Chapter 18 -- Development and Language Environments Chapter 18 Development and Language Environments CONTENTS An Overview of Development Environments Cyberleaf WebFORCE Extending the Web Through New Languages Inferno and Limbo Virtual Reality Modeling Language (VRML) Broadway Environments and Languages Check The converters and editors described in the preceding chapter can be a boon for developers who want to quickly translate documents in other formats to HTML or to create and edit HTML files. These filters and editors are at the file or package level of granularity in web development (see Chapter 7, "Web Design" and Chapter 11, "Design and Implementation Style and Techniques"). As described in Part II, "Web-Development Processes," web development involves many considerations other than just creating individual HTML files. Web development involves larger processes of planning, analysis, design, promotion, and innovation at the web and systems level. Tools are available that take a piecemeal approach to these processes, with mostly a focus on web implementation. This chapter focuses on some of these higher-level development tools and environments. Because web development also has been expanded by languages specifically designed for network communication, this chapter gives you an overview of the role of two of these languages, Java and Limbo, as well as the Broadway project. An Overview of Development Environments Software systems that integrate several tools to provide more comprehensive support for web developers are being introduced. These environments offer steps toward more integrated support environments for web development and information delivery. Although still very much oriented toward the technical construction of HTML rather than the content-development processes, these systems might be the first steps toward even more sophisticated help for the web developer. Ideally, a development environment seamlessly integrates a set of powerful, flexible tools. For software developers, the UNIX operating system is a good example of a very flexible and powerful environment for software development. A skilled programmer using UNIX can create more tools and build applications using them. This "tool to build a tool" capability is key to large-scale, high-level development. Environments are not easy to create, however. Software engineers have tried to create Computer-Aided Software Engineering (CASE) tools for years, with only moderate success. Although developing information for the Web is in some ways analogous to creating software, web development is not software development. Web development may involve programming (gateway programming, as discussed in Part IV) as well as programming-like activities such as creating HTML files, checking HTML syntax, and managing computer files on networked computer systems. The first generation of web development environments, however, mainly addresses these file-management tasks. Cyberleaf Cyberleaf from Interleaf, Inc. (http://www.ileaf.com/ip.html) is a software system for web development that approaches the document-production process of web development-specifically, the needs for large-scale document production. Cyberleaf is not an HTML editor; it uses word processors as the basis for creating information. Cyberleaf converts documents from many standard word processing formats to others and uses a filtering system that includes file management and style conversion. Cyberleaf is offered on many platforms, including Digital Equipment, Hewlett-Packard, IBM, and Sun. Cyberleaf works with text, graphics, and tables from standard word processors such as Word RTF (rich text format), WordPerfect, Interleaf, and FrameMaker and can convert these files to HTML with GIF illustrations. The conversion is done using a system of style matching that takes into account GIF picture size. Cyberleaf can perform multiple document conversions in batch processing. Figure 18.1 shows a sample screen of Cyberleaf showing the checks available at each stage in this conversion process. Figure 18.1 : A Cyberleaf software sample screen (courtesy of Interleaf, Inc.). Cyberleaf's system takes a web-level approach to managing information, not just page-level conversion and formatting. After updating source documents, styles, or parameters, an entire web can be regenerated. This allows for incremental changes as well as web-wide changes in style or links. This generation process also identifies broken links and file system changes using relative path names. Cyberleaf's particular strengths are its openness (it requires only standard word processors as the authoring interface), its sophistication of style conversion, and functions for life-cycle file management. Although not a total integration of all web-development processes, Interleaf's Cyberleaf is a step toward a more integrated approach. WebFORCE Integration is also the theme of Silicon Graphics, Inc.'s commercial product called WebFORCE (http://www.sgi.com/Products/WebFORCE/). WebFORCE's tagline is "to author and to serve,"-an appropriate motto because the software approaches both the development "authoring" as well as the dissemination "serving" sides of web development on both the Internet and on intranets. WebFORCE for authoring provides developers with WebMagic-a graphical user interface for hypermedia development. Bundled with WebMagic are professional-grade tools for multimedia, image, and illustration, such as Adobe Photoshop and Illustrator. WebMagic is intended to be a WYSIWYG interface for HTML document creation. Integrated with Indigo Magic, a user environment for graphical development; the Digital Media Tools Suite for multimedia development; and the InPerson software for group communication; the WebFORCE tools for authoring approach many tasks. Figure 18.2 shows the Indigo Magic user environment, illustrating (1)  The teleconferencing options using InPerson (2)  Audio and video (3)  Shared whiteboard (4)  Shared files (5) through (14)  The multimedia tools for movies, work spaces called desks, digital video recording and teleconferencing, movie