外文翻译对象技术.doc

Object TechnologyObject technology is a new approach to developing software that allows programmers to create objects, a combination of data and program instructions. This new technology 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.1.1 Introduction to OTThere exists 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 dynamic marketplace. It is called Object Technology (OT).By allowing the integration of disparate and non compatible source, OT has the potential to precipitate a revolution in information systems 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, the recent emphasis task of enterprise information technology integration has brought OT into the spotlight.OT promises to provide component-level software objects that can be quickly combined to 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 by sending messages that can be understood by other objects, large integrated systems are easier to assemble.Each object is responsible for a specific function within either an application or a distributed 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 timing and extendibility of new systems.1.2 OT-based Products The current market for OT-based products can be divided into four major segments: Languages and programming tools Developers toolkits Object-Oriented database Object-Oriented CASE toolsThe largest 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, as well as extensions to PASCAL and Modula-2.Products in this category are available from a variety of vendors. Increasingly, the trend in this group is to offer the language compliers with associated development tools as part of a complete development environment.Developers toolkits account for the next largest part of the OT market. These products are designed to develop a program that 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.By providing developers 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.An important 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 collecting is a key problem.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 Digital and HP, and start-ups such as Object Design, Servio, and Objectivity, have all produced products.These products, dubbed ”Objectbases”, fill an important need by storing complex objects as 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.The value of an objectbase, as opposed to a database, is best described as following:“Object databases offer a better way to store objects because they provide all of the traditional database services without the overhead 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. Its like having to disassembling your car each night rather than just putting it into the garage!”Over the next few years, a shift from proprietary CASE implementations to those based on the object paradigm can be expected. This 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 are methodological in nature.1.3 0bject-oriented Programming Object-oriented programming (OOP) is a new approach to developing software that allows programmers to create objects, 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 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, such as when a new field is added to a file. With OOP, the program instructions and data are combined into objects that can be used repeatedly by programmers whenever they need them. Specific instructions, called methods define how the object acts when it is used by a program.With OOP, programmers define 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 unique 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 object. When an OOP object is sent an instruction to do something, called a message, unlike a traditional program, the message does not have to tell the OOP object exactly what to do. What to do is defined by the methods that the OOP object contains or has inherited.Objectoriented 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 gains 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 easily. It could mean two different user interfaces within an application, one for the user who likes to type, and another for the users who just want to shout at the terminal.Objects can be viewed as reusable components, and once the programmer has developed 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 creating 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 object he needs. But if productivity is your aim, reusability is worth the risks.