The major characteristics of Java are:

• The programs you create are portable in a network. Your program is compiled into Java bytecode that can be run anywhere in a network on a server or client that has a Java virtual machine. The Java virtual machine interprets the bytecode into code that will run on the real computer hardware. This means that individual computer platform differences such as instruction lengths can be recognized and accommodated locally just as the program is being executed. Platform-specific versions of your program are no longer needed.
• The code is "robust," here meaning that, unlike programs written in C++ and perhaps some other languages, the Java objects can contain no references to data external to themselves or other known objects. This ensures that an instruction can not contain the address of data storage in another application or in the operating system itself, either of which would cause the program and perhaps the operating system itself to terminate or "crash." The Java virtual machine makes a number of checks on each object to ensure integrity.
• Java is object-oriented, which means that, among other characteristics, similar objects can take advantage of being part of the same class and inherit common code. Objects are thought of as "nouns" that a user might relate to rather than the traditional procedural "verbs." A method can be thought of as one of the object's capabilities or behaviors.
• In addition to being executed at the client rather than the server, a Java applet has other characteristics designed to make it run fast.
• Relative to C++, Java is easier to learn. (However, it is not a language you'll pick up in an evening!)

   Java was introduced by Sun Microsystems in 1995 and instantly created a new sense of the interactive possibilities of the Web. Both of the major Web browsers include a Java virtual machine. Almost all major operating system developers (IBM, Microsoft, and others) have added Java compilers as part of their product offerings.

   The Java virtual machine includes an optional just-in-time (JIT) compiler that dynamically compiles bytecode into executable code as an alternative to interpreting one bytecode instruction at a time. In many cases, the dynamic JIT compilation is faster than the virtual machine interpretation.

   JavaScript should not be confused with Java. JavaScript, which originated at Netscape, is interpreted at a higher level, is easier to learn than Java, but lacks some of the portability of Java and the speed of bytecode. Because Java applets will run on almost any operating system without requiring recompilation and because Java has no operating system-unique extensions or variations, Java is generally regarded as the most strategic language in which to develop applications for the Web. (However, JavaScript can be useful for very small applications that run on the Web client or server.)

   JavaScript is used in Web site development to do such things as:

• Automatically change a formatted date on a Web page (see our "Today" page)
• Cause a linked-to page to appear in a popup window (see our "Make a WordPop!" page)
• Cause text or a graphic image to change during a mouse rollover

   JavaScript uses some of the same ideas found in Java, the compiled object-oriented language derived from C++. JavaScript code can be imbedded in HTML pages and interpreted by the Web browser (or client). JavaScript can also be run at the server as in Microsoft's Active Server Pages (ASPs) before the page is sent to the requestor. Both Microsoft and Netscape browsers support JavaScript, but sometimes in slightly different ways.

   Along with the Graphic Interchange Format (GIF) file, the JPEG is a file type supported by the World Wide Web protocol, usually with the file suffix of ".jpg". You can create a progressive JPEG that is similar to an interlaced GIF.

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