player, sound editor, video capture, sound filter, and image editor. Figure 18.2 : A sample work space from Indigo Magic user environment (courtesy of Silicon Graphics, Inc.). WebFORCE's integration of hypermedia plus teleconferencing provides a very broad range of not just the technical requirements for creating webs, but support for the computer-mediated communication and cooperative work involved. WebFORCE is certainly a much higher-end product than those discussed so far, but it offers comprehensive support for multimedia development. Combined with Silicon Graphic's offering of WebSpace, the first commercial 3-D viewer for the Web (http://webspace.sgi.com/), this integration is poised also for the future. Extending the Web Through New Languages In addition to emerging software environments, Web developers have the prospect of using new languages that will work with Web software. These languages make it possible to deliver innovative content in new formats. Virtual Reality Modeling Language (VRML), which offers the beginnings of three-dimensional representation integrated with Web information, is discussed in more detail in Chapter 28, "Virtual Reality Modeling Language." Java, a language for providing distributed executable applications, also extends the kinds of information the Web can deliver. Although creating tools to build tools is the next step in sophistication for web development, new technologies have rapidly been introduced that offer still more methods of expression. This book traces how HTML and its new levels give rich possibilities for hypertext and hypermedia expression. New kinds of expressions that enliven the visual and interactive possibilities for the Web, however, have emerged: Java, a language for creating distributed applications, and VRML, a language for three-dimensional representation on the Web. Java Although the Web's system of hypertext and hypermedia gives users a high degree of selectivity over the information they choose to view, their level of interactivity with that information is typically low. Java, a computer programming language developed by Sun Microsystems, brings this missing interactivity to the Web. With a Java-enabled Web browser, you can encounter animations and interactive applications. Java programmers can make customized media formats and information protocols that can be displayed in any Java-enabled browser. Java's features enrich the communication, information, and interaction on the Web by enabling users to distribute executable content-rather than just HTML pages and multimedia files-to Web users. This capability to distribute executable content is the power of Java. Java's origins are in Sun Microsystems' work to create a programming language to create software that can run on many different kinds of devices. Today, Java is a language for distributing executable content through the Web. A Selection of Sample Java Applications What Java makes possible for developers and users is impossible to show in a paper book: animated applications that can be downloaded across the network and operate on multiple platforms on heterogeneous, distributed networks. By giving the browser the capability to download and run executables, developers can create information in many new formats without having to worry about which helper application a user has installed. Instead of requiring helper applications for multimedia display, a smart browser has the capability to learn how to deal with new protocols and data formats dynamically. Information developers therefore can serve data with proprietary protocols because the browser, in essence, can be instructed on how to deal with them. Figure 18.3 shows an example of the kind of animation application that is possible with Java. In the figure, the little black-and-white character is Duke, the mascot of Java. Duke tumbles across a Web page displayed in the browser, cycling through a set of graphics images that loop while the user has this page loaded. Figure 18.3 : Tumbling Duke, mascot of Java (courtesy of Arhur van Hoff, formerly of Sun Microsystems). Animation isn't limited to cartoon figures, however. Pages can have animated logos or text that moves or shimmers across the screen. Java animations also do not need to be just decorative, pre-generated figures; instead, they can be graphics generated based on computation. Figure 18.4 shows a bar chart applet. Figure 18.4 : A bar charts applet (courtesy of Sun Microsystems). Although the animations shown can be static images that are drawn or generated, or animated images that can behave according to a preset algorithm (such as the tumbling Duke in Fig. 18.3), animation also can be made interactive, where the user has some input on its appearance. Figure 18.5 shows a 3-D rendering of chemical models. Using the mouse, you can spin these models and view them from many angles. Unlike the source code for the graph applet shown in Figure 18.4, of course, the source code for the chemical modeling is more complicated. To the user, however, the chemical models seem 3-D, giving an insight into the nature of the atomic structure of these elements like no book could. Figure 18.5 : Three-dimensional, manipulable chemical models (courtesy of Sun Microsystes). The chemical models in Figure 18.5 respond to user clicks of the mouse. Another variation on this animation involves giving the user a way to interact with an interface in order to get feedback. The impressionist drawing canvas in Figure 18.6 is an excellent example of this. Paul Haeberli at Silicon graphics developed an impressionist Java applet at http://reality.sgi.com/grafica/impression/imppaint.html. He originally developed this technique for creating these kinds of graphics in 1988 for a Silicon Graphics IRIS workstation. Later patented, this technique drives his Java applet. The result is that you can draw using various-size brushes on a canvas and reveal one of several pictures. Figure 18.6 : An interactive, impressionist drawing (courtesy of Paul Haeberli at Silicon Graphics). Another variation on interactivity is real-time