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 used for many years, during which company needs may change and make software modifications necessary.Software reliability can be improved by object-oriented programming. Since the objects are repeatedly 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 improve with parallel processing.1.4 Object-oriented DBMSA shift toward object-oriented DBMSs does not have to replace relational DNMS. As its name implies, it is orientation rather than a full-blown DBMS model. As such, it can blend with and build on the relational schema.Object-oriented DBMSs integrate a variety of real-world data types such as business procedures and 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 text.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, objects can be reused and replicated. You can query the data on its functions, attributes, and relationships.By contrast, most RDBMSs demand that the knowledge associated with the data be written into and maintained separately in each application program.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 its advantages without going through a major conversion.The first form of object-oriented DBMS focused largely on the computer-aided design (CAD) market, which needed to store complex data types such as the graphics involved with an aircraft design.The second form is made up of the leading RDBMS vendors who support the concept of integrating object management capabilities whit their current line 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 a servers database engine . Any client attempting a transaction that does not conform to these procedures is simply rejected by the database. That capability enables users to shorten the development cycle, since integrity logic and business rules no longer need to be programmed into each application.This approach reduces maintenance costs as well, since any changes in the procedure can be made once at the server level instead of several times within all the affected applications.Last, the server-level procedures increase the systems performance, since the operations are taking place closer to where the data is actually stored.对象技术对象技术是开发软件的一种新方法，它允许程序员生成数据和程序相结合的对象。这一新技术从19世纪60年代后期出现以来一直在稳步发展，而且有希望成为解决正在出现的软件危机的主要组成部分之一。1.1对象技术（OT）引论有一项关键技术正在改变着人们构想、制造、使用和发展计算机系统的方法。这是一种很多公司用来提高效率、降低成本和适应不断变化的市场的技术。这就是所谓的对象技术（OT）。与推出的计算机芯片而引起的计算机的硬件革命一样，对象技术能够把各不相关的、计算机互不兼容的信息资源集成起来，从而有潜力发动一场信息系统设计的革命。虽然，对象技术不是一项新事物，从它很多年前问世以来，就在不断的开发和推出新产品。但是由于最近对整个企业内信息技术的集成这一复杂任务的重视，才使得对象技术成为热点。对象技术有望提供部件级的软件对象，这种软件对象能快速的组合起来，以构成能适应不断变化的业务的新应用软件。对象一旦被创建，就可以在其它应用软件中重复使用，从而降低开发成本和加快开发过程。由于对象是通过发送能被其它对象理解的消息进行交流的，故而可较容易地构成大型的集成系统。在一个应用软件或者一个分布式系统中，每个对象负责一个特定的功能。这就意味着，随着业务的变化，可以很容易的对单个对象升级、扩充或者更换，而系统的其它部分不受影响，这就能直接降低维护成本，加快新系统的问世，并提高其可扩充性。1.2 基于对象技术的产品目前，基于对象技术的产品市场可以分为四大类：语言与编程工具；开发人员工具箱；面向对象的数据库；面向对象的CASE工具。当前，对象技术产品市场中，最大的一类就是语言与编程工具。这种产品包括C、Smalltalk、Common Lisp System（CLOS）、Eiffel、Ada和ObjectiveC的语言编辑器以及PASCAL和Modula2的升级版本。这类产品可以从各种供应商那里得到，其趋势是越来越多地在提供语言编辑器地同时，提供相应的开发工具，作为整个开发环境的一部分。开发人员工具箱是对象技术市场的第二大类产品。这些产品被设计为用来开发程序，是开发人员很容易地把程序与分布环境连接起来，或者通过一个产品来开发图形屏幕。这类产品通过向开发人员提供高级的描述语言和可复用部件，给予开发人员开始编制面向对象系统的一种简单高效的方法。 在这类产品中，有一个相对较新的重要领域就是最终用户工具。这类工具之所以重要是因为组织和分析计算机系统能够收集的日益增长的大量数据是个关键问题。面向对象数据库管理系统是对象技术市场上的最令人振奋和高速增长的部分之一。一些公司，也包括DEC和HP等系统供应商以及诸如Object Design，Servio和Objectivity等新建公司，都开发出了新产品。这些称作“对象库”的产品通过把复杂对象按单个实体储存起来以满足某个重要的需求。对象库产品使得对象的存储、检索和共享的方式非常类似于存储在关系数据库管理系统中的数据。相对于数据库而言，对象库的价值可作如下最佳的描述：“对象数据库提供了一种更好的存储对象的方法，因为它们可以完成传统数据库的全部功能，却不需要在每次存储和检索对象时对其进行调整。与对象数据库相比，在关系数据库中存储复杂的对象，即使最好的情况也是很麻烦人的。这好比每晚必须把汽车拆开而不是直接放进车库一样。”今后几年，可望能看到专用的CASE实现工具将转到基于对象范例的工具上，这一领域的增长已滞后于早期的预测。基于对象技术的CASE工具将不得不作为解决大规模开发大系统的一套可行产品出现。这类产品还包括本质上属于方法学的那些工具。1.3 面向对象的程序设计面向对象的程序设计（OOP）是一种开发软件的新方法，它允许用户生成包括数据和程序指令的对象。传统的程序设计方法使数据（如文件）独立于使用这些数据进行工作的程序。因此，每一个传统程序都必须定义在该具体程序中数据将被如何使用。这样常常会导致产生多余的程序代码，当数据结构被改变（如将一个新字段增加到文件中）时，就必须对这些程序代码进行修改。而面向对象的程序设计技术把程序指令和数据组合到一起放入程序员需要时可以重复使用的对象中。具体指令（称为方法）用来定义对象被一个程序使用时是如何工作的。使用面向对象的程序审计技术时，程序员可以定义对象的类。每个类包括该类所独有的方法。每个类还可以有一个或者多个子类。每一个子类包括它的上一级类的方法以及该子类所独有的方法。面向对象的程序设计的这种把方法向下一级传送的能力称为“继承”。对象的具体实例（instance）包括来自它的上一级类中的所有方法以及该对象所独