外文翻譯-對象技術(shù)

1、<p>　　Object Technology</p><p>　　Object technology is a new approach to developing software that allows programmers to create objects, a combination of data and program instructions. This new technology

2、 has been steadily developed since the late 1960s and promises to be one of the major ingredients in the response to the ongoing software crisis.</p><p>　　1.1 Introduction to OT</p><p>　　There e

3、xists a critical technology that is changing the way we conceive, build, use and evolve our computer systems. It is a technology that many companies are adopting to increase their efficiency, reduce costs and adapt to a

4、dynamic marketplace. It is called Object Technology (OT).</p><p>　　By allowing the integration of disparate and non compatible source, OT has the potential to precipitate a revolution in information systems

5、design on a par with that caused in computer hardware by the introduction of the computer chip. Yet OT is not a new phenomenon. Development and product releases have been ongoing since its origin many years ago. However,

6、 the recent emphasis task of enterprise information technology integration has brought OT into the spotlight.</p><p>　　OT promises to provide component-level software objects that can be quickly combined to

7、build new applications that respond to changing business conditions. Once used, objects may be reused in other applications, lowering development costs and speeding up the development process. Because objects communicate

8、 by sending messages that can be understood by other objects, large integrated systems are easier to assemble.</p><p>　　Each object is responsible for a specific function within either an application or a di

9、stributed system. That means that as the business changes, individual object may be easily upgraded, augmented or replaced, leaving the rest of the system untouched. This directly reduces the cost of maintenance and the

10、timing and extendibility of new systems.</p><p>　　1.2 OT-based Products </p><p>　　The current market for OT-based products can be divided into four major segments:</p><p>　　·L

11、anguages and programming tools</p><p>　　·Developers’ toolkits</p><p>　　·Object-Oriented database</p><p>　　·Object-Oriented CASE tools</p><p>　　The larges

12、t segment of the current market for OT-based products is languages and programming tools. Products in this area include language compliers for C++, Smalltalk, Common Lisp Object System (CLOS), Eiffel, Ada and Objective-C

13、, as well as extensions to PASCAL and Modula-2.</p><p>　　Products in this category are available from a variety of vendors. Increasingly, the trend in this group is to offer the language compliers with assoc

14、iated development tools as part of a complete development environment.</p><p>　　Developers’ toolkits account for the next largest part of the OT market. These products are designed to develop a program that

15、enables a developer to easily do one of two things. The first is interfacing an application to distributed environment. The second is developing a graphical screen through a product.</p><p>　　By providing de

16、velopers with higher level description language and reusable components, products in this category give developers an easy and cost effective way to begin producing object-oriented systems.</p><p>　　An impor

17、tant component in this category is the relatively new area of end-users tools. This element is important because organizing and analying the increasingly large amounts of data that computer systems are capable of collect

18、ing is a key problem.</p><p>　　Object-oriented database management systems are one of the most interesting and rapidly growing segments of the OT market. A number of companies, including systems vendors like

19、 Digital and HP, and start-ups such as Object Design, Servio, and Objectivity, have all produced products.</p><p>　　These products, dubbed ”O(jiān)bjectbases”, fill an important need by storing complex objects as

20、a single entity. The objectbase products allow objects to be stored, retrieved and shared in much the same way as data is stored in a relational database management system.</p><p>　　The value of an objectbas

21、e, as opposed to a database, is best described as following:</p><p>　　“Object databases offer a better way to store objects because they provide all of the traditional database services without the overhead

22、of disassembling and reassembling objects every time they are stored and retrieved. Compared with an object database, storing complex objects in a relational database is tedious at best. It’s like having to disassembling

23、 your car each night rather than just putting it into the garage!”</p><p>　　Over the next few years, a shift from proprietary CASE implementations to those based on the object paradigm can be expected. This

24、area has lagged growth from earlier projections. OT-based CASE tools will have to emerge as a viable product category to address the wide scale development of large systems. This category also include those tools that ar

25、e methodological in nature.</p><p>　　1.3 0bject-oriented Programming </p><p>　　Object-oriented programming (OOP) is a new approach to developing software that allows programmers to create o