interactivity. Figure 18.7 shows an interactive application that involves moving graphics that the user manipulates. This is the game of Tetris, in which you try to line up the falling tile shapes to completely fill the rectangle. Using designated keys for playing, you interact with the interface to steer the falling shapes. This Tetris implementation demonstrates the possibilities for arcade-like games using Java technology. Figure 18.7 : Tetris (courtesy of Nathan Williams). The chemical model, impressionist canvas, and Tetris game demonstrate how interactivity and animation can work together. These applets customized their animated output based on user input, so they actually were performing computation. An example that shows this computational capability in more concrete terms is in Figure 18.8-a simple spreadsheet. Figure 18.8 : A simple spreadsheet (courtesy of Sami Shain, Sun Microsystems). This spreadsheet demonstrates the computational possibilities of Java. The spreadsheet shown allows you to change the contents of any of the 24 cells (A1 through D6) by replacing its label, value, or formula. This is just like a real spreadsheet, which is more of an environment in which the user can work than a fixed game. This capability to create an environment for computation is a profound one; using Java, a user can obtain an entire environment for open-ended interaction rather than a fixed set of options for interaction-opening up the Web page into a Web stage. Java also can be used to support mass communication in new ways. The Nando Times is a Web-based news service that has been very innovative in news delivery on the Web. Using Java, this news agency now provides a tickertape of headlines across its front page. The text under the Nando banner in Figure 18.9 scrolls continuously to show the world, national, sports, and political top stories at the moment. The four pictures below the labels for these categories also change, giving a slide show that is very effective in displaying new information without requiring the user to select it for viewing. This transforms the Web into something that people can watch to get new information. Figure 18.9 : Headline feed on The Nando Times (courtesy of The Nando Times). Java Applets On-Line Check out Gamelan, the Java directory on-line at http://www.gamelan.com, for a great collection of Java applets and information. Java's Potential The previous examples illustrate only some of the potential of Java. A few of these examples are "toy" demonstrations meant to show the possibilities of Java. What kind of communication might Java foster? The Nando Times example shows an innovative application for providing information in a way that allows you to sit back and observe rather than select hypertext links. Java opens up a new degree of interactivity and customizability of interaction for the Web. Earlier web-development techniques of creating pages and linking them together still are necessary in a Java-flavored Web. Java creates possibilities for richer kinds of content to be developed, however. The user can interact with and change the appearance of a Web page along with the state of a database using a Java-enabled browser. Java profoundly changes the texture of the Web in the following ways: Java creates places to stop on the paths of the Web.  A well-done Java application on a single hypertext page can engage a user for a long time. Instead of just offering text, sound, images, or videos to observe, a Java page can offer a place to play, learn, or communicate and interact with others in a way that isn't necessarily based on going somewhere else on the Web through hyperlinks. If the hypertext links of the Web are like paths, the Java pages are like the towns, villages, and cities to stop on these paths and do something other than just observe or "surf." Java increases the dynamism and competitiveness of the Web.  Just as new browser technology prompted Web developers to create still more applications and pages to exploit these features, so too does Java technology promise a new round of content development on the Web. Java enriches the interactivity of the Web.  Java's interactivity is far richer, more immediate, and more transparent than the interactivity possible through gateway programming. Gateway programming still should have a role in Web applications, just as page design and multimedia presentation still will play a role. Java's inter-activity brings new possibilities to what can happen on the Web, however. With Java, transactions on the Web can be more customized, with immediate, continuous, and ongoing feedback to the user. Java transforms the Web into a software delivery system.  Java's essential design as a language to deliver executable content makes it possible for programmers to create software of any kind and deliver it to users of Java-enabled browsers. Instead of focusing on the interface, the Java programmer focuses on the interaction desired and lets the built-in features of the graphics take care of the rest of the implementation. The result is that very simple programs like the drawing and spreadsheet applications can be created quickly and distributed worldwide. The true potential of Java to transform the Web is still in its initial stages. New potential applications for commerce, information delivery, and user interaction still await the imagination and skill of future Java developers. Java's Technical Model for Distributable, Executable Content Executable content is a general term that characterizes the important difference between the content that a Java-enabled Web browser downloads and the content a non-Java-enabled browser can download. Simply put, in a non-Java Web browser, the downloaded content is defined in terms of Multipurpose Internet Mail Extension (MIME) specifications, which include a variety of multimedia document formats. This content, after downloaded by the user's browser, is displayed in the browser. The browser may employ a helper application (such as in displaying images, sound, and video). The overall pattern for the use of this content