【導(dǎo)讀】它最初被命名為Oak，目標(biāo)設(shè)置在家用電器等小型系統(tǒng)的編程語(yǔ)言，來(lái)解。決諸如電視機(jī)、電話、鬧鐘、烤面包機(jī)等家用電器的控制和通訊問(wèn)題。這些智能化家電的市場(chǎng)需求沒(méi)有預(yù)期的高，Sun放棄了該項(xiàng)計(jì)劃。用前景，于是改造了Oak，在1995年5月以“Java”的名稱正式發(fā)布了。語(yǔ)言的風(fēng)格十分接近C++語(yǔ)言。Java不同于一般的編譯運(yùn)行計(jì)算機(jī)語(yǔ)言和解釋執(zhí)行計(jì)算機(jī)語(yǔ)言。這同時(shí)也在一定程度上降低了Java程序的運(yùn)行效率。Java的運(yùn)行速度有了大幅提升。與傳統(tǒng)程序不同Sun公司在推出Java之際就。全球數(shù)以萬(wàn)計(jì)的Java開(kāi)發(fā)公司被要求所設(shè)計(jì)的Java. 口號(hào)之一，并獲得了廣大軟件開(kāi)發(fā)商的認(rèn)同。1992年12月3日，這臺(tái)設(shè)備進(jìn)行了展示。Java和“Java”由于商標(biāo)搜索顯示，Oak已被一家顯卡制。年10月，HotJava和Java平臺(tái)為公司高層進(jìn)行演示。年3月23日SunWorld大會(huì)上進(jìn)行的。升陽(yáng)公司的科學(xué)指導(dǎo)約翰·蓋吉聲明。起進(jìn)行的，宣布網(wǎng)景將在其瀏覽器中包含對(duì)Java的支持。公司成立了Java業(yè)務(wù)集團(tuán)，專門開(kāi)發(fā)Java技術(shù)。Java在瀏覽器中的地位被逐步侵蝕。

　　

【正文】 ese add overhead and plexity. Note that garbage collection does not prevent logical memory leaks, . those where the memory is still referenced but never used. Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object。 this can cause a program to stall momentarily. Explicit memory management is not possible in Java. Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security. As in C++ and some other objectoriented languages, variables of Java39。s primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as monly true for objects (but see Escape analysis). This was a conscious decision by Java39。s designers for performance reasons. Because of this, Java was not considered to be a pure objectoriented programming language. However, as of Java , autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class. Java contains multiple types of garbage collectors. By default, HotSpot uses the Concurrent Mark Sweep collector, also known as the CMS Garbage Collector. However, there are also several other garbage collectors that can be used to manage the Heap. For 90% of applications in Java, the CMS Garbage Collector is good enough. A class that is not declared public may be stored in any .java file. The piler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name were the concatenation of the name of their enclosing class, a $, and an integer. The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is located. The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any method variables that are not static. The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call () explicitly. The method name main is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise JavaBean do not use or need a main() method. A java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from. The main method must accept an array of String objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array of String objects), but allows an alternative syntax for creating and passing the array. The Java launcher launches Java by loading a given class (specified on the mand line or as an attribute in a JAR) and starting its public static void main(String[]) method. Standalone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a mand line. Criticism of Java A number of criticisms have been leveled at Java programming language for various design choices in the language and platform. Such criticisms include the implementation of generics, the handling of unsigned numbers, the implementation of floatingpoint arithmetic, and security vulnerabilities. Class libraries Java Platform and Class libraries diagramJava libraries are the piled bytecodes of source code developed by the JRE implementor to support application development in Java. Examples of these libraries are: The core libraries, which include: Collection libraries that implement data structures such as lists, dictionaries, trees, sets, queues and doubleended queue, or stacks XML Processing (Parsing, Transforming, Validating) libraries Security Internationalization and localization libraries The integration libraries, which allow the application writer to municate with external systems. These libraries include: The Java Database Connectivity (JDBC) API for database access Java Naming and Directory Interface (JNDI) for lookup and discovery RMI and CORBA for distributed application development JMX for managing and monitoring applications User interface libraries, which include: The (heavyweight, or native) Abstract Window Toolkit (AWT), which provides GUI ponents, the means for laying out those ponents and the means for handling events from those ponents The (lightweight) Swing libraries, which are built on AWT but provide (nonnative) implementations of the AWT widgetry APIs for audio capture, processing, and playback A platform dependent implementation of Java Virtual Machine (JVM) that is the means by which the byte codes of the Java libraries and third party applications are executed Pl