【文章內(nèi)容簡(jiǎn)介】 n environment that provides services such as security, portability across operating systems, and automatic garbage collection. As a programmer, you will want a language with a pleasant syntax and prehensible semantics (., not C++). Java fits the bill, as do dozens of other fine languages. Some languages give you portability, garbage collection, and the like, but they don39。t have much of a library, forcing you to roll your own if you want fancy graphics or working or database access. Well, Java has everything— a good language, a highquality execution environment, and a vast library. That bination is what makes Java an irresistible proposition to so many programmers. Features of Java We wanted to build a system that could be programmed easily without a lot of esoteric training and which leveraged today39。s 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 prehensible. Java omits 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 cleanedup version of the syntax for C++. There is no need for header files, pointer arithmetic (or even a pointer syntax), structures, unions, operator 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 syntax of the switch statement is unchanged in Java. If you know C++, you will find the transition 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 lot more programming in Java. The beauty of Visual Basic is that its visual design environment almost automatically provides 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, thirdparty development environments that provide draganddropstyle program development. Another aspect of being simple is being small. One of the goals of Java is to enable the construction of software that can run standalone in small machines. The size of the basic interpreter and class support is about 40K bytes。 adding the basic standard libraries and thread support (essentially a selfcontained microkernel) adds an additional 175K. 2. Object Oriented Simply stated, objectoriented 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 objectoriented carpenter would be mostly concerned with the chair he was building, and secondarily with the tools used to make it。 a nonobjectoriented carpenter would think primarily of his tools. The objectoriented facilities of Java are essentially those of C++. Object orientation has proven its worth in the last 30 years, and it is inconceivable that a modern programming language would not use it. Indeed, the objectoriented features of Java are parable to those of C++. The major difference between Java and C++ lies in multiple inheritance, which Java has replaced with the simpler concept of interfaces, and in the Java metaclass model. The reflection mechanism and object serialization feature make it much easier to implement persistent objects and GUI builders that can integrate offtheshelf ponents. 3. Distributed Java 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. We have found the working capabilities of Java to be both strong and easy to use. Anyone who has tried to do Inter programming using another language will revel in how simple Java makes onerous tasks like opening a socket connection. (We cover working in Volume 2 of this book.) The remote method invocation mechanism enables munication between distributed objects (also covered in Volume 2).There is now a separate architecture, the Java 2 Enterprise Edition (J2EE), that supports very large scale distributed applications. 4. Robust Java is intended for 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 errorprone.? The single biggest difference between Java and C/C++ is that Java has a pointer model that eliminates the possibility of overwriting memory and corrupting data. This feature is also very useful. The Java piler detects many problems that, in other languages, would show up only at run time. As for the second point, anyone who has spent hours chasing memory corruption caused by a pointer bug will be very happy with this feature of Java. If you are ing from a language like Visual Basic that doesn39。t explicitly use pointers, you are probably wondering why this is so important. C programmers are not so lucky. They need pointers to access strings, arrays, objects, and even files. In Visual Basic, you do not use pointers for any of these entities, nor do you need to worry about memory allocation for them. On the other hand, many data structures are difficult to implement in a pointerless language. Java gives you the best of both worlds. You do not need pointers for everyday constructs like strings and arrays. You have the power of pointers if you need it, for example, for linked lists. And you always have plete safety, because you can never access a bad