毕业设计论文外文中英文翻译讲解

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毕业设计中英文翻译【范本模板】

英文The road (highway)The road is one kind of linear construction used for travel。

It is made of the roadbed，the road surface, the bridge, the culvert and the tunnel. In addition, it also has the crossing of lines, the protective project and the traffic engineering and the route facility。

The roadbed is the base of road surface, road shoulder，side slope, side ditch foundations. It is stone material structure, which is designed according to route's plane position .The roadbed, as the base of travel, must guarantee that it has the enough intensity and the stability that can prevent the water and other natural disaster from corroding.The road surface is the surface of road. It is single or complex structure built with mixture。

The road surface require being smooth，having enough intensity，good stability and anti—slippery function. The quality of road surface directly affects the safe, comfort and the traffic。

毕业设计外文翻译英文加中文

A Comparison of Soft Start Mechanisms for Mining BeltConveyors1800 Washington Road Pittsburgh, PA 15241 Belt Conveyors are an important method for transportation of bulk materials in the mining industry. The control of the application of the starting torque from the belt drive system to the belt fabric affects the performance, life cost, and reliability of the conveyor. This paper examines applications of each starting method within the coal mining industry.INTRODUCTIONThe force required to move a belt conveyor must be transmitted by the drive pulley via friction between the drive pulley and the belt fabric. In order to transmit power there must be a difference in the belt tension as it approaches and leaves the drive pulley. These conditions are true for steady state running, starting, and stopping. Traditionally, belt designs are based on static calculations of running forces. Since starting and stopping are not examined in detail, safety factors are applied to static loadings (Harrison, 1987). This paper will primarily address the starting or acceleration duty of the conveyor. The belt designer must control starting acceleration to prevent excessive tension in the belt fabric and forces in the belt drive system (Suttees, 1986). High acceleration forces can adversely affect the belt fabric, belt splices, drive pulleys, idler pulleys, shafts, bearings, speed reducers, and couplings. Uncontrolled acceleration forces can cause belt conveyor system performance problems with vertical curves, excessive belt take-up movement, loss of drive pulley friction, spillage of materials, and festooning of the belt fabric. The belt designer is confronted with two problems, The belt drive system must produce a minimum torque powerful enough to start the conveyor, and controlled such that the acceleration forces are within safe limits. Smooth starting of the conveyor can be accomplished by the use of drive torque control equipment, either mechanical or electrical, or a combination of the two (CEM, 1979).SOFT START MECHANISM EVALUATION CRITERIONWhat is the best belt conveyor drive system? The answer depends on many variables. The best system is one that provides acceptable control for starting, running, and stopping at a reasonable cost and with high reliability (Lewdly and Sugarcane, 1978). Belt Drive System For the purposes of this paper we will assume that belt conveyors are almost always driven byelectrical prime movers (Goodyear Tire and Rubber, 1982). The belt "drive system" shall consist of multiple components including the electrical prime mover, the electrical motor starter with control system, the motor coupling, the speed reducer, the low speed coupling, the belt drive pulley, and the pulley brake or hold back (Cur, 1986). It is important that the belt designer examine the applicability of each system component to the particular application. For the purpose of this paper, we will assume that all drive system components are located in the fresh air, non-permissible, areas of the mine, or in non-hazardous, National Electrical Code, Article 500 explosion-proof, areas of the surface of the mine.Belt Drive Component Attributes SizeCertain drive components are available and practical in different size ranges. For this discussion, we will assume that belt drive systems range from fractional horsepower to multiples of thousands of horsepower. Small drive systems are often below 50 horsepower. Medium systems range from 50 to 1000 horsepower. Large systems can be considered above 1000 horsepower. Division of sizes into these groups is entirely arbitrary. Care must be taken to resist the temptation to over motor or under motor a belt flight to enhance standardization. An over motored drive results in poor efficiency and the potential for high torques, while an under motored drive could result in destructive overspending on regeneration, or overheating with shortened motor life (Lords, et al., 1978).Torque ControlBelt designers try to limit the starting torque to no more than 150% of the running torque (CEMA, 1979; Goodyear, 1982). The limit on the applied starting torque is often the limit of rating of the belt carcass, belt splice, pulley lagging, or shaft deflections. On larger belts and belts with optimized sized components, torque limits of 110% through 125% are common (Elberton, 1986). In addition to a torque limit, the belt starter may be required to limit torque increments that would stretch belting and cause traveling waves. An ideal starting control system would apply a pretension torque to the belt at rest up to the point of breakaway, or movement of the entire belt, then a torque equal to the movement requirements of the belt with load plus a constant torque to accelerate the inertia of the system components from rest to final running speed. This would minimize system transient forces and belt stretch (Shultz, 1992). Different drive systems exhibit varying ability to control the application of torques to the belt at rest and at different speeds. Also, the conveyor itself exhibits two extremes of loading. An empty belt normally presents the smallest required torque for breakaway and acceleration, while a fully loaded belt presents the highest required torque. A mining drive system must be capable of scaling the applied torque from a 2/1 ratio for a horizontal simple belt arrangement, to a 10/1 ranges for an inclined or complex belt profile.Thermal RatingDuring starting and running, each drive system may dissipate waste heat. The waste heat may be liberated in the electrical motor, the electrical controls,, the couplings, the speed reducer, or the belt braking system. The thermal load of each start Is dependent on the amount of belt load and the duration of the start. The designer must fulfill the application requirements for repeated starts after running the conveyor at full load. Typical mining belt starting duties vary from 3 to 10 starts per hour equally spaced, or 2 to 4 starts in succession. Repeated starting may require the dreading or over sizing of system components. There is a direct relationship between thermal rating for repeated starts and costs. Variable Speed. Some belt drive systems are suitable for controlling the starting torque and speed, but only run at constant speed. Some belt applications would require a drive system capable of running for extended periods at less than full speed. This is useful when the drive load must be shared with other drives, the belt is used as a process feeder for rate control of the conveyed material, the belt speed is optimized for the haulage rate, the belt is used at slower speeds to transport men or materials, or the belt is run a slow inspection or inching speed for maintenance purposes (Hager, 1991). The variable speed belt drive will require a control system based on some algorithm to regulate operating speed. Regeneration or Overhauling Load. Some belt profiles present the potential for overhauling loads where the belt system supplies energy to the drive system. Not all drive systems have the ability to accept regenerated energy from the load. Some drives can accept energy from the load and return it to the power line for use by other loads. Other drives accept energy from the load and dissipate it into designated dynamic or mechanical braking elements. Some belt profiles switch from motoring to regeneration during operation. Can the drive system accept regenerated energy of a certain magnitude for the application? Does the drive system have to control or modulate the amount of retarding force during overhauling? Does the overhauling occur when running and starting? Maintenance and Supporting Systems. Each drive system will require periodic preventative maintenance. Replaceable items would include motor brushes, bearings, brake pads, dissipation resistors, oils, and cooling water. If the drive system is conservatively engineered and operated, the lower stress on consumables will result in lower maintenance costs. Some drives require supporting systems such as circulating oil for lubrication, cooling air or water, environmental dust filtering, or computer instrumentation. The maintenance of the supporting systems can affect the reliability of the drive system.CostThe drive designer will examine the cost of each drive system. The total cost is the sum of the first capital cost to acquire the drive, the cost to install and commission the drive, thecost to operate the drive, and the cost to maintain the drive. The cost for power to operate the drive may vary widely with different locations. The designer strives to meet all system performance requirements at lowest total cost. Often more than one drive system may satisfy all system performance criterions at competitive costs.ComplexityThe preferred drive arrangement is the simplest, such as a single motor driving through a single head pulley.However,mechanical, economic,and functional requirements often necessitate the use of complex drives.The belt designer must balance the need for sophistication against the problems that accompany complex systems. Complex systems require additional design engineering for successful deployment. An often-overlooked cost in a complex system is the cost of training onsite personnel, or the cost of downtime as a result of insufficient training.SOFT START DRIVE CONTROL LOGICEach drive system will require a control system to regulate the starting mechanism. The most common type of control used on smaller to medium sized drives with simple profiles is termed "Open Loop Acceleration Control". In open loop, the control system is previously configured to sequence the starting mechanism in a prescribed manner, usually based on time. In open loop control, drive-operating parameters such as current, torque, or speed do not influence sequence operation. This method presumes that the control designer has adequately modeled drive system performance on the conveyor. For larger or more complex belts, "Closed Loop" or "Feedback" control may he utilized. In closed loop control, during starting, the control system monitors via sensors drive operating parameters such as current level of the motor, speed of the belt, or force on the belt, and modifies the starting sequence to control, limit, or optimize one or wore parameters. Closed loop control systems modify the starting applied force between an empty and fully loaded conveyor. The constants in the mathematical model related to the measured variable versus the system drive response are termed the tuning constants. These constants must be properly adjusted for successful application to each conveyor. The most common schemes for closed loop control of conveyor starts are tachometer feedback for speed control and load cell force or drive force feedback for torque control. On some complex systems, It is desirable to have the closed loop control system adjust itself for various encountered conveyor conditions. This is termed "Adaptive Control". These extremes can involve vast variations in loadings, temperature of the belting, location of the loading on the profile, or multiple drive options on the conveyor. There are three commonadaptive methods. The first involves decisions made before the start, or 'Restart Conditioning'. If the control system could know that the belt is empty, it would reduce initial force and lengthen the application of acceleration force to full speed. If the belt is loaded, the control system would apply pretension forces under stall for less time and supply sufficient torque to adequately accelerate the belt in a timely manner. Since the belt only became loaded during previous running by loading the drive, the average drive current can be sampled when running and retained in a first-in-first-out buffer memory that reflects the belt conveyance time. Then at shutdown the FIFO average may be use4 to precondition some open loop and closed loop set points for the next start. The second method involves decisions that are based on drive observations that occur during initial starting or "Motion Proving'. This usually involves a comparison In time of the drive current or force versus the belt speed. if the drive current or force required early in the sequence is low and motion is initiated, the belt must be unloaded. If the drive current or force required is high and motion is slow in starting, the conveyor must be loaded. This decision can be divided in zones and used to modify the middle and finish of the start sequence control. The third method involves a comparison of the belt speed versus time for this start against historical limits of belt acceleration, or 'Acceleration Envelope Monitoring'. At start, the belt speed is measured versus time. This is compared with two limiting belt speed curves that are retained in control system memory. The first curve profiles the empty belt when accelerated, and the second one the fully loaded belt. Thus, if the current speed versus time is lower than the loaded profile, it may indicate that the belt is overloaded, impeded, or drive malfunction. If the current speed versus time is higher than the empty profile, it may indicate a broken belt, coupling, or drive malfunction. In either case, the current start is aborted and an alarm issued.CONCLUSIONThe best belt starting system is one that provides acceptable performance under all belt load Conditions at a reasonable cost with high reliability. No one starting system meets all needs. The belt designer must define the starting system attributes that are required for each belt. In general, the AC induction motor with full voltage starting is confined to small belts with simple profiles. The AC induction motor with reduced voltage SCR starting is the base case mining starter for underground belts from small to medium sizes. With recent improvements, the AC motor with fixed fill fluid couplings is the base case for medium to large conveyors with simple profiles. The Wound Rotor Induction Motor drive is the traditional choice for medium to large belts with repeated starting duty or complex profilesthat require precise torque control. The DC motor drive, Variable Fill Hydrokinetic drive, and the Variable Mechanical Transmission drive compete for application on belts with extreme profiles or variable speed at running requirements. The choice is dependent on location environment, competitive price, operating energy losses, speed response, and user familiarity. AC Variable Frequency drive and Brush less DC applications are limited to small to medium sized belts that require precise speed control due to higher present costs and complexity. However, with continuing competitive and technical improvements, the use of synthesized waveform electronic drives will expand.REFERENCES[1]Michael L. Nave, P.E.1989.CONSOL Inc.煤矿业带式输送机几种软起动方式的比较1800 年华盛顿路匹兹堡, PA 15241带式运送机是采矿工业运输大批原料的重要方法。