is user choice, browser download, and browser display. A Java-enabled browser also follows this pattern, but adds another crucial step. First, the Java-enabled browser, following requests by the user, downloads content defined by MIME specifications and displays it. However, a Java-enabled browser recognizes a special hypertext tag called APPLET. When downloading a Web page containing an APPLET tag, the Java-enabled browser knows that a special kind of Java program called an applet is associated with that Web page. The browser then downloads another file of information, as named in an attribute of the APPLET tag, that describes the execution of that applet. This file of information is written in what are called bytecodes. The Java-enabled browser interprets these bytecodes and runs them as an executable program on the user's host. The resulting execution on the user's host then drives the animation, interaction, or further communication. This execution of content on the user's host is what sets Java content apart from the hypertext and other multimedia content of the Web (see Fig. 18.10). Figure 18.10 : Java's technical model. The process of using executable content in a Java-enabled browser, for users, is seamless. The downloading and start of the execution of content happens automatically. Users do not have to specifically request this content or start its execution. This executable content is platform-independent; Java programmers do not need to create separate versions of the applets for different computer platforms, as long as the users have a Java interpreter (or Java-enabled browser) installed on their computer. So, when surfing the Web with a Java-enabled browser, you might find not only all the hypertext content that the pre-Java age Web offered, but also animated, executable, and distributed content. Moreover, this executable content can include instructions for handling new forms of media and new information protocols. The Java Language According to the information provided by Sun Microsystems (http://java.sun.com/), Java is a "simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language." This characterization identifies the key technical features of Java, which are discussed in this section. Simple The developers of Java based it on the C++ programming language but removed many of the language features that rarely are used or often are used poorly. C++ is a language for object-oriented programming and offers very powerful features. As is the case with many languages designed to have power, however, some features often cause problems. Programmers can create code that contains errors in logic or is incomprehensible to other programmers trying to read it. Because most of the cost of software engineering often is code maintenance rather than code creation, this shift to understandable code rather than powerful but poorly understood code can help reduce software costs. Specifically, Java differs from C++ (and C) in these ways: Java does not support the struct, union, and pointer data types. Java does not support typedef or #define. Java differs in its handling of certain operators and does not permit operator overloading. Java does not support multiple inheritance. Java handles command-line arguments differently than C or C++. Java has a String class as part of the java.lang package. This differs from the null-terminated array of characters used in C and C++. Java has an automatic system for allocating and freeing memory (garbage collection), so it is unnecessary to use memory allocation and deallocation functions as in C and C++. Object-Oriented Like C++, Java can support an object-oriented approach to writing software. Ideally, object-oriented design can permit the creation of software components that can be reused. Object-oriented programming is based on modeling the world in terms of software components called objects. An object consists of data and operations that can be performed on that data called methods. These methods can encapsulate, or protect, an object's data because programmers can create objects in which the methods are the only way to change the state of the data. Another quality of object-orientation is inheritance. Objects can use characteristics of other objects without having to reproduce the functionality in those objects that supports those characteristics. Inheritance therefore helps in software reuse, because programmers can create methods just once that do a specific job. Another benefit of inheritance is software organization and understandability. By organizing objects according to classes, each object in a class inherits characteristics from parent objects. This makes the job of documenting, understanding, and benefiting from previous generations of software easier, because the functionality of the software grows incrementally as more objects are created. Objects at the end of a long inheritance chain can be very specialized and powerful. Figure 18.11 summarizes the general qualities of data encapsulation, methods, and inheritance of an object-oriented language. Figure 18.11 : Object-oriented systems. Technically, Java's object-oriented features are those of C++ with extensions from Objective C for dynamic method resolution. Distributed Unlike the languages C++ and C, Java is designed specifically to work within a networked environment. Java has a large library of classes for communicating using the Internet's TCP/IP protocol suite, including protocols such as HTTP and FTP. Java code can manipulate resources via URLs as easily as programmers are used to accessing a local file system using C or C++. Interpreted When the Java compiler translates a Java class source file to bytecodes, this bytecode class file can be run on any machine that runs a Java interpreter or Java-enabled browser. This allows the Java code to be written independent of the users' platforms. Interpretation also eliminates the compile and run cycle for the client, because the bytecodes are not specific to a given machine but are interpreted. Robust Robust software doesn't "break" easily because of