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1、附 录 1. 外文文献Introduction To Objects1、The progress of abstractionAll programming languages provide abstractions. It can be argued that the complexity of the problems youre able to solve is directly related to the kind and quality of abstraction. By “kind” I mean, “What is it that you are abstracting?”
2、 Assembly language is a small abstraction of the underlying machine. Many so-called “imperative” languages that followed (such as FORTRAN, BASIC, and C) were abstractions of assembly language. These languages are big improvements over assembly language, but their primary abstraction still requires y
3、ou to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer must establish the association between the machine model (in the “solution space,” which is the place where youre modeling that problem, such as a computer) and the
4、model of the problem that is actually being solved (in the “problem space,” which is the place where the problem exists). The effort required to perform this mapping, and the fact that it is extrinsic to the programming language, produces programs that are difficult to write and expensive to maintai
5、n, and as a side effect created the entire “programming methods” industry. The alternative to modeling the machine is to model the problem youre trying to solve. Early languages such as LISP and APL chose particular views of the world (“All problems are ultimately lists” or “All problems are algorit
6、hmic,” respectively). PROLOG casts all problems into chains of decisions. Languages have been created for constraint-based programming and for programming exclusively by manipulating graphical symbols. (The latter proved to be too restrictive.) Each of these approaches is a good solution to the part
7、icular class of problem theyre designed to solve, but when you step outside of that domain they become awkward. The object-oriented approach goes a step further by providing tools for the programmer to represent elements in the problem space. This representation is general enough that the programmer
8、 is not constrained to any particular type of problem. We refer to the elements in the problem space and their representations in the solution space as “objects.” (You will also need other objects that dont have problem-space analogs.) The idea is that the program is allowed to adapt itself to the l
9、ingo of the problem by adding new types of objects, so when you read the code describing the solution, youre reading words that also express the problem. This is a more flexible and powerful language abstraction than what weve had before. Thus, OOP allows you to describe the problem in terms of the
10、problem, rather than in terms of the computer where the solution will run. Theres still a connection back to the computer: each object looks quite a bit like a little computerit has a state, and it has operations that you can ask it to perform. However, this doesnt seem like such a bad analogy to ob
11、jects in the real worldthey all have characteristics and behaviors. Alan Kay summarized five basic characteristics of Smalltalk, the first successful object-oriented language and one of the languages upon which Java is based. These characteristics represent a pure approach to object-oriented program
12、ming: Everything is an object. Think of an object as a fancy variable; it stores data, but you can “make requests” to that object, asking it to perform operations on itself. In theory, you can take any conceptual component in the problem youre trying to solve (dogs, buildings, services, etc.) and re
13、present it as an object in your program. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you “send a message” to that object. More concretely, you can think of a message as a request to call a method that belongs to a particular obje
14、ct. Each object has its own memory made up of other objects. Put another way, you create a new kind of object by making a package containing existing objects. Thus, you can build complexity into a program while hiding it behind the simplicity of objects. Every object has a type. Using the parlance,
15、each object is an instance of a class, in which “class” is synonymous with “type.” The most important distinguishing characteristic of a class is “What messages can you send to it?” All objects of a particular type can receive the same messages. This is actually a loaded statement, as you will see l
16、ater. Because an object of type “circle” is also an object of type “shape,” a circle is guaranteed to accept shape messages. This means you can write code that talks to shapes and automatically handle anything that fits the description of a shape. This substitutability is one of the powerful concepts in OOP. Booch offers an even more succinct description of an object:An object has state, behavior and identity.This means that an object can have internal data (which gives it state),