本科毕业设计外文翻译(中文)

本科生毕业设计（论文）外文翻译外文原文题目：Real-time interactive optical micromanipulation of a mixture of high- and low-index particles中文翻译题目：高低折射率微粒混合物的实时交互式光学微操作毕业设计（论文）题目：阵列光镊软件控制系统设计姓名：任有健学院：生命学院班级：06210501指导教师：李勤高低折射率微粒混合物的实时交互式光学微操作Peter John Rodrigo Vincent Ricardo Daria Jesper Glückstad丹麦罗斯基勒DK-4000号，Risø国家实验室光学和等离子研究系jesper.gluckstad@risoe.dkhttp://www.risoe.dk/ofd/competence/ppo.htm摘要：本文论证一种对于胶体的实时交互式光学微操作的方法，胶体中包含两种折射率的微粒，与悬浮介质（0n ）相比，分别低于（0L n n <）、高于（0H n n >）悬浮介质的折射率。

球形的高低折射率微粒在横平板上被一批捕获激光束生成的约束光势能捕获，捕获激光束的横剖面可以分为“礼帽形”和“圆环形”两种光强剖面。

这种应用方法在光学捕获的空间分布和个体几何学方面提供了广泛的可重构性。

我们以实验为基础证实了同时捕获又独立操作悬浮于水（0 1.33n =）中不同尺寸的球形碳酸钠微壳（ 1.2L n ≈）和聚苯乙烯微珠（ 1.57H n =）的独特性质。

1 引言光带有动量和角动量。

伴随于光与物质相互作用的动量转移为我们提供了在介观量级捕获和操作微粒的方法。

过去数十年中的巨大发展已经导致了在生物和物理领域常规光学捕获的各种应用以及下一代光学微操作体系的出现[1-5]。

建筑学Modern-Architecture现代建筑大学毕业论文外文文献翻译及原文

建筑学Modern-Architecture现代建筑⼤学毕业论⽂外⽂⽂献翻译及原⽂毕业设计（论⽂）外⽂⽂献翻译⽂献、资料中⽂题⽬：现代建筑⽂献、资料英⽂题⽬：Modern Architecture⽂献、资料来源：⽂献、资料发表（出版）⽇期：院（部）：专业：班级：姓名：学号：指导教师：翻译⽇期： 2017.02.14建筑学毕业设计的外⽂⽂献及译⽂⽂献、资料题⽬：《Advanced Encryption Standard》⽂献、资料发表（出版）⽇期：2004.10.25外⽂⽂献：Modern ArchitectureModern architecture, not to be confused with 'contemporary architecture', is a term given to a number of building styles with similar characteristics, primarily the simplification of form and the elimination of ornament. While the style was conceived early in the 20th century and heavily promoted by a few architects, architectural educators and exhibits, very few Modern buildings were built in the first half of the century. For three decades after the Second World War, however, it became the dominant architectural style for institutional and corporate building.1. OriginsSome historians see the evolution of Modern architecture as a social matter, closely tied to the project of Modernity and hence to the Enlightenment, a result of social and political revolutions.Others see Modern architecture as primarily driven by technological and engineering developments, and it is true that the availability of new building materials such as iron, steel, concrete and glass drove the invention of new building techniques as part of the Industrial Revolution. In 1796, Shrewsbury mill owner Charles Bage first used his ‘fireproof’ design, which relied on cast iron and brick with flag stone floors. Such construction greatly strengthened the structure of mills, which enabled them to accommodate much bigger machines. Due to poor knowledge of iron's properties as a construction material, a number of early mills collapsed. It was not until the early 1830s that Eaton Hodgkinson introduced the section beam, leading to widespread use of iron construction, this kind of austere industrial architecture utterly transformed the landscape of northern Britain, leading to the description, "Dark satanic mills" of places like Manchester and parts of West Yorkshire. The Crystal Palace by Joseph Paxton at the Great Exhibition of 1851 was an early example of iron and glass construction; possibly the best example is the development of the tall steel skyscraper in Chicago around 1890 by William Le Baron Jenney and Louis Sullivan. Early structures to employ concrete as the chief means of architectural expression (rather than for purely utilitarian structure) include Frank Lloyd Wright's Unity Temple, built in 1906 near Chicago, and Rudolf Steiner's Second Goetheanum, built from1926 near Basel, Switzerland.Other historians regard Modernism as a matter of taste, a reaction against eclecticism and the lavish stylistic excesses of Victorian Era and Edwardian Art Nouveau.Whatever the cause, around 1900 a number of architects around the world began developing new architectural solutions to integrate traditional precedents (Gothic, for instance) with new technological possibilities. The work of Louis Sullivan and Frank Lloyd Wright in Chicago, Victor Horta in Brussels, Antoni Gaudi in Barcelona, Otto Wagner in Vienna and Charles Rennie Mackintosh in Glasgow, among many others, can be seen as a common struggle between old and new.2. Modernism as Dominant StyleBy the 1920s the most important figures in Modern architecture had established their reputations. The big three are commonly recognized as Le Corbusier in France, and Ludwig Mies van der Rohe and Walter Gropius in Germany. Mies van der Rohe and Gropius were both directors of the Bauhaus, one of a number of European schools and associations concerned with reconciling craft tradition and industrial technology.Frank Lloyd Wright's career parallels and influences the work of the European modernists, particularly via the Wasmuth Portfolio, but he refused to be categorized with them. Wright was a major influence on both Gropius and van der Rohe, however, as well as on the whole of organic architecture.In 1932 came the important MOMA exhibition, the International Exhibition of Modern Architecture, curated by Philip Johnson. Johnson and collaborator Henry-Russell Hitchcock drew together many distinct threads and trends, identified them as stylistically similar and having a common purpose, and consolidated them into the International Style.This was an important turning point. With World War II the important figures of the Bauhaus fled to the United States, to Chicago, to the Harvard Graduate School of Design, and to Black Mountain College. While Modern architectural design never became a dominant style in single-dwelling residential buildings, in institutional and commercial architecture Modernism became the pre-eminent, and in the schools (for leaders of the profession) the only acceptable, design solution from about 1932 to about 1984.Architects who worked in the international style wanted to break with architectural tradition and design simple, unornamented buildings. The most commonly used materials are glass for the facade, steel for exterior support, and concrete for the floors and interior supports; floor plans were functional and logical. The style became most evident in the design of skyscrapers. Perhaps its most famous manifestations include the United Nations headquarters (Le Corbusier, Oscar Niemeyer, Sir Howard Robertson), the Seagram Building (Ludwig Mies van der Rohe), and Lever House (Skidmore, Owings, and Merrill), all in New York. A prominent residential example is the Lovell House (Richard Neutra) in Los Angeles.Detractors of the international style claim that its stark, uncompromisingly rectangular geometry is dehumanising. Le Corbusier once described buildings as "machines for living", but people are not machines and it was suggested that they do not want to live in machines. Even Philip Johnson admitted he was "bored with the box." Since the early 1980s many architects have deliberately sought to move away from rectilinear designs, towards more eclectic styles. During the middle of the century, some architects began experimenting in organic forms that they felt were more human and accessible. Mid-century modernism, or organic modernism, was very popular, due to its democratic and playful nature. Alvar Aalto and Eero Saarinen were two of the most prolific architects and designers in this movement, which has influenced contemporary modernism.Although there is debate as to when and why the decline of the modern movement occurred, criticism of Modern architecture began in the 1960s on the grounds that it was universal, sterile, elitist and lacked meaning. Its approach had become ossified in a "style" that threatened to degenerate into a set of mannerisms. Siegfried Giedion in the 1961 introduction to his evolving text, Space, Time and Architecture (first written in 1941), could begin "At the moment a certain confusion exists in contemporary architecture, as in painting; a kind of pause, even a kind of exhaustion." At the Metropolitan Museum of Art, a 1961 symposium discussed the question "Modern Architecture: Death or Metamorphosis?" In New York, the coup d'état appeared to materialize in controversy around the Pan Am Building that loomed over Grand Central Station, taking advantage of the modernist real estate concept of "air rights",[1] In criticism by Ada Louise Huxtable and Douglas Haskell it was seen to "sever" the Park Avenue streetscape and "tarnish" the reputations of its consortium of architects: Walter Gropius, Pietro Belluschi and thebuilders Emery Roth & Sons. The rise of postmodernism was attributed to disenchantment with Modern architecture. By the 1980s, postmodern architecture appeared triumphant over modernism, including the temple of the Light of the World, a futuristic design for its time Guadalajara Jalisco La Luz del Mundo Sede International; however, postmodern aesthetics lacked traction and by the mid-1990s, a neo-modern (or hypermodern) architecture had once again established international pre-eminence. As part of this revival, much of the criticism of the modernists has been revisited, refuted, and re-evaluated; and a modernistic idiom once again dominates in institutional and commercial contemporary practice, but must now compete with the revival of traditional architectural design in commercial and institutional architecture; residential design continues to be dominated by a traditional aesthetic.中⽂译⽂：现代建筑现代建筑,不被混淆与'当代建筑' , 是⼀个词给了⼀些建筑风格有类似的特点, 主要的简化形式,消除装饰等. 虽然风格的设想早在20世纪,并⼤量造就了⼀些建筑师、建筑教育家和展品,很少有现代的建筑物,建于20世纪上半叶. 第⼆次⼤战后的三⼗年, 但最终却成为主导建筑风格的机构和公司建设.1起源⼀些历史学家认为进化的现代建筑作为⼀个社会问题, 息息相关的⼯程中的现代性,从⽽影响了启蒙运动,导致社会和政治⾰命.另⼀些⼈认为现代建筑主要是靠技术和⼯程学的发展, 那就是获得新的建筑材料,如钢铁, 混凝⼟和玻璃驱车发明新的建筑技术,它作为⼯业⾰命的⼀部分. 1796年, shrewsbury查尔斯bage⾸先⽤他的'⽕'的设计, 后者则依靠铸铁及砖与⽯材地板. 这些建设⼤⼤加强了结构,使它们能够容纳更⼤的机器. 由于作为建筑材料特性知识缺乏,⼀些早期建筑失败. 直到1830年初,伊顿Hodgkinson预计推出了型钢梁, 导致⼴泛使⽤钢架建设，⼯业结构完全改变了这种窘迫的⾯貌,英国北部领导的描述, "⿊暗魔⿁作坊"的地⽅如曼彻斯特和西约克郡. ⽔晶宫由约瑟夫paxton的重⼤展览, 1851年,是⼀个早期的例⼦,钢铁及玻璃施⼯; 可能是⼀个最好的例⼦,就是1890年由William乐男爵延长和路易沙利⽂在芝加哥附近发展的⾼层钢结构摩天楼. 早期结构采⽤混凝⼟作为⾏政⼿段的建筑表达(⽽⾮纯粹功利结构) ,包括建于1906年在芝加哥附近,劳埃德赖特的统⼀宫, 建于1926年瑞⼠巴塞尔附近的鲁道夫斯坦纳的第⼆哥特堂,.但⽆论原因为何, 约有1900多位建筑师,在世界各地开始制定新的建筑⽅法,将传统的先例(⽐如哥特式)与新的技术相结合的可能性.路易沙利⽂和赖特在芝加哥⼯作,维克多奥尔塔在布鲁塞尔,安东尼⾼迪在巴塞罗那, 奥托⽡格纳和查尔斯景mackintosh格拉斯哥在维也纳,其中之⼀可以看作是⼀个新与旧的共同⽃争.2现代主义风格由1920年代的最重要⼈物,在现代建筑⾥确⽴了⾃⼰的名声. 三个是公认的柯布西耶在法国, 密斯范德尔德罗和⽡尔特格罗⽪乌斯在德国. 密斯范德尔德罗和格罗⽪乌斯为董事的包豪斯, 其中欧洲有不少学校和有关团体学习调和⼯艺和传统⼯业技术.赖特的建筑⽣涯中,也影响了欧洲建筑的现代艺术,特别是通过⽡斯穆特组合但他拒绝被归类与他们. 赖特与格罗⽪乌斯和Van der德罗对整个有机体系有重⼤的影响.在1932年来到的重要moma展览,是现代建筑艺术的国际展览,艺术家菲利普约翰逊. 约翰逊和合作者亨利-罗素阁纠集许多鲜明的线索和趋势, 内容相似,有⼀个共同的⽬的,巩固了他们融⼊国际化风格这是⼀个重要的转折点. 在⼆战的时间包豪斯的代表⼈物逃到美国,芝加哥，到哈佛⼤学设计⿊⼭书院. 当现代建筑设计从未成为主导风格单⼀的住宅楼，在成为现代卓越的体制和商业建筑, 是学校(专业领导)的唯⼀可接受的, 设计解决⽅案,从约1932年⾄约1984年.那些从事国际风格的建筑师想要打破传统建筑和简单的没有装饰的建筑物。