programming bugs or logic errors in it. A programming language that encourages robust software often places more restrictions on programmers when they are writing the source code. These restrictions include those on data types and the use of pointers. The C programming language is notoriously lax in its checking of compatible data types during compilation and runtime. C++ was designed to be more strongly typed than C; however, C++ retains some of C's approach toward typing. In Java, typing is more rigorous; a programmer cannot turn an arbitrary integer into a pointer by casting, for example. Also, Java does not support pointer arithmetic but has arrays instead. These simplifications eliminate some of the tricks that C programmers could use to access arbitrary areas of memory. In particular, Java does not allow programmers to overwrite memory and corrupt other data through pointers. In contrast, C programmers often can accidentally (or deliberately) overwrite or corrupt data. Secure Because Java works in networked environments, the issue of security is one that should be of concern to developers. Plans are in the works for Java to use public-key encryption techniques to authenticate data. In its present form, Java puts limits on pointers so that developers cannot forge access to memory where not permitted. These aspects of Java enable a more secure software environment. "Java Security," later in this chapter, outlines the layers of Java's security in more detail. Architecture Neutral The Java compiler creates bytecodes that are sent to the requesting browser and interpreted on the browser's host machine, which has the Java interpreter or a Java-enabled browser installed. With this model, programmers can write code once and know that it will run the same way on any hardware architecture with a ported interpreter. Portable The quality of being architecture neutral allows for a great deal of portability. However, another aspect of portability is how the hardware interprets arithmetic operations. In C and C++, source code might run slightly differently on different hardware platforms because of how these platforms implement arithmetic operations. In Java, this method has been simplified. An integer type in Java, int, is a signed, two's complement 32-bit integer. A real number, float, is always a 32-bit, floating-point number defined by the IEEE 754 standard. These consistencies make it possible to have the assurance that any result on one computer with Java can be replicated on another. High Performance Although Java bytecodes are interpreted, the performance sometimes isn't as fast as direct compilation and execution on a particular hardware platform. Java compilation includes an option to translate the bytecodes into machine code for a particular hardware platform. This can provide the same efficiency as a traditional compile-and-load process. According to Sun Microsystems testing, performance of this bytecode-to-machine-code translation is "almost indistinguishable" from direct compilation from C or C++ programs. Multithreaded Java is a language that can be used to create applications in which several things happen at once. Based on a system of routines that allow for multiple threads of events based on C.A.R. Hoare's monitor-and-condition paradigm, Java presents the programmer with a way to support real-time, interactive behavior in programs. Dynamic Unlike C++ code, which often requires complete recompilation if a parent class is changed, Java uses a method of interfaces to relieve this dependency. The result is that Java programs can allow for new methods and instance variables in objects in a library without affecting their dependent client objects. HotJava Marked the Emergence of a New Kind of Web Browser The HotJava browser that showcases Java marks the start of a new generation of smart browsers for the Web. Not constrained to a fixed set of functionality, the HotJava browser can adjust and learn new protocols and formats dynamically. Developers of Web information using Java no longer are constrained to the text, graphics, and relatively low-quality multimedia of the fixed set available for Web browsers in the pre-Java age. Instead, the HotJava browser opens new possibilities for new protocols and new media formats never before seen on the Web. Through the past half-decade of development of the World Wide Web, new browser technologies often have altered the common view of what the Web and on-line communication could be. When the Mosaic browser was released in 1993, it rocketed the Web to the attention of the general public because of the graphical, seamless appearance it gave to the Web. Instead of a disparate set of tools to access a variety of information spaces, Mosaic dramatically and visually integrated Internet information. Its point-and-click operation changed ideas about what a Web browser could be, and its immediate successor, Netscape, likewise has grown in popularity and has continued to push the bounds of what is presented on the Web. HotJava, however, marks a very new stage of technological evolution of browsers. HotJava breaks the model of Web browsers as only filters for displaying network information; a Java-age browser acts more like an intelligent interpreter of executable content and a displayer for new protocol and media formats. Release 2.0 and later of Netscape Communications' Navigator browser is Java-enabled. Netscape justifiably characterizes its browser as "platforms" for development and applications rather than just a Web browser. Pre-Java Browsers The earliest browser of the Web was the line-mode browser from CERN. The subsequent Mosaic-class browsers (Mosaic and Netscape from 1993 to mid-1995) dramatically opened the graphical view of the Web. The Mosaic-type browsers, however, acted as an information filter to Internet-based information. Encoded into these browsers was knowledge of the fundamental Internet protocols and media formats (such as HTTP, NNTP, Gopher, FTP, HTML, and GIF). The browsers matched this knowledge with the protocols and media formats found on the Net and then