【正文】 ributed environments. Paramount among these challenges is secure delivery of applications that consume the minimum of system resources, can run on any hardware and software platform, and can be extended dynamically. Java originated as part of a research project to develop advanced software for a wide variety of work devices and embedded systems. The goal was to develop a small, reliable, portable, distributed, realtime operating platform. When the project started, C++ was the language of choice. But over time the difficulties encountered with C++ grew to the point where the problems could best be addressed by creating an entirely new language platform. Design and architecture decisions drew from a variety of languages such as Eiffel, SmallTalk, Objective C, and Cedar/Mesa. The result is a language platform that has proven ideal for developing secure, distributed, work based enduser applications in environments ranging from workembedded devices to the WorldWide Web and the desktop. The design requirements of Java are driven by the nature of the puting environments in which software must be deployed. The massive growth of the Inter and the WorldWide Web leads us to a pletely new way of looking at development and distribution of software. To live in the world of electronic merce and distribution, Java must enable the development of secure, high performance, and highly robust applications on multiple platforms in heterogeneous, distributed works. Operating on multiple platforms in heterogeneous works invalidates the traditional schemes of binary distribution, release, upgrade, patch, and so on. To survive in this jungle, Java must be architecture neutral, portable, and dynamically adaptable. The Java system that emerged to meet these needs is simple, so it can be easily programmed by most developers。 familiar, so that current developers can easily learn Java。 multithreaded, for high performance in applications that need to perform multiple concurrent activities, such as multimedia。 with the burden of memory management taken off the programmer’s shoulders, storage allocation errors go away. Another reason monly given that languages like Lisp, TCL, and SmallTalk are good for prototyping is that they don’t require you to pin down decisions early on— these languages are semantically rich. Java has exactly the opposite property: it forces you to make explicit choices. Along with these choices e a lot of assistance— you can write method invocations and, if you get something wrong, you get told about it at pile time. You don’t have to worry about method invocation error. The Java beginner must grasp 30 basic concepts Basic concept OOP the only relations is what the object’s interface, such as the puter seller he leaves alone internal structure of electrical source, but he is only concerned about that whether the power will be supplied to you, also so long as know can or not but is not how and procedures are make up of certain attribute and the behavior object, the different object visit invokes through the function invocation, between the object all intermunion are invoke through the method invocation, By object data encapsulation, enhances reuse rate very much.. OOP the most important thought is class, the class is the template ,is a blueprint, construct an object from a class, namely construct an instance of the class. 3. Encapsulation: is that bines the data and the behavior in a package) and hides the data the realization process to the object user, in an object data is called its instance field. expands a class to obtain a new class is called inheritance, but all classes are constructed by the object super root class of expansion, super root class of as follows can make the introduction. 3 principal characteristics Behavior explained this object can make what. Tate when the object exerts the method object reflection. Dentity and other similar behavior objects discrimination symbols. Each object has only indentity and among three characteristics they affect mutually. 6. Relations among classes: Usea:Dependent relation Hasa:Polymerization relation Isa: inheritor relation example:A class has inherited B class, this time A class not only has B class of method, but also has its own method(Individuality exists in general character) object use structure:Structure proposing, the structure is one special method, the structure object and to its initialization. Example:A Data class of structure calls Data New Data () structure a new object, also initialize current time. Data happyday=new Data () an object evaluates an variable happyday, thus enables this object to be used many times, here be stated the cause variable and the object variable are different. New returns the value is a quotation. Constructor characteristic:The constructor may have 0, one or many parameters The constructor and the class have the same name A cl