毕业设计论文电子秤中英文外文资料文献翻译

外文参考资料二：Abstract: In order to solve the weight problem often encountered in measuring the low-mass objects in the trade and daily life of the modern business, the design of a new pocket-sized electronic scales. This pocket-sized electronic scales Force Sensor gravity signals into electrical signals to measure, and measuring the results of the digital display. The pocket-sized electronic scales with a small size, light weight, easy to carry, intuitive display, measurement and high precision; complex structure, the cost disadvantages. This article focuses on the load cell works, error compensation, the main parameters of selection. And the technical and economic analysis.Keywords: pocket-sized electronic scales; weighing; sensor; error compensationCLC number: TH715. 1] sign code: A Article ID: 100 522 895 (2007) 022*******1 A needs analysisIn modern commerce and trade and everyday life, often encounter the problem of measuring the weight of the low-mass objects. Although the traditional steelyard can solve this problem, but inconvenient to carry, the efficiency is not high, the display is not intuitive and low measurement accuracy; mechanical spring balance can solve this problem, but the inertia inherent low frequency, high sensitivity, measurement accuracy is not high. With the progress and update of the micro-computer technology, integrated circuit technology, sensor technology, electronic scales rapiddevelopment, it has a responsive, high accuracy, fully functional, the display is intuitive, compact, easy to use and so on.For these reasons, in order to solve the low-mass objects weighing problem, if a small size, light weight, portable, digital display electronic scales, will be popular. Discussed below, that is, the scope of a weighing 5 kg compact electronic scales.2 DesignThe principle of the structure of the pocket-sized electronic scales shown in Figure 1. The main task is to design compact electronic scales weighing the choice of the force sensors. Dynamometer sensor types to achieve the weighing and digital display, the key is to want to force (gravity) signals into electrical signals to measure method is mainly divided into two categories: one is the direct method, namely the use of pressure magnetic sensor, piezoelectric sensor, Piezoresistive sensors directly to the force signal converted into electrical signals; the other is the indirect method, the elastic element as the sensor will pull, pressure changes in strain, displacement, or frequency, and then strain sensors, displacement sensors, or frequency sensor strain, displacement or frequency changes for power. Comprehensive comparison of a variety of sensors, use the indirect method of strain gauge force transducer.Pocket-sized electronic scale structural principle:Sensor → amplifier → CPU→ display → AöD converter3 sensor device design3.1 worksThe working principle of the strain gauge force transducer strain gauge pasted into force, force-sensitive elasticelement, the corresponding strain when the elastic element force deformation, the strain gauge into a resistance change, which led to the voltage measuring circuit changes by measuring the output voltage value, and then through the conversion can be obtained by the measurement of body weight. Since the pocket-sized electronic scales require small size, weighing in scope, precision and angle even consistency, sensor and display integration, it is selected parallel to the two holes cantilever beam strain gauge load-bearing sensor. Its characteristics are: high precision, ease of processing, simple and compact structure, strong resistance to partial load, high natural frequency.Strain gauge choice of a metal palisade metal mooring paste on the insulating substrate parked strain gages, mechanical strain resistance strain gauge feelings generally 10 - 10 - 2mm, the resistance rate of change of the attendant about 10 - 6 10 - 2 orders of magnitude, such a small change in resistance measured using the general resistance of the instrument is hard to measure out, you must use some form of measurement.Circuit into small changes in resistance rates to changes in voltage or current, in order to secondary instrument display. Bridge measurement circuit to meet this requirement. In the load cell, R 1, R 2, R 3, R 44 strain gauge resistor bridge measurement circuit shown in Figure 3. R m is the temperature compensation resistor, e is the excitation voltage, V is the output voltage.外文参考资料三：The load cell is a quality signal into a measurable electrical signal outputdevice. Must consider the actual working environment of the sensor which sensor Yin, this is essential for the correct selection of the load cell, and it is related to the sensor can work as well as its safety and service life, and the whole weighing the reliability and safety sex. On the basic concepts and methods of evaluation of the major technical indicators of the load cell, the new and old GB qualitative differences.The traditional concept, the load sensor weighing sensors, force sensors, collectively referred to using a single parameter to evaluate its measurement properties. Old GB will be completely different application objects and the use of environmental conditions "weighing" and "measured force" two sensors into one to consider, not given to distinguish between the test and evaluation methods. Old GB total of 21 indicators, were tested at room temperature; and non-linearity, hysteresis error, repeatability error, creep, the additional error of the zero temperature and the maximum error in the six indicators of the rated output additional temperature error, to determine said The level of accuracy of the weight sensor, respectively 0.02,0.03,0.05 said.Proportion to convert the output signal can be measured. Taking into account the different place of use of the acceleration due to gravity and air buoyancy on the conversion, the main performance indicators of the load cell linearity error, hysteresis error, repeatability error, creep, zero-temperature characteristics and temperature sensitivity characteristics. In a variety of weighing and measuring the quality of the system, usually the integrated error accuracy of the integrated control sensors, and integrated error band or scale error band (Figure 1) linked so that selection corresponds to a certain accuracy weighing weighing sensors. International Organization of Legal Metrology(OIML) requirements, sensor error with total weighing instrument error δ with Δ of 70% of the load cell linearity error, hysteresis error within the specified temperature range due to the effect of temperature on the sensitivity of the error the sum can not exceed the error band of δ. This allows the manufacturer of the components that make up the total measurement error adjustment to obtain the desired accuracy.The load cell conversion method is divided into photoelectric, hydraulic, electromagnetic force type, capacitive, magnetic poles change the form of vibratory gyroscope ceremony, resistance strain type, to the most extensive use of resistance strain.Electromagnetic force sensorIt uses a load-bearing stage load and the principle of electromagnetic force Equilibrium (Figure 5). Put the loading stage, the measured object at one end of the lever upward tilt; photoelectric detect the tilt signal, amplified into the coil, the electromagnetic force, so that the lever to return to equilibrium. Currents produce electromagnetic counterweight digital converter, you can determine the quality of the measured object. The electromagnetic force sensor accuracy, up to 1/2000 ~ 1/60000, but the weighing range is only tens of mg to 10 kg.Capacitive sensorsItcapacitor oscillator circuit of the oscillation frequency f and the plate spacing d is directly proportional relationship between the work (Figure 6). There are two plates, one fixed and the other one can move. Bearing load measured object, the leaf spring deflection, the distance between the twoplates changes, the oscillation frequency of the circuit also changes. The measured frequency change can be calculated to the quality of the load-bearing stage, the measured object. Capacitive sensor power consumption, low cost, accuracy of 1/200 to 1/500.Pole change the form of sensorFerromagneticcomponents in the measured object gravity under mechanical deformation, internal stress and cause changes in permeability, and also changes so that the induced voltage of the secondary coil wound on both sides of the ferromagnetic component (pole). Measure the voltage variation can be calculated added to the force on the pole, and then determine the quality of the measured object. Pole to change the form of sensor accuracy is not high, usually 1/100, applicable to the large tonnage weighing, weighing ranging from tens to tens of thousands of kilograms.Vibration sensorThe force of the elastic element, the natural vibration frequency of the force is proportional to the square root of. Measure the natural frequency changes, you can find the measured object role in the elastic component of the force, and then calculate the quality. The vibration sensor vibrating wire and tuning fork.The elastic component of the vibrating wire sensor string wire. When the load-bearing stage, plus the measured object, the intersection of the V-shaped string wire is pulled down, and left strings of tension increases, the right string tension decreases. The natural frequency of the two strings of different changes. Calculate the frequency difference between the two strings, you can find the quality of the measured object. The higher the accuracy of the vibrating wire sensor, up to 1/1000 ~ 1/10000, weighing 100 g to hundreds of kilograms, but the structure is complex anddifficult process, and high cost.The elastic component of the tuning fork sensor is a tuning fork. Fixed tuning fork end of the piezoelectric element, the natural frequency of oscillation of a tuning fork, it can be measured oscillation frequency. When the load-bearing stage and the measured object, the tuning fork direction of tensile force while the increase in natural frequency, increasing levels of applied force is proportional to the square root. Measure the changes of natural frequency can be calculated heavy loads imposed on the tuning fork on the force, and then calculate the quality of heavy objects. The tuning fork sensor power consumption, measurement accuracy up to 1/10000 to 1/200 000, weighing range of 500g ~ 10kg.外文参考文献中文翻译参考资料二：摘要: 为解决现代商业贸易和日常生活中经常遇到的测量小质量物体的重量问题, 介绍了一种新型的袖珍式电子秤的设计。