26、bjects, a combination of data and program instructions. Traditional programming methods keep data, such as files, independent of the programs that work with the data. Each traditional program, t5herfore, must define how

27、the data will be used for that particular program. This often results in redundant programming code that must be changed every time the structure of the data is changed, </p><p>　　With OOP, programmers defin

28、e classes of objects. Each class contains the methods that are unique to that class. Each class can have one or more subclasses. Each subclass contains the methods of its higher level classes plus whatever methods are un

29、ique to the subclass. The OOP capability to pass methods to lower levels is called “inheritance”. A specific instance of an object contains all methods from its higher level classes plus any methods that a unique to the

30、object. When an OOP object is sen</p><p>　　Object—oriented programming can bring many advantages to users. It can bring productivity gains as high as 1000 to 1500 percent instead of the 10 or 15 percent gain

31、s available from structured programming methods. It allows large complex systems to be built which are not economically feasible using traditional programming techniques. It allows program modifications to be made more e

32、asily. It could mean two different user interfaces within an application, one for the user who likes to type, and an</p><p>　　Objects can be viewed as reusable components, and once the programmer has develop

33、ed a library of these components, he can minimize the amount of new coding required. One user envisions a commercial library of objects which could be purchased by programmers and reused for various applications. But cr

34、eating a library is no simple task because the integrity of the original software design is critical. Reusability can be a mixed blessing for users, too, as a programmers has to be able to find the o</p><p>

35、　　The long-term productivity of systems is enhanced by object-oriented programming. Because of the modular nature of the code, programs are more malleable. This is particularly beneficial for applications that will be us

36、ed for many years, during which company needs may change and make software modifications necessary.</p><p>　　Software reliability can be improved by object-oriented programming. Since the objects are repeate

37、dly tested in a variety of applications, bugs are more likely to be found and corrected. Object-oriented programming also has potential benefits in parallel processing. Execution speed under object oriented methods will

38、improve with parallel processing.</p><p>　　1.4 Object-oriented DBMS</p><p>　　A shift toward object-oriented DBMSs does not have to replace relational DNMS. As its name implies, it is orientation

39、 rather than a full-blown DBMS model. As such, it can blend with and build on the relational schema.</p><p>　　Object-oriented DBMSs integrate a variety of real-world data types –such as business procedures a

40、nd policies, graphics, pictures, voice, and an non-tated text. Current relational products are not equipped to handle them efficiently. Data types in RDBMSs are more commonly record-oriented and expressed in numbers and

41、text.</p><p>　　Object orientation also makes contributions to application development efficiency. It makes the data function, attributes, and relationships an integral part of the object. In this way, object

42、s can be reused and replicated. You can query the data on its functions, attributes, and relationships.</p><p>　　By contrast, most RDBMSs demand that the knowledge associated with the data be written into an

43、d maintained separately in each application program.</p><p>　　Object orientation is going to be available in two forms: one for those who need and want a radical change, and one for those who want some of it

44、s advantages without going through a major conversion.</p><p>　　The first form of object-oriented DBMS focused largely on the computer-aided design (CAD) market, which needed to store complex data types such

45、 as the graphics involved with an aircraft design.</p><p>　　The second form is made up of the leading RDBMS vendors who support the concept of integrating object management capabilities whit their current li

46、ne of relational products. Sybase, Inc, the first vendor to introduce an object-oriented capability, offers Sybase , which enables the user to program a limited number of business procedures along with the data types in

47、a server’s database engine . Any client attempting a transaction that does not conform to these procedures is simply rejected by the </p><p>　　This approach reduces maintenance costs as well, since any chang

48、es in the procedure can be made once at the server level instead of several times within all the affected applications.</p><p>　　Last, the server-level procedures increase the system’s performance, since the

49、 operations are taking place closer to where the data is actually stored.</p><p><b>　　對象技術(shù)</b></p><p>　　對象技術(shù)是開發(fā)軟件的一種新方法，它允許程序員生成數(shù)據(jù)和程序相結(jié)合的對象。這一新技術(shù)從19世紀(jì)60年代后期出現(xiàn)以來一直在穩(wěn)步發(fā)展，而且有希望成為解決正在出現(xiàn)的

