Why Open Source Software / Free Software (OSS/FS)? Look at the Numbers!

This article briefly introduces Java, provides a bit of Java history, and describes various Javaimplementations. Introduction: What's Java? Sun Microsystems has developed a set of technologies called Java (TM). Using Java you can create various kinds of programs including traditional applications and "applets" that automatically run whena user views a web page. Many users are very attracted to Java's goal of "write once, run anywhere"(WORA), that is, using this technology the same program can be highly portable and run on manydifferent platforms (e.g. Windows, Mac, Unix, and Linux). The Java technologies can be divided intofour components: The Java Virtual Machine (JVM) which runs class files. The JVM is an abstract computer thatexecutes programs stored in "class" files. The JVM can be implemented on real computers inmany different ways, and that's the point: as long as your computer faithfully recreates thisabstract computer, it can run programs stored in class files. For example, the JVM might beimplemented as an interpreter built into a web browser, or as a separate program that interpretsthe class files. Your computer could implement the JVM by transforming the class files into anexecutable program specific for that machine just before running them (this is called a"just-in-time" compiler). In fact, your computer hardware might implement the JVM directly.As long as you have an implementation of the JVM, you can run Java programs, compiled intoclass files. Class files are also called bytecode files.1. The Java language. The Java language is an object oriented computer programming languagethat resembles C++ and Objective-C in syntax, with its semantics being more like a crossbetween Ada (for typesafety) and Smalltalk. It supports automatic garbage collection and singleinheritance (with multiple inheritance of interfaces) but lacks enumerations and templates.2. A compiler that generates class files. The JVM runs class files, so you need a way to createthem. Sun has developed a compiler that takes programs written in the Java language andgenerates Java class files. Other vendors have also developed compilers that generate classfiles. Note that you don't need to use the Java language to generate Java class files.3. The Java library. The Java technology includes a set of components for a platform-independentgraphical user interface (GUI) as well as other useful components.4. Sometimes the terminology can rather confusing. Many people, including Sun, use the term "Java" foreach of these different components and for the technology as a whole. You'll need to determine whatthey mean by its context. The key point is that when people "run a Java program", they're actuallyrunning a set of class files on their version of the JVM. Note: the term "Java" is trademarked by Sun. For purposes of this page, "Java" will also includeanything implementing (or trying to implement) Sun's specifications of Java, but this is not intendedto infringe on the status of Sun's trademark.Alternative Java Implementations http://www.dwheeler.com/java-imp.html (1 de 10) [1/10/2001 10:37:00]