毕业论文外文翻译--虚拟现实技术的发展过程及研究现状（适用于毕业论文外文翻译+中英文对照）

虚拟现实技术的发展过程及研究现状虚拟现实技术是近年来发展最快的技术之一，它与多媒体技术、网络技术并称为三大前景最好的计算机技术。

与其他高新技术一样，客观需求是虚拟现实技术发展的动力。

近年来，在仿真建模、计算机设计、可视化计算、遥控机器人等领域，提出了一个共同的需求，即建立一个比现有计算机系统更为直观的输入输出系统，成为能与各种船感器相联、更为友好的人机界面、人能沉浸其中、超越其上、进出自如、交互作用的多维化信息环境。

VR技术是人工智能、计算机图形学、人机接口技术、多媒体技术、网络技术、并行计算技术等多种技术的集成。

它是一种有效的模拟人在自然环境中视听、动等行为的高级人机交互技术。

虚拟现实(Virtual Reality )：是一种最有效的模拟人在自然环境中视、听、动等行为的高级人机交互技术，是综合计算机图形技术、多媒体技术、并行实时计算技术、人工智能、仿真技术等多种学科而发展起来的20世纪90年代计算机领域的最新技术。

VR以模拟方式为使用者创造一个实时反映实体对象变化与相互作用的三维图像世界，在视、听、触、嗅等感知行为的逼真体验中，使参与者可直接探索虚拟对象在所处环境中的作用和变化；仿佛置身于虚拟的现实世界中，产生沉浸感(immersive)、想象(imaginative和实现交互性interactive) 。

VR技术的每一步都是围绕这三个特征而前进的。

这三个特征为沉浸特征、交互特征和构想特征。

这三个重要特征用以区别相邻近的技术，如多媒体技术、计算机可视化技术沉浸特征，即在VR提供的虚拟世界中，使用户能感觉到是真实的进入了一个客观世界；交互特征，要求用户能用人类熟悉的方式对虚拟环境中的实体进行观察和操纵；构想特征：即“从定性和定量综合集成环境中得到感性和理性的认识：从而化概念和萌发新意”。

1.VR技术发展的三个阶段VR技术的发展大致可分为三个阶段：20世纪50年代至70年代VR技术的准备阶段；80年代初80年代中期，是VR 技术系统化、开始走出实验室进入实际应用的阶段；80年代末至90年代初，是VR技术迅猛发展的阶段。