50、軟件危機的主要組成部分之一。</p><p>　　1.1對象技術(shù)（OT）引論</p><p>　　有一項關(guān)鍵技術(shù)正在改變著人們構(gòu)想、制造、使用和發(fā)展計算機系統(tǒng)的方法。這是一種很多公司用來提高效率、降低成本和適應(yīng)不斷變化的市場的技術(shù)。這就是所謂的對象技術(shù)（OT）。</p><p>　　與推出的計算機芯片而引起的計算機的硬件革命一樣，對象技術(shù)能夠把各不相關(guān)的、計算機互不

51、兼容的信息資源集成起來，從而有潛力發(fā)動一場信息系統(tǒng)設(shè)計的革命。雖然，對象技術(shù)不是一項新事物，從它很多年前問世以來，就在不斷的開發(fā)和推出新產(chǎn)品。但是由于最近對整個企業(yè)內(nèi)信息技術(shù)的集成這一復(fù)雜任務(wù)的重視，才使得對象技術(shù)成為熱點。</p><p>　　對象技術(shù)有望提供部件級的軟件對象，這種軟件對象能快速的組合起來，以構(gòu)成能適應(yīng)不斷變化的業(yè)務(wù)的新應(yīng)用軟件。對象一旦被創(chuàng)建，就可以在其它應(yīng)用軟件中重復(fù)使用，從而降低開發(fā)成本和

52、加快開發(fā)過程。由于對象是通過發(fā)送能被其它對象理解的消息進行交流的，故而可較容易地構(gòu)成大型的集成系統(tǒng)。</p><p>　　在一個應(yīng)用軟件或者一個分布式系統(tǒng)中，每個對象負(fù)責(zé)一個特定的功能。這就意味著，隨著業(yè)務(wù)的變化，可以很容易的對單個對象升級、擴充或者更換，而系統(tǒng)的其它部分不受影響，這就能直接降低維護成本，加快新系統(tǒng)的問世，并提高其可擴充性。</p><p>　　1.2 基于對象技術(shù)的產(chǎn)品&

53、lt;/p><p>　　目前，基于對象技術(shù)的產(chǎn)品市場可以分為四大類：</p><p><b>　　·語言與編程工具；</b></p><p><b>　　·開發(fā)人員工具箱；</b></p><p>　　·面向?qū)ο蟮臄?shù)據(jù)庫；</p><p>　　

54、83;面向?qū)ο蟮腃ASE工具。</p><p>　　當(dāng)前，對象技術(shù)產(chǎn)品市場中，最大的一類就是語言與編程工具。這種產(chǎn)品包括C＋＋、Smalltalk、Common Lisp System（CLOS）、Eiffel、Ada和Objective－C的語言編輯器以及PASCAL和Modula－2的升級版本。</p><p>　　這類產(chǎn)品可以從各種供應(yīng)商那里得到，其趨勢是越來越多地在提供語言編輯器地

55、同時，提供相應(yīng)的開發(fā)工具，作為整個開發(fā)環(huán)境的一部分。</p><p>　　開發(fā)人員工具箱是對象技術(shù)市場的第二大類產(chǎn)品。這些產(chǎn)品被設(shè)計為用來開發(fā)程序，是開發(fā)人員很容易地把程序與分布環(huán)境連接起來，或者通過一個產(chǎn)品來開發(fā)圖形屏幕。</p><p>　　這類產(chǎn)品通過向開發(fā)人員提供高級的描述語言和可復(fù)用部件，給予開發(fā)人員開始編制面向?qū)ο笙到y(tǒng)的一種簡單高效的方法。</p><p&g

56、t;　　在這類產(chǎn)品中，有一個相對較新的重要領(lǐng)域就是最終用戶工具。這類工具之所以重要是因為組織和分析計算機系統(tǒng)能夠收集的日益增長的大量數(shù)據(jù)是個關(guān)鍵問題。</p><p>　　面向?qū)ο髷?shù)據(jù)庫管理系統(tǒng)是對象技術(shù)市場上的最令人振奮和高速增長的部分之一。一些公司，也包括DEC和HP等系統(tǒng)供應(yīng)商以及諸如Object Design，Servio和Objectivity等新建公司，都開發(fā)出了新產(chǎn)品。</p><

