modularity which partitions the system into well
defined boundaries that can be individually/independently understood, designed, and revised. However,
despite “classification theory”, choosing what objects represent a system is not always that straight
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System Safety Handbook系统安全手册下(78)