毕业设计论文翻译（译文+原文）

Hacking tricks toward security on network environments Tzer-Shyong Chen1, Fuh-Gwo Jeng 2, and Yu-Chia Liu 11 Department of Information Management, Tunghai University, Taiwan2 Department of Applied Mathematics, National Chiayi University, TaiwanE-Mail:****************.edu.twAbstractMounting popularity of the Internet has led to the birth of Instant Messaging, an up-and-coming form of Internet communication. Instant Messaging is very popular with businesses and individuals since it has instant communication ability. As a result, Internet security has become a pressing and important topic for discussion. Therefore, in recent years, a lot of attention has been drawn towards Internet security and the various attacks carried out by hackers over the Internet. People today often handle affairs via the Internet. For instance, instead of the conventional letter, they communicate with others by e-mails; they chat with friends through an instant messenger; find information by browsing websites instead of going to the library; perform e-commerce transactions through the Internet, etc. Although the convenience of the Internet makes our life easier, it is also a threat to Internet security. For instance, a business email intercepted during its transmission may let slip business confidentiality; file transfers via instant messengers may also be intercepted, and then implanted with backdoor malwares; conversations via instant messengers could be eavesdropped. Furthermore, ID and password theft may lose us money when using Internet bank service. Attackers on the Internet use hacking tricks to damage systems while users are connected to the Internet. These threats along with possible careless disclosure of business information make Instant Messaging a very unsafe method of communication for businesses. The paper divides hacking tricks into three categories: (1) Trojan programs that share files via instant messenger. (2) Phishing or fraud via e-mails. (3) Fake Websites. Keywords:Hacking tricks, Trojan programs, Phishing, Firewall, Intrusion detection system.1. IntroductionIncreasingly more people are using instant messengers such as MSN Messenger, Yahoo! Messenger, ICQ, etc as the media of communication. These instant messengers transmit alphanumeric message as well as permit file sharing. During transfer, a file may be intercepted by a hacker and implanted with backdoor malware. Moreover, the e-mails users receive every day may include Spam, advertisements, and fraudulent mail intended to trick uninformed users. Fake websites too are prevalent. Websites which we often visit could be counterfeited by imitating the interface and the URL of the original, tricking users. The paper classifies hacking tricks into three categories which are explained in the following sections.2. Hacking TricksThe paper divides hacking tricks into three categories: (1) Trojan programs that share files via instant messenger. (2) Phishing (3) Fake Websites.2.1 Trojan programs that share files via instant messengerInstant messaging allows file-sharing on a computer [9]. All present popular instant messengers have file sharing abilities, or allow users to have the above functionality by installing patches or plug-ins; this is also a major threat to present information security. These communication softwares also makeit difficult for existing hack prevention methods to prevent and control information security. Therefore, we shall discuss how to control the flow of instant messages and how to identify dangerous user behavior.Hackers use instant communication capability to plant Trojan program into an unsuspected program; the planted program is a kind of remotely controlled hacking tool that can conceal itself and is unauthorized. The Trojan program is unknowingly executed, controlling the infected computer; it can read, delete, move and execute any file on the computer. The advantages of a hacker replacing remotely installed backdoor Trojan programs [1] with instant messengers to access files are:When the victim gets online, the hacker will be informed. Thus, a hacker can track and access the infected computer, and incessantly steal user information.A hacker need not open a new port to perform transmissions; he can perform his operations through the already opened instant messenger port.Even if a computer uses dynamic IP addresses, its screen name doesn’t change.Certain Trojan programs are designed especially for instant messengers. These Trojans can change group settings and share all files on the hard disk of the infected computer. They can also destroy or modify data, causing data disarray. This kind of program allows a hacker access to all files on an infected computer, and thus poses a great threat to users. The Trojan program takes up a large amount of the resources of the computer causing it to become very slow and often crashes without a reason.Trojan programs that access a user computer through an instant messenger are probably harder to detect than classic Trojan horse programs. Although classic Trojan intrudes a computer by opening a listening or outgoing port which is used to connect toa remote computer, a desktop firewall can effectively block such Trojans. Alternatively, since it is very difficult for the server’s firewall to spot intrusion by controlling an instant messenger’s flow, it is extremely susceptible to intrusion.Present Trojan programs have already successfully implemented instant messengers. Some Trojan programs are Backdoor Trojan, AIMVision, and Backdoor. Sparta.C. Backdoor Trojans use ICQ pager to send messages to its writer. AIMVision steals AIM related information stored in the Windows registry, enabling a hacker to setup an AIM user id. Backdoor. Sparta.C uses ICQ to communicate with its writer and opens a port on an infected host and send its IP Address to the hacker, and at the same time attempts to terminate the antivirus program or firewall of the host.2.1.1 Hijacking and ImpersonationThere are various ways through which a hacker can impersonate other users [7]. The most commonly used method is eavesdropping on unsuspecting users to retrieve user accounts, passwords and other user related information.The theft of user account number and related information is a very serious problem in any instant messenger. For instance, a hacker after stealing a user’s information impersonate the user; the user’s contacts not knowing that the user’s account has been hacked believe that the person they’re talking to is the user, and are persuaded to execute certain programs or reveal confidential information. Hence, theft of user identity not only endangers a user but also surrounding users. Guarding against Internet security problems is presently the focus of future research; because without good protection, a computer can be easily attacked, causing major losses.Hackers wishing to obtain user accounts may do so with the help of Trojans designed to steal passwords. If an instant messenger client stores his/her password on his/her computer, then a hacker can send a Trojan program to the unsuspecting user. When the user executes the program, the program shall search for the user’s password and send it to the hacker. There are several ways through which a Trojan program can send messages back to the hacker. The methods include instant messenger, IRC, e-mails, etc.Current four most popular instant messengers are AIM, Yahoo! Messenger, ICQ, and MSN Messenger, none of which encrypts its flow. Therefore, a hackercan use a man-in-the-middle attack to hijack a connection, then impersonate the hijacked user and participate in a chat-session. Although difficult, a hacker can use the man-in-the-middle attack to hijack the connection entirely. For example, a user may receive an offline message that resembles that sent by the server, but this message could have been sent by the hacker. All at once, the user could also get disconnected to the server. Furthermore, hackers may also use a Denial of Service (DoS) tool or other unrelated exploits to break the user’s connection. However, the server keeps the connection open, and does not know that the user has been disconnected; thus allowing the hacker to impersonate the user. Moreover, since the data flow is unencrypted and unauthenticated, a hacker can use man-in-the-middle attacks that are similar to that of ARP fraud to achieve its purpose.2.1.2 Denial of Service (DoS)There are many ways through which a hacker can launch a denial of service (DoS) attack [2] on an instant messenger user. A Partial DoS attack will cause a user end to hang, or use up a large portion of CPU resources causing the system to become unstable.Another commonly seen attack is the flooding of messages to a particular user. Most instant messengers allow the blocking of a particular user to prevent flood attacks. However, a hacker can use tools that allow him to log in using several different identities at the same time, or automatically create a large number of new user ids, thus enabling a flood attack. Once a flood attack begins, even if the user realizes that his/her computer has been infected, the computer will not be able to respond. Thus, the problem cannot be solved by putting a hacker’s user id on the ignore list of your instant messenger.A DoS attack on an instant messenger client is only a common hacking tool. The difficulty of taking precautions against it could turn this hacking tool into dangerous DoS type attacks. Moreover, some hacking tools do not just cause an instant messenger client to hang, but also cause the user end to consume large amount of CPU time, causing the computer to crash.2.1.3 Information DisclosureRetrieving system information through instant messenger users is currently the most commonly used hacking tool [4]. It can effortlessly collect user network information like, current IP, port, etc. IP address retriever is an example. IP address retrievers can be used to many purposes; for instance, a Trojan when integrated with an IP address retriever allows a hacker to receive all information related to the infected computer’s IP address as soon as the infected computer connects to the internet. Therefore, even if the user uses a dynamic IP address, hackers can still retrieve the IP address.IP address retrievers and other similar tools can also be used by hackers to send data and Trojans to unsuspecting users. Hackers may also persuade unsuspecting users to execute files through social engineering or other unrelated exploits. These files when executed search for information on the user’s computer and sends them back to the hacker through the instant messenger network.Different Trojan programs were designed for different instant messaging clients. For example, with a user accounts and password stealing Trojans a hacker can have full control of the account once the user logs out. The hacker can thus perform various tasks like changing the password and sending the Trojan program to all of the user’s contacts.Moreover, Trojans is not the only way through which a hacker can cause information disclosure. Since data sent through instant messengers are unencrypted, hackers can sniff and monitor entire instant messaging transmissions. Suppose an employee of an enterprise sends confidential information of the enterprise through the instant messenger; a hacker monitoring the instant messaging session can retrieve the data sent by the enterprise employee. Thus, we must face up to the severity of the problem.2.2 PhishingThe word “Phishing” first appeared in 1996. It is a variant of ‘fishing’, and formed by replacing the ‘f’ in ‘fishing’ with ‘ph’ from phone. It means tricking users of their money through e-mails.Based on the statistics of the Internet Crime Complaint Center, loss due to internet scam was as high as $1.256 million USD in 2004. The Internet Crime Complaint Center has listed the above Nigerian internet scam as one of the ten major internet scams.Based on the latest report of Anti-Phishing Working Group (APWG) [8], there has been a 28% growth of Phishing scams in the past 4 months, mostly in the US and in Asia. Through social engineering and Trojans, it is very difficult for a common user to detect the infection.To avoid exploitation of your compassion, the following should be noted:(1)When you need to enter confidentialinformation, first make sure that theinformation is entered via an entirely secureand official webpage. There are two ways todetermine the security of the webpage:a.The address displayed on the browserbegins with https://, and not http://. Payattention to if the letter ‘s’ exists.b.There is a security lock sign on the lowerright corner of the webpage, and whenyour mouse points to the sign, a securitycertification sign shall appear.(2)Consider installing a browser security softwarelike SpoofStick which can detect fake websites.(3)If you suspect the received e-mail is a Phishinge-mail, do not open attachments attached to theemail. Opening an unknown attachment couldinstall malicious programs onto your computer.(4)Do not click on links attached to your emails. Itis always safer to visit the website through theofficial link or to first confirm the authenticityof the link. Never follow or click on suspiciouslinks in an e-mail. It is advisable to enter theURL at the address bar of the web browser,and not follow the given link.Generally speaking, Phishing [3] [5] is a method that exploits people’s sympathy in the form of aid-seeking e-mails; the e-mail act as bait. These e-mails usually request their readers to visit a link that seemingly links to some charitable organization’s website; but in truth links the readers to a website that will install a Trojan program into the reader’s computer. Therefore, users should not forward unauthenticated charity mails, or click on unfamiliar links in an e-mail. Sometimes, the link could be a very familiar link or an often frequented website, but still, it would be safer if you’d type in the address yourself so as to avoid being linked to a fraudulent website. Phisher deludes people by using similar e-mails mailed by well-known enterprises or banks; these e-mails often asks users to provide personal information, or result in losing their personal rights; they usually contain a counterfeit URL which links to a website where the users can fillin the required information. People are often trapped by phishing due to inattentionBesides, you must also be careful when using a search engine to search for donations and charitable organizations.2.3 Fake WebsitesFake bank websites stealing account numbers and passwords have become increasingly common with the growth of online financial transactions. Hence, when using online banking, we should take precautions like using a secure encrypted customer’s certificate, surf the net following the correct procedure, etc.There are countless kinds of phishing baits, for instance, messages that say data expired, data invalid, please update data, or identity verification intended to steal account ID and matching password. This typeof online scam is difficult for users to identify. As scam methods become finer, e-mails and forged websites created by the impostor resemble their original, and tremendous losses arise from the illegal transactions.The following are methods commonly used by fake websites. First, the scammers create a similar website homepage; then they send out e-mails withenticing messages to attract visitors. They may also use fake links to link internet surfers to their website. Next, the fake website tricks the visitors into entering their personal information, credit card information or online banking account number and passwords. After obtaining a user’s information, the scammers can use the information to drain the bank accounts, shop online or create fake credit cards and other similar crimes. Usually, there will be a quick search option on these fake websites, luring users to enter their account number and password. When a user enters their account number and password, the website will respond with a message stating that the server is under maintenance. Hence, we must observe the following when using online banking:(1)Observe the correct procedure for entering abanking website. Do not use links resultingfrom searches or links on other websites.(2)Online banking certifications are currently themost effective security safeguard measure. (3)Do not easily trust e-mails, phone calls, andshort messages, etc. that asks for your accountnumber and passwords.Phishers often impost a well-known enterprise while sending their e-mails, by changing the sender’s e-mail address to that of the well known enterprise, in order to gain people’s trust. The ‘From’ column of an e-mail is set by the mail software and can be easily changed by the web administrator. Then, the Phisher creates a fake information input website, and send out e-mails containing a link to this fake website to lure e-mail recipients into visiting his fake website.Most Phishers create imitations of well known enterprises websites to lure users into using their fake websites. Even so, a user can easily notice that the URL of the website they’re entering has no relation to the intended enterprise. Hence, Phishers may use different methods to impersonate enterprises and other people. A commonly used method is hiding the URL. This can easily be done with the help of JavaScript.Another way is to exploit the loopholes in an internet browser, for instance, displaying a fake URL in the browser’s address bar. The security loophole causing the address bar of a browser to display a fake URL is a commonly used trick and has often been used in the past. For example, an e-mail in HTML format may hold the URL of a website of a well-known enterprise, but in reality, the link connects to a fake website.The key to successfully use a URL similar to that of the intended website is to trick the visual senses. For example, the sender’s address could be disguised as that of Nikkei BP, and the link set to http://www.nikeibp.co.jp/ which has one k less than the correct URL which is http://www.nikkeibp.co.jp/. The two URLs look very similar, and the difference barely noticeable. Hence people are easily tricked into clicking the link.Besides the above, there are many more scams that exploit the trickery of visual senses. Therefore, you should not easily trust the given sender’s name and a website’s appearance. Never click on unfamiliar and suspicious URLs on a webpage. Also, never enter personal information into a website without careful scrutiny.3. ConclusionsBusiness strategy is the most effective form of defense and also the easiest to carry out. Therefore, they should be the first line of defense, and not last. First, determine if instant messaging is essential in the business; then weigh its pros and cons. Rules and norms must be set on user ends if it is decided that the business cannot do without instant messaging functionality. The end server should be able to support functions like centralized logging and encryption. If not, then strict rules must be drawn, and carried out by the users. Especially, business discussions must not be done over an instant messenger.The paper categorized hacking tricks into three categories: (1) Trojan programs that share files via instant messenger. (2) Phishing (3) Fake Websites. Hacking tricks when successfully carried out could cause considerable loss and damage to users. The first category of hacking tricks can be divided into three types: (1) Hijacking and Impersonation; (2) Denial of Service; (3) Information Disclosure.Acknowledgement:This work was supported by the National Science Council, Taiwan, under contract No. NSC 95-2221-E-029-024.References[1] B. Schneier, “The trojan horse race,”Communications of ACM, Vol. 42, 1999, pp.128.[2] C. L. Schuba, “Analysis of a denial of serviceattack on TCP,” IEEE Security and PrivacyConference, 1997, pp. 208-223.[3] E. Schultz, “Phishing is becoming moresophisticated,” Computer and Security, Vol.24(3), 2005, pp. 184-185.[4]G. Miklau, D. Suciu, “A formal analysis ofinformation disclosure in data exchange,”International Conference on Management ofData, 2004, pp. 575-586.[5]J. Hoyle, “'Phishing' for trouble,” Journal ofthe American Detal Association, Vol. 134(9),2003, pp. 1182-1182.[6]J. Scambray, S. McClure, G. Kurtz, Hackingexposed: network security secrets and solutions,McGraw-Hill, 2001.[7]T. Tsuji and A. Shimizu, “An impersonationattack on one-time password authenticationprotocol OSPA,” to appear in IEICE Trans.Commun, Vol. E86-B, No.7, 2003.[8]Anti-Phishing Working Group,.[9]/region/tw/enterprise/article/icq_threat.html.有关网络环境安全的黑客技术摘要：现在人们往往通过互联网处理事务。