57、;p>　　這些稱作“對象庫”的產(chǎn)品通過把復(fù)雜對象按單個實體儲存起來以滿足某個重要的需求。對象庫產(chǎn)品使得對象的存儲、檢索和共享的方式非常類似于存儲在關(guān)系數(shù)據(jù)庫管理系統(tǒng)中的數(shù)據(jù)。</p><p>　　相對于數(shù)據(jù)庫而言，對象庫的價值可作如下最佳的描述：</p><p>　　“對象數(shù)據(jù)庫提供了一種更好的存儲對象的方法，因為它們可以完成傳統(tǒng)數(shù)據(jù)庫的全部功能，卻不需要在每次存儲和檢索對象時對其進

58、行調(diào)整。與對象數(shù)據(jù)庫相比，在關(guān)系數(shù)據(jù)庫中存儲復(fù)雜的對象，即使最好的情況也是很麻煩人的。這好比每晚必須把汽車拆開而不是直接放進車庫一樣?！?lt;/p><p>　　今后幾年，可望能看到專用的CASE實現(xiàn)工具將轉(zhuǎn)到基于對象范例的工具上，這一領(lǐng)域的增長已滯后于早期的預(yù)測?；趯ο蠹夹g(shù)的CASE工具將不得不作為解決大規(guī)模開發(fā)大系統(tǒng)的一套可行產(chǎn)品出現(xiàn)。這類產(chǎn)品還包括本質(zhì)上屬于方法學(xué)的那些工具。</p><p

59、>　　1.3 面向?qū)ο蟮某绦蛟O(shè)計</p><p>　　面向?qū)ο蟮某绦蛟O(shè)計（OOP）是一種開發(fā)軟件的新方法，它允許用戶生成包括數(shù)據(jù)和程序指令的對象。傳統(tǒng)的程序設(shè)計方法使數(shù)據(jù)（如文件）獨立于使用這些數(shù)據(jù)進行工作的程序。因此，每一個傳統(tǒng)程序都必須定義在該具體程序中數(shù)據(jù)將被如何使用。這樣常常會導(dǎo)致產(chǎn)生多余的程序代碼，當(dāng)數(shù)據(jù)結(jié)構(gòu)被改變（如將一個新字段增加到文件中）時，就必須對這些程序代碼進行修改。而面向?qū)ο蟮某绦蛟O(shè)計

60、技術(shù)把程序指令和數(shù)據(jù)組合到一起放入程序員需要時可以重復(fù)使用的對象中。具體指令（稱為方法）用來定義對象被一個程序使用時是如何工作的。</p><p>　　使用面向?qū)ο蟮某绦驅(qū)徲嫾夹g(shù)時，程序員可以定義對象的類。每個類包括該類所獨有的方法。每個類還可以有一個或者多個子類。每一個子類包括它的上一級類的方法以及該子類所獨有的方法。面向?qū)ο蟮某绦蛟O(shè)計的這種把方法向下一級傳送的能力稱為“繼承”。對象的具體實例（instance

61、）包括來自它的上一級類中的所有方法以及該對象所獨有的方法。當(dāng)一個OOP對象發(fā)送一條指令（稱為消息）來執(zhí)行某一操作時，和傳統(tǒng)的程序不同，消息并不需要準(zhǔn)確地告訴OOP對象做什么。做什么被OOP對象所包括或者繼承的方法定義。</p><p>　　面向?qū)ο蟮某绦蛟O(shè)計可以為用戶帶來很多的好處。它可以使生產(chǎn)率增益高達(dá)百分之一千到一千五，而結(jié)構(gòu)化程序設(shè)計方法的增益只有百分之十到十五。它使那些用傳統(tǒng)編程技術(shù)在經(jīng)濟上不可行的大型復(fù)

