基于JSP校园二手交易系统的设计与实现外文翻译.doc
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1、1. SimpleWe wanted to build a system that could be programmed easily without a lot of eso-teric training and which leveraged todays standard practice. So even though we found that C+ was unsuitable, we designed Java as closely to C+ as possible in order to make the system more comprehensible. Java o
2、mits many rarely used, poorly understood, confusing features of C+ that, in our experience, bring more grief than benefit.The syntax for Java is, indeed, a cleaned-up version of the syntax for C+. There is no need for header files, pointer arithmetic (or even a pointer syntax), structures, unions, o
3、perator overloading, virtual base classes, and so on. (See the C+ notes interspersed throughout the text for more on the differences between Java and C+.) The designers did not, however, attempt to fix all of the clumsy features of C+. For example, the syn-tax of the switch statement is unchanged in
4、 Java. If you know C+, you will find the tran- sition to the Java syntax easy. If you are used to a visual programming environment (such as Visual Basic), you will not find Java simple. There is much strange syntax (though it does not take long to get the hang of it). More important, you must do a l
5、ot more programming in Java. The beauty of Visual Basic is that its visual design environment almost automatically pro-vides a lot of the infrastructure for an application. The equivalent functionality must be programmed manually, usually with a fair bit of code, in Java. There are, however, third-p
6、arty development environments that provide “drag-and-drop”-style program development.2.Object OrientedSimply stated, object-oriented design is a technique for programming that focuses on the data (= objects) and on the interfaces to that object. To make an analogy with carpentry, an “object-oriented
7、” carpenter would be mostly concerned with the chair he was building, and secondarily with the tools used to make it; a “non-object-oriented” carpenter would think primarily of his tools. The object-oriented facilities of Java are essentially those of C+.Object orientation has proven its worth in th
8、e last 30 years, and it is inconceivable that a modern programming language would not use it. Indeed, the object-oriented features of Java are comparable to those of C+. The major difference between Java and C+ lies in multiple inheritance, which Java has replaced with the simpler concept of interfa
9、ces, and in the Java metaclass model.3.Network-SavvyJava has an extensive library of routines for coping with TCP/IP protocols like HTTP and FTP. Java applications can open and access objects across the Net via URLs with the same ease as when accessing a local file system.4.RobustJava is intended fo
10、r writing programs that must be reliable in a variety of ways. Java puts a lot of emphasis on early checking for possible problems, later dynamic (runtime) checking, and eliminating situations that are error-prone. . . . The single biggest difference between Java and C/C+ is that Java has a pointer
11、model that elim- inates the possibility of overwriting memory and corrupting data.5.SecureJava is intended to be used in networked/distributed environments. Toward that end, a lot of emphasis has been placed on security. Java enables the construction of virus-free, tamper-free systems. From the begi
12、nning, Java was designed to make certain kinds of attacks impossible,among them: Overrunning the runtime stacka common attack of worms and viruses Corrupting memory outside its own process space Reading or writing files without permission6.Architecture NeutralThe compiler generates an architecture-n
13、eutral object file formatthe compiled code is executable on many processors, given the presence of the Java runtime sys-tem. The Java compiler does this by generating bytecode instructions which have nothing to do with a particular computer architecture. Rather, they are designed to be both easy to
14、interpret on any machine and easily translated into native machinecode on the fly.7.PortableUnlike C and C+, there are no “implementation-dependent” aspects of the specifi- cation. The sizes of the primitive data types are specified, as is the behavior of arith-metic on them. For example, an int in
15、Java is always a 32-bit integer. In C/C+, int can mean a 16-bit integer, a 32-bit integer, or any other size that the compiler vendor likes. The only restriction is that the int type must have at least as many bytes as a short int and cannot have more bytes than a long int. Having a fixed size for n
16、umber types eliminates a major porting headache. Binary data is stored and transmitted in a fixed format, eliminating confusion about byte ordering. Strings are saved in a standard Unicode format.8.InterpretedThe Java interpreter can execute Java bytecodes directly on any machine to which the interp
17、reter has been ported. Since linking is a more incremental and lightweight process, the development process can be much more rapid and exploratory.9.High PerformanceWhile the performance of interpreted bytecodes is usually more than adequate, there are situations where higher performance is required
18、. The bytecodes can be translated on the fly (at runtime) into machine code for the particular CPU the application is running on.10.MultithreadedThe benefits of multithreading are better interactive responsiveness and real-time behavior.11.DynamicIn a number of ways, Java is a more dynamic language
19、than C or C+. It was designed to adapt to an evolving environment. Libraries can freely add new meth- ods and instance variables without any effect on their clients. In Java, finding out runtime type information is straightforward. This is an important feature in those situations in which code needs
20、 to be added to a running program. A prime example is code that is downloaded from the Internet to run in a browser. In Java 1.0, finding out runtime type information was anything but straightforward, but current versions of Java give the programmer full insight into both the structure and behavior
21、of its objects. This is extremely useful for systems that need to analyze objects at runtime, such as Java GUI builders, smart debuggers, pluggable components, and object databases.Common Misconceptions about Java1.Java is an extension of HTML.Java is a programming language; HTML is a way to describ
22、e the structure of a web page. They have nothing in common except that there are HTML extensions for placing Java applets on a web page.2.I use XML, so I dont need Java.Java is a programming language; XML is a way to describe data. You can process XML data with any programming language, but the Java
23、 API contains excellent support for XML processing. In addition, many important third-party XML tools are implemented in Java. See Volume II for more information.3.Java is an easy programming language to learn.No programming language as powerful as Java is easy. You always have to distinguish betwee
24、n how easy it is to write toy programs and how hard it is to do serious work. Also, consider that only four chapters in this book discuss the Java language.4.Java will become a universal programming language for all platforms.This is possible, in theory, and it is certainly the case that every vendo
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