毕业设计的论文中英翻译

Anti-Aircraft Fire Control and the Development of IntegratedSystems at SperryT he dawn of the electrical age brought new types of control systems. Able to transmit data between distributed components and effect action at a distance, these systems employed feedback devices as well as human beings to close control loops at every level. By the time theories of feedback and stability began to become practical for engineers in the 1930s a tradition of remote and automatic control engineering had developed that built distributed control systems with centralized information processors. These two strands of technology, control theory and control systems, came together to produce the large-scale integrated systems typical of World War II and after.Elmer Ambrose Sperry (I860-1930) and the company he founded, the Sperry Gyroscope Company, led the engineering of control systems between 1910 and 1940. Sperry and his engineers built distributed data transmission systems that laid the foundations of today‟s command and control systems. Sperry‟s fire control systems included more than governors or stabilizers; they consisted of distributed sensors, data transmitters, central processors, and outputs that drove machinery. This article tells the story of Sperry‟s involvement in anti-aircraft fire control between the world wars and shows how an industrial firm conceived of control systems before the common use of control theory. In the 1930s the task of fire control became progressively more automated, as Sperry engineers gradually replaced human operators with automatic devices. Feedback, human interface, and system integration posed challenging problems for fire control engineers during this period. By the end of the decade these problems would become critical as the country struggled to build up its technology to meet the demands of an impending war.Anti-Aircraft Artillery Fire ControlBefore World War I, developments in ship design, guns, and armor drove the need for improved fire control on Navy ships. By 1920, similar forces were at work in the air: wartime experiences and postwar developments in aerial bombing created the need for sophisticated fire control for anti-aircraft artillery. Shooting an airplane out of the sky is essentially a problem of “leading” the target. As aircraft developed rapidly in the twenties, their increased speed and altitude rapidly pushed the task of computing the lead out of the range of human reaction and calculation. Fire control equipment for anti-aircraft guns was a means of technologically aiding human operators to accomplish a task beyond their natural capabilities.During the first world war, anti-aircraft fire control had undergone some preliminary development. Elmer Sperry, as chairman of the Aviation Committee of the Naval Consulting Board, developed two instruments for this problem: a goniometer,a range-finder, and a pretelemeter, a fire director or calculator. Neither, however, was widely used in the field.When the war ended in I918 the Army undertook virtually no new development in anti-aircraft fire control for five to seven years. In the mid-1920s however, the Army began to develop individual components for anti-aircraft equipment including stereoscopic height-finders, searchlights, and sound location equipment. The Sperry Company was involved in the latter two efforts. About this time Maj. Thomas Wilson, at the Frankford Arsenal in Philadelphia, began developing a central computer for firecontrol data, loosely based on the system of “director firing” that had developed in naval gunn ery. Wilson‟s device resembled earlier fire control calculators, accepting data as input from sensing components, performing calculations to predict the future location of the target, and producing direction information to the guns.Integration and Data TransmissionStill, the components of an anti-aircraft battery remained independent, tied together only by telephone. As Preston R. Bassett, chief engineer and later president of the Sperry Company, recalled, “no sooner, however, did the components get to the point of functioning satisfactorily within themselves, than the problem of properly transmitting the information from one to the other came to be of prime importance.”Tactical and terrain considerations often required that different fire control elements be separated by up to several hundred feet. Observers telephoned their data to an officer, who manually entered it into the central computer, read off the results, and telephoned them to the gun installations. This communication system introduced both a time delay and the opportunity for error. The components needed tighter integration, and such a system required automatic data communications.In the 1920s the Sperry Gyroscope Company led the field in data communications. Its experience came from Elmer Spe rry‟s most successful invention, a true-north seeking gyro for ships. A significant feature of the Sperry Gyrocompass was its ability to transmit heading data from a single central gyro to repeaters located at a number of locations around the ship. The repeaters, essentially follow-up servos, connected to another follow-up, which tracked the motion of the gyro without interference. These data transmitters had attracted the interest of the Navy, which needed a stable heading reference and a system of data communication for its own fire control problems. In 1916, Sperry built a fire control system for the Navy which, although it placed minimal emphasis on automatic computing, was a sophisticated distributed data system. By 1920 Sperry had installed these systems on a number of US. battleships.Because of the Sperry Company‟s experience with fire control in the Navy, as well as Elmer Sperry‟s earlier work with the goniometer and the pretelemeter, the Army approached the company for help with data transmission for anti-aircraft fire control. To Elmer Sperry, it looked like an easy problem: the calculations resembled those in a naval application, but the physical platform, unlike a ship at sea, anchored to the ground. Sperry engineers visited Wilson at the Frankford Arsenal in 1925, and Elmer Sperry followed up with a letter expressing his interest in working on the problem. He stressed his company‟s experience with naval problems, as well as its recent developments in bombsights, “work from the other end of the pro position.” Bombsights had to incorporate numerous parameters of wind, groundspeed, airspeed, and ballistics, so an anti-aircraft gun director was in some ways a reciprocal bombsight . In fact, part of the reason anti-aircraft fire control equipment worked at all was that it assumed attacking bombers had to fly straight and level to line up their bombsights. Elmer Sperry‟s interests were warmly received, and in I925 and 1926 the Sperry Company built two data transmission systems for the Army‟s gun directors.The original director built at Frankford was designated T-1, or the “Wilson Director.” The Army had purchased a Vickers director manufactured in England, but encouraged Wilson to design one thatcould be manufactured in this country Sperry‟s two data tran smission projects were to add automatic communications between the elements of both the Wilson and the Vickers systems (Vickers would eventually incorporate the Sperry system into its product). Wilson died in 1927, and the Sperry Company took over the entire director development from the Frankford Arsenal with a contract to build and deliver a director incorporating the best features of both the Wilson and Vickers systems. From 1927 to 193.5, Sperry undertook a small but intensive development program in anti-aircraft systems. The company financed its engineering internally, selling directors in small quantities to the Army, mostly for evaluation, for only the actual cost of production [S]. Of the nearly 10 models Sperry developed during this period, it never sold more than 12 of any model; the average order was five. The Sperry Company offset some development costs by sales to foreign govemments, especially Russia, with the Army‟s approval 191.The T-6 DirectorSperry‟s modified version of Wilson‟s director was designated T-4 in development. This model incorporated corrections for air density, super-elevation, and wind. Assembled and tested at Frankford in the fall of 1928, it had problems with backlash and reliability in its predicting mechanisms. Still, the Army found the T-4 promising and after testing returned it to Sperry for modification. The company changed the design for simpler manufacture, eliminated two operators, and improved reliability. In 1930 Sperry returned with the T-6, which tested successfully. By the end of 1931, the Army had ordered 12 of the units. The T-6 was standardized by the Army as the M-2 director.Since the T-6 was the first anti-aircraft director to be put into production, as well as the first one the Army formally procured, it is instructive to examine its operation in detail. A technical memorandum dated 1930 explained the theory behind the T-6 calculations and how the equations were solved by the system. Although this publication lists no author, it probably was written by Earl W. Chafee, Sperry‟s director of fire control engineering. The director was a complex mechanical analog computer that connected four three-inch anti-aircraft guns and an altitude finder into an integratedsystem (see Fig. 1). Just as with Sperry‟s naval fire control system, the primary means of connection were “data transmitters,” similar to those that connected gyrocompasses to repeaters aboard ship.The director takes three primary inputs. Target altitude comes from a stereoscopic range finder. This device has two telescopes separated by a baseline of 12 feet; a single operator adjusts the angle between them to bring the two images into coincidence. Slant range, or the raw target distance, is then corrected to derive its altitude component. Two additional operators, each with a separate telescope, track the target, one for azimuth and one for elevation. Each sighting device has a data transmitter that measures angle or range and sends it to the computer. The computer receives these data and incorporates manual adjustments for wind velocity, wind direction, muzzle velocity, air density, and other factors. The computer calculates three variables: azimuth, elevation, and a setting for the fuze. The latter, manually set before loading, determines the time after firing at which the shell will explode. Shells are not intended to hit the target plane directly but rather to explode near it, scattering fragments to destroy it.The director performs two major calculations. First, pvediction models the motion of the target and extrapolates its position to some time in the future. Prediction corresponds to “leading” the target. Second, the ballistic calculation figures how to make the shell arrive at the desired point in space at the future time and explode, solving for the azimuth and elevation of the gun and the setting on the fuze. This calculation corresponds to the traditional artillery man‟s task of looking up data in a precalculated “firing table” and setting gun parameters accordingly. Ballistic calculation is simpler than prediction, so we will examine it first.The T-6 director solves the ballistic problem by directly mechanizing the traditional method, employing a “mechanical firing table.” Traditional firing tables printed on paper show solutions for a given angular height of the target, for a given horizontal range, and a number of other variables. The T-6 replaces the firing table with a Sperry ballistic cam.” A three-dimensionally machined cone shaped device, the ballistic cam or “pin follower” solves a pre-determined function. Two independent variables are input by the angular