62、雜系統(tǒng)得以實現(xiàn)，也能使程序修改更加的容易。這可能意味著在一個應(yīng)用程序中有兩個不同的用戶接口，一個是供喜歡使用鍵盤的用戶用，另一個是供只想與終端對話的用戶用。</p><p>　　對象可以看作是可重復(fù)使用的部件，一旦程序員開發(fā)了這種部件的一個庫，他就能使新編寫程序的量減至最小。用戶想往商品化的對象庫，可由程序員購買，并可用于不同的應(yīng)用中。但構(gòu)造這樣一個對象庫不是一件簡單的事情，因為初始軟件設(shè)計的完整性至關(guān)重要。對用

63、戶來說，可復(fù)用性也可能是一件好壞難說的事情，因為程序員必須能夠找到他所需要的對象。但若提高生產(chǎn)率是你的目標(biāo)，那么可復(fù)用性是值得冒一下的風(fēng)險。</p><p>　　面向?qū)ο蟮木幊烫岣吡讼到y(tǒng)的長期生產(chǎn)率。由于這種程序的模塊化本質(zhì)，程序更容易適應(yīng)。這一點對那些需要使用很多年的應(yīng)用程序來說尤其有益，因為在這期間，公司的需要可能有變化，軟件的修改是必不可少的。</p><p>　　面向?qū)ο蟮木幊棠芴?/p>

64、高軟件的可靠性。由于對象在不同的應(yīng)用中反復(fù)測試，更有可能查出和糾正其錯誤。面向?qū)ο蟮木幊淘诓⑿刑幚碇幸灿袧撛趦?yōu)勢，使用面向?qū)ο蟮姆椒ǎ⑿刑幚淼膱?zhí)行速度將得到提高。</p><p>　　1.4 面向?qū)ο蟮臄?shù)據(jù)庫管理系統(tǒng) </p><p>　　向面向?qū)ο蟮臄?shù)據(jù)庫管理系統(tǒng)（OODBMS）的轉(zhuǎn)變不必取代原有的關(guān)系型數(shù)據(jù)庫管理系統(tǒng)（DBMS），因為顧名思義，它只是一個方向，而不是一個完善的DBMS

65、模型。它本身能與關(guān)系模式混合，并在其上構(gòu)建。</p><p>　　面向?qū)ο驞BMS把真實世界中的各種數(shù)據(jù)類型集中在一起，如業(yè)務(wù)過程與政策、圖形、圖像、聲音和注釋過的文本。目前的關(guān)系產(chǎn)品不能有效的處理這些數(shù)據(jù)類型。關(guān)系DBMS中的數(shù)據(jù)類型一般是面向記錄，并只用數(shù)字和文本表示。</p><p>　　面向?qū)ο笠灿兄谔岣邞?yīng)用開發(fā)的效率。它使數(shù)據(jù)功能、屬性和關(guān)系成為對象的一個組成部分。這樣，對象可

66、以重復(fù)使用和復(fù)制，可以按功能、屬性和關(guān)系來查詢數(shù)據(jù)。</p><p>　　與此相反，多數(shù)DBMS要求將與數(shù)據(jù)有關(guān)的知識寫進每個應(yīng)用程序中，并分別在其中維護。</p><p>　　面向?qū)ο笥袃煞N形式：一種適用于那些需要徹底變革的人，另一種適用于那些需要不經(jīng)大的改變就能得到很好使用的人們。</p><p>　　面向?qū)ο驞BMS的第一種形式主要著眼于計算機輔助設(shè)計（CAD

67、）市場，它需要存儲復(fù)雜的數(shù)據(jù)類型，如飛機設(shè)計中的圖形。</p><p>　　第二種形式占主導(dǎo)地位的關(guān)系DBMS廠商所提供的產(chǎn)品組成，這些廠商支持把對象管理能力集成到其當(dāng)前的關(guān)系產(chǎn)品系列中，Sybase公司（首家推出具有面向?qū)ο蠊δ艿漠a(chǎn)品）提供的Sybase數(shù)據(jù)庫系統(tǒng)，能使用戶對有限數(shù)目的業(yè)務(wù)過程與服務(wù)器的數(shù)據(jù)庫系統(tǒng)中的數(shù)據(jù)類型一起進行編程。任何一個想處理與這些過程不符合的事務(wù)處理的客戶機都將被數(shù)據(jù)庫拒絕。此功能使