displayed the results. Figure 18.12 illustrates this operation as the browser finds material on the Net and interprets it according to its internal programming for protocols or common media formats. These browsers also used helper applications to display specialized media formats, such as movies or sound. Figure 18.12 : Pre-Java browsers acted as filters. Pre-Java browsers were very knowledgeable about the common protocols and media formats on the network (and therefore very bulky). Unfortunately, pre-Java browsers could not handle protocols for which they had not been programmed or media formats for which no helper applications were available. These are the technical shortcomings that Java-age browsers address. Java-Age Browsers A Java-age browser is very lightweight because it actually has no predefined protocols or media formats programmed into its core functionality. Instead, the core functionality of a HotJava browser consists of the capability to learn how to interpret any protocol or media format. Of course, the HotJava browser is told about the most common protocols and formats as part of its distribution package. In addition, a HotJava browser can learn any new format or protocol that a Java programmer might devise. As Figure 18.13 shows, a Java-age browser is lightweight; it doesn't come with a monolithic store of knowledge of the Web, but with the most important capability of all-the capability to learn. Figure 18.13 : The Java-age browser can learn. Java in Operation Another way to put the Java language, a Java-enabled browser, and the larger context of on-line communications into perspective is to review the processes that occur when a user with a Java-enabled browser requests a page containing a Java applet: The user sends a request for an HTML document to the information provider's server. The HTML document is returned to the user's browser. The document contains the APPLET tag, which identifies the applet. The corresponding applet bytecode is transferred to the user's host. This bytecode was created previously by the Java compiler using the Java source code file for that applet. The Java-enabled browser on the user's host interprets the bytecodes and provides the display. The user may have further interaction with the applet but with no further downloading from the provider's Web server. This is because the bytecode contains all the information necessary to interpret the applet. Java Software Components Another aspect of the technical makeup of the Java environment is the software components that comprise its environment. See the Sun Microsystems Java site (http://java.sun.com/) for complete details on obtaining the Java Developer's Kit (JDK). Programmer's need to learn the vocabulary of the pieces of the JDK as well as terms for what can be created with it. Java Language Constructs Java is the programming language used to develop executable, distributed applications for delivery to a Java-enabled browser or the Java interpreter. A Java programmer can create the following: applets  Programs that are referenced in HTML pages through the APPLET tag and displayed in a Java-enabled browser. The simple "hello world" program is an applet. applications  Stand-alone programs written in Java and executed independently of a browser. This execution is done using the Java interpreter, java, included in the Java code distribution. The input and output of these applications do not need to be through the command line or text only. The HotJava browser itself is a Java application. protocol handlers  Programs that are loaded into the user's HotJava browser and interpret a protocol. These protocols include standard ones, such as HTTP or programmer-defined protocols. content handlers  A program loaded into the user's HotJava browser that interprets files of a type defined by the Java programmer. The Java programmer provides the necessary code for the user's HotJava browser to display/interpret this special format. native methods  Methods that are declared in a Java class but implemented in C. These native methods essentially allow a Java programmer to access C code from Java. Java Distribution Software The Java Developer's Kit available from Sun Microsystems includes the following pieces: Java applet viewer  Lets you run and test applets without having to create an HTML page to refer to it. Note that the beta release of the JDK included an applet viewer instead of an updated HotJava browser. Java compiler  The software used to translate the human-readable Java source code to machine-readable bytecodes. The Java compiler is invoked by using the javac command. Java language runtime  The environment for interpreting Java applications. Java debugger API and prototype debugger  A command-line debugger that uses this API. The Java API The Java API is a set of classes distributed with the JDK that programmers can use in Java applications. The documentation of the API provided on-line is key reference material for Java programmers. The API consists of the packages in the Java language. The API documentation includes a list of All packages: java.applet java.awt java.awt.image java.awt.peer java.io java.lang java.net java.util Information on each package, describing the following: Interfaces Classes Exceptions Documentation on each class in the package that describes the class: Variables Constructors Methods The Java Virtual Machine Specification A document available from the Sun Microsystems Java site (http://java.sun.com/) called The Java Virtual Machine specifies how the Java language is designed to exchange executable content across networks. The aim of this specification is to describe Java as a nonproprietary, open language that may be implemented by many companies and sold as a package. The Java Virtual Machine specification describes in abstract terms how Java operates. This leaves the details of implementation up to the programmers who create Java interpreters and compilers. The Java Virtual Machine specification also concretely defines the specific interchange format for Java code. This is called The Java Interchange