rotation of the cam and the longitudinal position of a pin that rests on top of the cam. As the pin moves up and down the length of the cam, and as the cam rotates, the height of the pin traces a function of two variables: the solution to the ballistics problem (or part of it). The T-6 director incorporates eight ballistic cams, each solving for a different component of the computation including superelevation, time of flight, wind correction, muzzle velocity. air density correction. Ballistic cams represented, in essence, the stored data of the mechanical computer. Later directors could be adapted to different guns simply by replacing the ballistic cams with a new set, machined according to different firing tables. The ballistic cams comprised a central component of Sperry‟s mechanical computing technology. The difficulty of their manufacture would prove a major limitation on the usefulness of Sperry directors.The T-6 director performed its other computational function, prediction, in an innovative way as well. Though the target came into the system in polar coordinates (azimuth, elevation, and range), targets usually flew a constant trajectory (it was assumed) in rectangular coordinates-i.e. straight andlevel. Thus, it was simpler to extrapolate to the future in rectangular coordinates than in the polar system. So the Sperry director projected the movement of the target onto a horizontal plane, derived the velocity from changes in position, added a fixed time multiplied by the velocity to determine a future position, and then converted the solution back into polar coordinates. This method became known as the “plan prediction method”because of the representation of the data on a flat “plan” as viewed from above; it was commonly used through World War II. In the plan prediction method, “the actual movement of the target is mechanically reproduced on a small scale within the Computer and the desired angles or speeds can be measured directly from the movements of these elements.”Together, the ballistic and prediction calculations form a feedback loop. Operators enter an estimated “time of flight” for the shell when they first begin tracking. The predictor uses this estimate to perform its initial calculation, which feeds into the ballistic stage. The output of the ballistics calculation then feeds back an updated time-of-flight estimate, which the predictor uses to refine the initial estimate. Thus “a cumulative cycle of correction brings the predicted future position of the target up to the point indicated by the actual future time of flight.”A square box about four feet on each side (see Fig. 2) the T-6 director was mounted on a pedestal on which it could rotate. Three crew would sit on seats and one or two would stand on a step mounted to the machine. The remainder of the crew stood on a fixed platform; they would have had to shuffle around as the unit rotated. This was probably not a problem, as the rotation angles were small. The direc tor‟s pedestal mounted on a trailer, on which data transmission cables and the range finder could be packed for transportation.We have seen that the T-6 computer took only three inputs, elevation, azimuth, and altitude (range), and yet it required nine operators. These nine did not include the operation of the range finder, which was considered a separate instrument, but only those operating the director itself. What did these nine men do?Human ServomechanismsTo the designers of the director, the operato rs functioned as “manual servomechanisms.”One specification for the machine required “minimum dependence on …human element.‟ The Sperry Company explained, “All operations must be made as mechanical and foolproof as possible; training requirements must visualize the conditions existent under rapid mobilization.” The lessons of World War I ring in this statement; even at the height of isolationism, with the country sliding into depression, design engineers understood the difficulty of raising large numbers of trained personnel in a national emergency. The designers not only thought the system should account for minimal training and high personnel turnover, they also considered the ability of operators to perform their duties under the stress of battle. Thus, nearly all the work for the crew was in a “follow-the-pointer”mode: each man concentrated on an instrument with two indicating dials, one the actual and one the desired value for a particular parameter. With a hand crank, he adjusted the parameter to match the two dials.Still, it seems curious that the T-6 director required so many men to perform this follow-the-pointer input. When the external rangefinder transmitted its data to the computer, it appeared on a dial and an operator had to follow the pointer to actually input the data into the computing mechanism. The machine did not explicitly calculate velocities. Rather, two operators (one for X and one for Y) adjusted variable-speed drives until their rate dials matched that of a constant-speed motor. When the prediction computation was complete, an operator had to feed the result into the ballistic calculation mechanism. Finally, when the entire calculation cycle was completed, another operator had to follow the pointer to transmit azimuth to the gun crew, who in turn had to match the train and elevation of the gun to the pointer indications.Human operators were the means of connecting “individual elements” into an integrated system. In one sense the men were impedance amplifiers, and hence quite similar to servomechanisms in other mechanical calculators of the time, especially Vannevar Bush‟s differential analyzer .The term “manual servomechanism”itself is an oxymoron: by the conventional definition, all servomechanisms are automatic. The very use of the term acknowledges the existence of an automatic technology that will eventually replace the manual method. With the T-6, this process was already underway. Though the director required nine operators, it had already eliminated two from the previous generation T-4. Servos replaced the operator who fed back superelevation data and the one who transmitted the fuze setting. Furthermore, in this early machine one man corresponded to one variable, and the machine‟s requirement for operators corresponded directly to the data flow of its computation. Thus the crew that operated the T-6 director was an exact reflection of the algorithm inside it.Why, then, were only two of the variables automated? This partial, almost hesitating automation indicates there was more to the human servo-motors than Sperry wanted to acknowledge. As much as the company touted “their duties are purely mechanical and little skill or judgment is required on the part of the operators,” men were still required to exercise some judgment, even if unconsciously. The data were noisy, and even an unskilled human eye could eliminate complications due to erroneous or corrupted data. The mechanisms themselves were rather delicate and erroneous input data, especially if it indicated conditions that were not physically possible, could lock up or damage the mechanisms. Theoperators performed as integrators in both senses of the term: they integrated different elements into a system.Later Sperry DirectorsWhen Elmer Sperry died in 1930, his engineers were at work on a newer generation director, the T-8. This machine was intended to be lighter and more portable than earlier models, as well as less expensive and “procurable in quantities in case of emergency.” The company still emphasized the need for unskilled men to operate the system in wartime, and their role as system integrators. The operators were “mechanical links in the apparatus, thereby making it possible to avoid mechanical complication which would be involved by the use of electrical or mechanical servo motors.” Still, army field experience with the T-6 had shown that servo-motors were a viable way to reduce the number of operators and improve reliability, so the requirements for the T-8 specified that wherever possible “electrical shall be used to reduce the number of operators to a minimum.” Thus the T-8 continued the process of automating fire control, and reduced the number of operators to four. Two men followed the target with telescopes, and only two were required for follow-the-pointer functions. The other follow-the-pointers had been replaced by follow-up servos fitted with magnetic brakes to eliminate hunting. Several experimental versions of the T-8 were built, and it was standardized by the Army as the M3 in 1934.Throughout the remain der of the …30s Sperry and the army fine-tuned the director system in the M3. Succeeding M3 models automated further, replacing the follow-the-pointers for target velocity with a velocity follow-up which employed a ball-and-disc integrator. The M4 series, standardized in 1939, was similar to the M3 but abandoned the constant altitude assumption and added an altitude predictor for gliding targets. The M7, standardized in 1941, was essentially similar to the M4 but added full power control to the guns for automatic pointing in elevation and azimuth. These later systems had eliminated errors. Automatic setters and loaders did not improve the situation because of reliability problems. At the start of World War II, the M7 was the primary anti-aircraft director available to the army.The M7 was a highly developed and integrated system, optimized for reliability and ease of operation and maintenance. As a mechanical computer, it was an elegant, if intricate, device, weighing 850 pounds and including about 11,000 parts. The design of the M7 capitalized on the strength of the Sperry Company: manufacturing of precision mechanisms, especially ballistic cams. By the time the U.S. entered the second world war, however, these capabilities were a scarce resource, especially for high volumes. Production of the M7 by Sperry and Ford Motor Company as subcontractor was a “real choke” and could not keep up with production of the 90mm guns, well into 1942. The army had also adopted an English system, known as the “Kerrison Director” or M5, which was less accurate than the M7 but easier to manufacture. Sperry redesigned the M5 for high-volume production in 1940, but passed in 1941.Conclusion: Human Beings as System IntegratorsThe Sperry directors we have examined here were transitional, experimental systems. Exactly for that reason, however, they allow us to peer inside the process of automation, to examine the displacement of human operators by servomechanisms while the process was still underway. Skilled asthe Sperry Company was at data transmission, it only gradually became comfortable with the automatic communication of data between subsystems. Sperry could brag about the low skill levels required of the operators of the machine, but in 1930 it was unwilling to remove them completely from the process. Men were the glue that held integrated systems together.As products, the Sperry Company‟s anti-aircraft gun directors were only partially successful. Still, we should judge a technological development program not only by the machines it produces but also by the knowledge it creates, and by how that knowledge contributes to future advances. Sperry‟s anti-aircraft directors of the 1930s were early examples of distributed control systems, technology that would assume critical importance in the following decades with the development of radar and digital computers. When building the more complex systems of later years, engineers at Bell Labs, MIT, and elsewhere would incorporate and build on the Sperry Company‟s experience,grappling with the engineering difficulties of feedback, control, and the augmentation of human capabilities by technological systems.在斯佩里防空炮火控和集成系统的发展电气时代的到来带来了新类型的控制系统。

