Java Programming
from the Grounds Up
by Mark C. Reynolds
With Java, it's possible to write some very sophisticated applets with a relatively small amount of code. Here's how.
Wildly popular due to its interactive multimedia capabilities, Java programming leads the list of Internet development skills in current commercial demand. In this first half of our two-part tutorial on Java applet development, we explore the essentials of Java's components. These include how Java development tools relate to each other and--most importantly--how they are used to provide content that executes on the client side instead of on your server.
Before Sun Microsystems introduced Java, most Web interactivity was accomplished via CGI (Common Gateway Interface) scripting. This is frequently utilized in forms or guestbooks where users type entries into text fields, then submit this information via their browser back to a host server. The host server then passes the information to an external program running on the Web server's machine. The output of this external program is then passed from the server back to the browser. CGIs must execute at least one round trip from the browser to the server and back.
In contrast, when a Java-compatible browser accesses a Java-powered page, an applet--a small program written in Java--is copied to the browser's machine and executes there. It does not execute on the server the way a CGI program does. This local execution makes possible a much greater level of Web interaction and multimedia effects, unhampered by HTML restrictions or network bandwidth. Java programs can display slick animations, invite users to play games, show step-by-step tutorial instructions, or run demonstration versions of computer software. When a browser accesses a standard HTML page, the result is usually a static display. When it runs a Java applet, however, the results are limited only by the creativity of the applet's developer. The applet is a nearly independent application running within the browser.
Getting Started
Java is an object-oriented programming language that resembles a simplified form of C++. Java code displays graphics, accesses the network, and interfaces with users via a set of capabilities--known as classes--that define similar states and common methods for an object's behavior. Unlike other programming languages, though, Java programs are not compiled into machine code; instead, they are converted into an architecture-neutral bytecode format. This collection of bytes represents the code for an abstract Java virtual machine (VM). In order for these bytes to execute on a physical machine, a Java interpreter running on that physical machine must translate those bytes into local actions, such as printing a string or drawing a button.
To run Java applets, you'll need a Java-enabled browser (such as Sun's HotJava, Netscape 2.0 or greater, or Internet Explorer 3.0 or greater) or you can use the appletviewer utility in Sun's Java Development Kit (JDK). The JDK also includes an interpreter for standalone Java applications (called simply java), as well as a debugger and compiler, called jdbg and javac respectively.
Java applets use a subset of the full Java VM, with a variety of features disabled for security reasons. You can add Java applets to your Web pages with the <APPLET> tag, which can include attributes for the applet's height, width, and other parameters. Java-capable browsers treat Java applets like other media objects in an HTML document: they are loaded with the rest of the page, then verified and executed.
NEXT
Mark Reynolds is a network protocol designer, Java programmer, computer animator, and fanatic mountaineer. He currently consults to Adaptive Optics Associates, a United Technologies company.
Reprinted from Web Developer® magazine, Vol. 2 No.1 Spring 1996 (c) 1996 internet.com Corporation. All rights reserved.
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