Graphic web design - Chapter 1 Introduction to Computers, the Internet and
Chapter 1 Introduction to Computers, the Internet and the Web 49 ties and research organizations, the Internet today is accessible by hundreds of millions of computers worldwide. The Web allows computer users to locate and view multimedia-intensive documents over the Internet. Java systems generally consist of several parts: an environment, the language, the Java Applications Programming Interface (API) and various class libraries. Java programs normally go through five phases to be executed edit, compile, load, verify and execute. Java program file names end with the .java extension. The Java compiler (javac) translates a Java program into bytecodes the language understood by the Java interpreter. If a program compiles correctly, the compiler produces a file with the .class extension. This is the file containing the bytecodes that are interpreted during the execution phase. A Java program must first be placed in memory before it can execute. This is done by the class loader, which takes the .class file (or files) containing the bytecodes and transfers it to memory. The .classfile can be loaded from a disk on your system or over a network. An application is a program that is normally stored and executed on the user s local computer. An applet is a small program that is normally stored on a remote computer that users connect to via a Web browser. Applets are loaded from a remote computer into the browser, executed in the browser and discarded when execution completes. Applications are loaded into memory, then executed by the java interpreter. Browsers are used to view HTML (Hypertext Markup Language) documents on the World Wide Web. When the browser sees an applet in an HTML document, the browser launches the Java class loader to load the applet. The browsers that support Java each have a built-in Java interpreter. Once the applet is loaded, the Java interpreter in the browser begins executing the applet. Applets can also be executed from the command line using the appletviewercommand provided with the Java 2 Software Development Kit (J2SDK). The appletviewer is commonly referred to as the minimum browser it knows only how to interpret applets. Before the bytecodes in an applet are executed by the Java interpreter built into a browser or the appletviewer, they are verified by the bytecode verifier to ensure that the bytecodes for downloaded classes are valid and that they do not violate Java s security restrictions. An intermediate step between interpreters and compilers is a just-in-time (JIT) compiler that, as the interpreter runs, produces compiled code for the programs and executes the programs in machine language rather than reinterpreting them. JIT compilers do not produce machine language that is as efficient as a full compiler. For organizations wanting to do heavy-duty information-systems development, Integrated Development Environments (IDEs) are available from the major software suppliers. The IDEs provide many tools for supporting the software-development process. Object orientation is a natural way of thinking about the world and of writing computer programs. The Unified Modeling Language (UML) is a graphical language that allows people who build systems to represent their object-oriented designs in a common notation. Humans think in terms of objects. We possess the marvelous ability of abstraction, which enables us to view screen images as people, planes, trees and mountains rather than as individual dots of color (called pixels for picture elements ). Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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