Specification. The other part of the Virtual Machine specification defines the abstractions that can be left to the implementer. These abstractions are not related to the interchange of Java code. These include, for example, management of runtime data areas, garbage-collection algorithms, the implementation of the compiler and other Java environment software, and optimization algorithms on compiled Java code. Java Security Because a HotJava browser downloads code across the network and then executes it on the user's host, security is a major concern for Java-enabled browser users and Java programmers. HotJava includes several layers of security, including the following: The Java language itself includes tight restrictions on memory access that are very different from the memory model used in the C language. These restrictions include removal of pointer arithmetic and removal of illegal cast operators. A bytecode verification routine in the Java interpreter verifies that bytecodes don't violate any language constructs (which might happen if an altered Java compiler were used). This verification routine checks to make sure that the code doesn't forge pointers, access restricted memory, or access objects other than according to their definition. This check also ensures that method calls include the correct number of arguments of the right type and that there are no stack overflows. During loading, each class name and its access restrictions are verified. An interface security system enforces security policies at many levels. At the file access level, if a bytecode attempts to access a file to which it has no permissions, a dialog box pops up that enables the user to continue or stop the execution. At the network level, future releases will have facilities to use public-key encryption and other cryptographic techniques to verify the source of the code and its integrity after having passed through the network. This encryption technology will be the key to secure financial transactions across the network. At runtime, information about the origin of the bytecode can be used to decide what that code can do. The security mechanism can tell whether a bytecode originated from inside a firewall. You can set a security policy that restricts code that you don't trust in some browsers (such as HotJava). Java Code The code-level details of Java reveal its object orientation, its similarity to C++, and its simplicity. A Java class is written in a source file using a syntax similar to C++. A file called HelloWorld.java, for example, might have the source code shown in Listing 18.1. Listing 18.1. The HelloWorld.java source code. import java.awt.Graphics; /**  Program:    HelloWorld  Purpose:    demonstrates a Java Applet;  @author     [e-mail]  @version    Java beta; 18 Oct 95  */ public class HelloWorld extends java.applet.Applet {    public void init() {       resize(600, 300);    }    public void paint(Graphics context) {       context.drawString("Hello, world!", 50, 100);    } } The java.applet.Applet class is the root class of all Java applets. The class defined in this sample file, HelloWorld, extends this root class by creating specialized methods for initialization (init) and display (paint). The source code for a Java class is connected to an HTML page through a new element: APPLET. Browsers that don't support Java should ignore this tag. The HelloWorld class defined in Listing 18.1, for example, can be used in an HTML page; Listing 18.2 shows the Hello.html code. Listing 18.2. Hello World HTML code. <HTML> <HEAD> <TITLE>Hello Java</TITLE> </HEAD> <BODY> <APPLET Code="HelloWorld.class" Width="150" Height="100"> </APPLET> </BODY> </HTML> When viewed through a Java-enabled browser, Hello.html displays the string "Hello world!". Programmers can create subclasses of the HelloWorld class and other subclasses of the root class Applet. The Java language can express many kinds of constructs, involving animation, event handling, and multimedia display. All of these class definitions, written in Java, are connected to HTML and compatible browsers through the APPLET element. Java On-Line Information Sources For pointers to more on-line information sources about Java, see http://www.december.com/works/java/info.html. Inferno and Limbo The development of the World Wide Web into a major means to distribute information on-line globally has strained the patchwork of operating systems and languages that support the Internet and the applications that are distributed on it. Java addresses the need for a write-once, run-anywhere (where a port has been completed) language for networks. Inferno is Lucent Technologies' (http://www.lucent.com/) entry into the network software arena. Inferno is actually a networked operating system that supports a programming language called Limbo. Limbo, in turn, relies on its virtual machine called Dis and a communications protocol called Styx. Inferno is a more comprehensive solution to network programming than Java. Inferno includes the full range of operating system functions plus language and other utilities to support application development. Inferno, like Java, approaches the problem of distributing software on a network. Inferno is a networked client-server environment with the goal of making resources available throughout heterogeneous networks on heterogeneous host computers. The components that work together-Inferno, Limbo, Dis, and Styx-each play a role in the computing environment. Figure 18.14 illustrates these roles. Figure 18.14 : Inferno system components. A computer programmer writes Limbo code that is compiled into a binary (object) file. Dis, Limbo's virtual machine, interprets the object file. The resulting application, using the Styx communications protocols, can communicate with other applications or client/server systems. Styx talks to Inferno, the operating system itself, which in turn is an interface to the computer's hardware. Inferno's outlook on computing is that all resources are files within a hierarchical file system that acts as a consistent namespace. This namespace allows