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本科毕业设计（论文）外文参考文献译文及原文

学院计算机学院专业网络工程年级班别 2008级（3）班学号 3108007132 学生姓名廖杰发指导教师黄益民

2012 年 5 月目录 1 对象的创建和存在时间 .................................................. 1 1.1 对象的创建及破坏方式 .............................................. 1 1.2 内存池中动态创建对象 .............................................. 1 1.3 对象的生命周期 .................................................... 1 1.4 其它内容 .......................................................... 2 1.4.1 集合与继承器 ................................................... 2 1.4.2 单根结构 ....................................................... 4 1.4.3 集合库与方便使用集合 ........................................... 5 2 Object landscapes and lifetimes ............................................ 7 2.1 objects created and destroyed ...................................................................................... 7 2.2 objects created dynamically ......................................................................................... 7 2.3 Objects’ Lifetime .......................................................................................................... 8 2.4 Other section ................................................................................................................ 8 2.4.1 Collections and iterators........................................................................................ 8 2.4.2 The singly rooted hierarchy ................................................................................ 10 2.4.3 Collection libraries and support for easy collection use ...................................... 11 1

1 对象的创建和存在时间从技术角度说，OOP（面向对象程序设计）只是涉及抽象的数据类型、继承以及多形性，但另一些问题也可能显得非常重要。本节将就这些问题进行探讨。 1.1 对象的创建及破坏方式对象需要的数据位于哪儿，如何控制对象的“存在时间”呢？针对这个问题，解决的方案是各异其趣的。C++认为程序的执行效率是最重要的一个问题，所以它允许程序员作出选择。为获得最快的运行速度，存储以及存在时间可在编写程序时决定，只需将对象放置在堆栈（有时也叫作自动或定域变量）或者静态存储区域即可。这样便为存储空间的分配和释放提供了一个优先级。某些情况下，这种优先级的控制是非常有价值的。然而，我们同时也牺牲了灵活性，因为在编写程序时，必须知道对象的准确的数量、存在时间、以及类型。如果要解决的是一个较常规的问题，如计算机辅助设计、仓储管理或者空中交通控制，这一方法就显得太局限了。 1.2 内存池中动态创建对象第二个方法是在一个内存池中动态创建对象，该内存池亦叫“堆”或者“内存堆”。若采用这种方式，除非进入运行期，否则根本不知道到底需要多少个对象，也不知道它们的存在时间有多长，以及准确的类型是什么。这些参数都在程序正式运行时才决定的。若需一个新对象，只需在需要它的时候在内存堆里简单地创建它即可。由于存储空间的管理是运行期间动态进行的，所以在内存堆里分配存储空间的时间比在堆栈里创建的时间长得多（在堆栈里创建存储空间一般只需要一个简单的指令，将堆栈指针向下或向下移动即可）。由于动态创建方法使对象本来就倾向于复杂，所以查找存储空间以及释放它所需的额外开销不会为对象的创建造成明显的影响。除此以外，更大的灵活性对于常规编程问题的解决是至关重要的。 C++允许我们决定是在写程序时创建对象，还是在运行期间创建，这种控制方法更加灵活。大家或许认为既然它如此灵活，那么无论如何都应在内存堆里创建对象，而不是在堆栈中创建。

1.3 对象的生命周期但还要考虑另外一个问题，亦即对象的“存在时间”或者“生存时间” 2

（Lifetime）。若在堆栈或者静态存储空间里创建一个对象，编译器会判断对象的持续时间有多长，到时会自动“破坏”或者“清除”它。程序员可用两种方法来破坏一个对象：用程序化的方式决定何时破坏对象，或者利用由运行环境提供的一种“垃圾收集器”特性，自动寻找那些不再使用的对象，并将其清除。当然，垃圾收集器显得方便得多，但要求所有应用程序都必须容忍垃圾收集器的存在，并能默许随垃圾收集带来的额外开销。但这并不符合C++语言的设计宗旨，所以未能包括到C++里。但Java确实提供了一个垃圾收集器（Smalltalk也有这样的设计；尽管Delphi默认为没有垃圾收集器，但可选择安装；而C++亦可使用一些由其他公司开发的垃圾收集产品）。 1.4 其它内容本节剩下的部分将讨论操纵对象时要考虑的另一些因素。 1.4.1 集合与继承器针对一个特定问题的解决，如果事先不知道需要多少个对象，或者它们的持续时间有多长，那么也不知道如何保存那些对象。既然如此，怎样才能知道那些对象要求多少空间呢？事先上根本无法提前知道，除非进入运行期。在面向对象的设计中，大多数问题的解决办法似乎都有些轻率——只是简单地创建另一种类型的对象。用于解决特定问题的新型对象容纳了指向其他对象的句柄。当然，也可以用数组来做同样的事情，那是大多数语言都具有的一种功能。但不能只看到这一点。这种新对象通常叫作“集合”（亦叫作一个“容器”，但AWT在不同的场合应用了这个术语，所以本书将一直沿用“集合”的称呼。在需要的时候，集合会自动扩充自己，以便适应我们在其中置入的任何东西。所以我们事先不必知道要在一个集合里容下多少东西。只需创建一个集合，以后的工作让它自己负责好了。幸运的是，设计优良的OOP语言都配套提供了一系列集合。在C++中，它们是以“标准模板库”（STL）的形式提供的。Object Pascal用自己的“可视组件库”（VCL）提供集合。Smalltalk提供了一套非常完整的集合。而Java也用自己的标准库提供了集合。在某些库中，一个常规集合便可满足人们的大多数要求；而在另一些库中（特别是C++的库），则面向不同的需求提供了不同类型的集合。例如，可以用一个矢量统一对所有元素的访问方式；一个链接列表则用于保证所有元素的插入统一。所以我们能根据自己的需要选择适当的类型。其中包括集、队列、散列表、树、堆栈等等。所有集合都提供了相应的读写功能。将某样东西置入集合时，采用的方式是十分明 3

显的。有一个叫作“推”（Push）、“添加”（Add）或其他类似名字的函数用于做这件事情。但将数据从集合中取出的时候，方式却并不总是那么明显。如果是一个数组形式的实体，比如一个矢量（Vector），那么也许能用索引运算符或函数。但在许多情况下，这样做往往会无功而返。此外，单选定函数的功能是非常有限的。如果想对集合中的一系列元素进行操纵或比较，而不是仅仅面向一个，这时又该怎么办呢？办法就是使用一个“继续器”（Iterator），它属于一种对象，负责选择集合内的元素，并把它们提供给继承器的用户。作为一个类，它也提供了一级抽象。利用这一级抽象，可将集合细节与用于访问那个集合的代码隔离开。通过继承器的作用，集合被抽象成一个简单的序列。继承器允许我们遍历那个序列，同时毋需关心基础结构是什么——换言之，不管它是一个矢量、一个链接列表、一个堆栈，还是其他什么东西。这样一来，我们就可以灵活地改变基础数据，不会对程序里的代码造成干扰。Java最开始（在1.0和1.1版中）提供的是一个标准继承器，名为Enumeration（枚举），为它的所有集合类提供服务。Java 1.2新增一个更复杂的集合库，其中包含了一个名为Iterator的继承器，可以做比老式的Enumeration更多的事情。从设计角度出发，我们需要的是一个全功能的序列。通过对它的操纵，应该能解决自己的问题。如果一种类型的序列即可满足我们的所有要求，那么完全没有必要再换用不同的类型。有两方面的原因促使我们需要对集合作出选择。首先，集合提供了不同的接口类型以及外部行为。堆栈的接口与行为与队列的不同，而队列的接口与行为又与一个集（Set）或列表的不同。利用这个特征，我们解决问题时便有更大的灵活性。其次，不同的集合在进行特定操作时往往有不同的效率。最好的例子便是矢量（Vector）和列表（List）的区别。它们都属于简单的序列，拥有完全一致的接口和外部行为。但在执行一些特定的任务时，需要的开销却是完全不同的。对矢量内的元素进行的随机访问（存取）是一种常时操作；无论我们选择的选择是什么，需要的时间量都是相同的。但在一个链接列表中，若想到处移动，并随机挑选一个元素，就需付出“惨重”的代价。而且假设某个元素位于列表较远的地方，找到它所需的时间也会长许多。但在另一方面，如果想在序列中部插入一个元素，用列表就比用矢量划算得多。这些以及其他操作都有不同的执行效率，具体取决于序列的基础结构是什么。在设计阶段，我们可以先从一个列表开始。最后调整性能的时候，再根据情况把它换成矢量。由于抽象是通过继承器进行的，所以能在两者方便地切换，对代码的影响则显得微不足道。