applications to communicate with each other and client/server relationships to occur. The namespace is virtual; files might be located anywhere on a local network or on a global network, but the applications just view all resources in terms of this coherent namespace. File operations between any two machines take place through requests and replies via Styx messages. Inferno can be deployed on computer hosts as the native operating system. In such cases, Inferno's components each serve a role in the operating system layers, as shown in Figure 18.15. Figure 18.15 : Inferno system layers. In the case of Inferno deployed on a host with a different operating system, an Inferno kernel emulation environment sits on top of this host operating system, so that the upper application layer remains consistent. The operating systems supported by the Inferno emulation kernel in release 1.0 include Solaris, Irix, Microsoft Windows NT, and Microsoft Windows 95. Hardware architectures supported include MIPS, SPARC, Intel 386, 486, Pentium, AMD 2900, and ARM. Inferno On-Line As of this writing, Inferno hasn't even been released to the public. For the latest on Inferno, check the Lucent Technologies Web site at http://www.lucent.com/inferno/. Virtual Reality Modeling Language (VRML) Like Java, Virtual Reality Modeling Language extends the kind of expression possible on the Web. VRML approaches the information-display issue by attempting to create a system for 3-D representation of objects on the Web. VRML issues include physical rendering, the language definition for VRML, and network references. The collection of material at http://vrml.wired.com/ gives an excellent on-line overview of some of the issues involved. Chapter 28 delves into VRML and how it interacts with the Web in more detail. Figure 18.16 illustrates a sample VRML environment, "The House of Immersion," at the U.S. National Institute of Standards and Technology's "Open Virtual Reality Testbed," a web devoted to demonstrating the capabilities of virtual reality. Figure 18.16 : A sample VRML image (courtesy of Sandy Ressler, U.S. National Institute of Standards and Technology). The new languages and information formats on the Web are sure to add new dimensions to the web-development process. Although satisfying user needs through a continuous-process approach still might be the best route for repeatable web-development success, the kind of expression possible on the Web is just in the beginning stages. Broadway Broadway is the name of a project to bring universal access to network applications. Under development by the X Consortium (http://www.x.org/), Broadway's goal is to allow users to "use any application, anywhere." Broadway grew out of the X Consortium's success with the X Window System (X), a device-independent windowing system that has been around for more than a decade. X provides a way for programmers to create graphical user interfaces that than can be run on a variety of hardware platforms, relieving the need to create interface code for every platform. The goal of Broadway is to extend this device independence to the Internet, while at the same time preventing a requirement to rewrite the vast amount of X software that exists. Broadway does this essentially by treating a Web browser as a desktop that can run X applications. The application itself remains on the host or server machine, not the client's. This is very different from the Java model, in which the executable content is downloaded to the client's host. Broadway's model is built on the capability to invoke remote applications. UNIX commands like rexec and others have made this possible, but not over all computing platforms. Broadway addresses the need to be able to run an application on a host but then present the user interface for that application on a client Web browser. Essentially, this makes the applications platform independent. Broadway On-Line Broadway is expected to be released in late 1996. See http://www.x.org/consortium/broadway.html for the latest on its release status. Environments and Languages Check The next generation of web-development environments and languages provides more expressive ways for a web developer to work. New languages such as Java and VRML give developers a way to create innovative information display and interactive applications. Language environments such as Cyberleaf approach HTML composition from a web- and systems-level perspective, in which file management and web-wide parameters and styles are under more control of the developer. Systems such as Silicon Graphics' WebFORCE software integrate many multimedia capabilities into a system for creating innovative hypermedia applications and serving those to the World Wide Web. Java, by recasting the browser as a flexible, multiprotocol viewer, transforms the Web from a set of fixed protocol and format experiences to one in which format and protocol can be negotiated dynamically between client and server. As a result, animations and hypermedia expressions in many formats can be created for use with capable browsers. The Java programming language works with a special kind of browser and bytecode interpreter. Java can exist within the context of World Wide Web communication and therefore "sits on top of" a set of applications on networks for data communications to support information retrieval. Inferno, developed by Lucent Technologies, addresses the need for a networked operating system to run on a variety of devices across many networks. Limbo is the name of the computer programming language used with Inferno. Broadway, growing from the X Consortium's successful X Window System software, like Inferno and Java, addresses the need for developers to be able to distribute applications over networks to many different kinds of hosts. Advances in web-development environments can free web developers to concentrate on the creative tasks of integrating all processes of web development to meet user needs. Advances in language environments can create new possibilities for new forms of creative human communication on the Web.