ch54.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

(<TT>http://java.sun.com/ tutorial/java/index.html</TT>) is an excellent introduction

to the language. The Java Developer's Kit is free if you are using it for personal

reasons only, but if you plan to publish Java-based Web pages, you might need a license.

Details are included with the JDK.</P>



<P>When installing the JDK, make sure that the path to the Java executables is in

your path. Even better, link the executables to <TT>/bin</TT>. The most common errors

Java programmers make while learning the language have to do with case sensitivity

and class use. Java, like UNIX, is case sensitive, so developers must be careful

to ensure that their code is correct. Class usage follows C++ methods, and it must

be adhered to properly. A good Java book is a must for learning the language, and

there are many books on learning Java.</P>

<P>For JavaScript, all you need is a JavaScript-enabled browser (such as Netscape

Navigator 2.02 or higher), a standard ASCII editor or an editor that can save in

ASCII, and a TCP/IP stack to communicate with other machines over the Internet or

an intranet.

<H3 ALIGN="CENTER"><A NAME="Heading5<FONT COLOR="#000077">The Java Language</FONT></H3>

<P>From the programmer's point of view, Java is a very stripped-down object-oriented

(OO) language. The principles of OO programming are suitable in most cases to client-server

applications, so using OO for Java made sense. By staying with OO, Java avoided the

need to set up procedural language structures, all of which cause code bloat to the

point that Java would not be nearly as fast. Also, by avoiding procedural language

principles, Java designers can ignore many of the startup and initialization aspects

of programming, again making the source code smaller and faster. Learning Java is

not especially difficult. If you know an OO language such as C++, you will probably

find Java easier to learn than if you've worked only in procedural languages such

as C or BASIC.</P>

<P>Java may be an OO language, but it's not strict about it. SmallTalk, for example,

is considered a pure OO language because all aspects of SmallTalk are dealt with

as either objects or messages, and all data are object classes. Java doesn't go quite

as far, primarily to avoid an overly large language. Java implements all the simple

C data types (integers, floating points, and characters) outside of the OO method,

but everything else is object-based. This is both a benefit, because it leads to

smaller, faster code, and a problem, because it means you can't subclass Java data

objects.</P>

<P>As mentioned earlier, Java is interpreted. When you develop Java applications,

you write the Java code as you would with any other language, then pass it through

a Java processor to produce the binary object file. The binary object file, called

a class file in Java, is not directly readable, but it can be transmitted faster

than the source code from which it was derived. When a Java-equipped browser receives

the class file, it runs the class file through the Java interpreter, which decodes

and executes the class file instructions. In one sense, Java is both a compiler and

an interpreter, in much the same way as many early languages that produced pseudo-code

requiring a runtime module were. The Java client that decodes the class file performs

a conversion from the generic instructions contained in the class file to machine-specific

instructions for the client's hardware and operating system.</P>

<P>Programming in the Java language takes a little while to become comfortable. The

code often seems like a hybrid of C and C++, and experience with both is handy. Because

Java is a fairly simple language, however, even nonprogrammers can pick it up with

a little practice. The most difficult aspects of learning Java for most people are

the need to understand object-oriented design and programming, and the somewhat awkward

syntax requirements (these are also familiar to C and C++ programmers). For example,

here's a simple Java program:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">    // HelloWorldApp.java

    class HelloWorldApp {

        public static void main (String args[]){

            system.out.println(&quot;Hello World!&quot;);

        }



    }

</FONT></PRE>

<P>This program (called <TT>HelloWorldApp</TT>) defines a single method called <TT>main</TT>

that returns nothing (<TT>void</TT>) and consists of a single Java instruction to

print the message <TT>Hello World!</TT> The first line in the code is a comment.

To compile this source code into a class file, you invoke the Java compiler with

the following command line:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">javac HelloWorldApp.java

</FONT></PRE>

<P>The compiler then grinds away and produces a class file (<TT>HelloWorldApp.class</TT>).

This class file can be run in a Java-equipped browser or from the command-line client

with the command<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">java HelloWorldApp

</FONT></PRE>

<P>which instructs Java to load and execute the <TT>.class</TT> file.</P>

<P>As you probably expect, Java is a lot more complicated than that in real life,

but if you have programmed before, you will see the similarities to other languages

(especially C and C++). The Java language is quite rich. You might need a couple

of weeks to wade through it, becoming familiar with its nuances as you go, but for

most people, Java is much more easily learned than other programming languages.

<H3 ALIGN="CENTER"><A NAME="Heading6<FONT COLOR="#000077">JavaScript and HTML</FONT></H3>

<P>You embed JavaScript commands inside HTML documents (see Chapter 53, &quot;HTML

Programming Basics,&quot; for more details on HTML) by enclosing them in the HTML

tags <TT>&lt;SCRIPT&gt;</TT> and <TT>&lt;/SCRIPT&gt;</TT>. The general syntax for

a JavaScript program inside an HTML document looks like this:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">&lt;SCRIPT language=&quot;JavaScript&quot;&gt;

    JavaScript statements

&lt;/SCRIPT&gt;

</FONT></PRE>

<P>The language option inside the <TT>&lt;SCRIPT&gt;</TT> tag is optional, but it

is a good idea to use it to make sure that the browser knows what is incoming. If

you want to load the JavaScript from a URL, you need to embed the URL in the <TT>&lt;SCRIPT&gt;</TT>

tag like this:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">&lt;SCRIPT language=&quot;JavaScript&quot; src=&quot;http://www.where.com&quot;&gt;

</FONT></PRE>

<P>If the JavaScript source is embedded in the HTML file, you can leave off the <TT>SRC</TT>

component. For example, here's a simple JavaScript applet in some HTML code (which

has been trimmed down to the essentials):<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">&lt;HTML&gt;

&lt;HEAD&gt;

...

&lt;SCRIPT language=&quot;JavaScript&quot;&gt;

   alert(&quot;Welcome to my Web site!&quot;);

&lt;/SCRIPT&gt;

&lt;/HEAD&gt;

&lt;/HTML&gt;

</FONT></PRE>

<P>The <TT>alert()</TT> function in JavaScript displays the message in a window with

an exclamation mark icon next to it. This is usually used to catch your attention

when you try to do something critical or illegal, or something that might cause problems.

JavaScript's functions, as you can see, are much like those in C. You can define

a complete function within a JavaScript file, similar to in a C program.</P>

<P>If you are calling a file with the JavaScript source in it from your HTML code,

the convention is to name the file with the <TT>.js</TT> filetype at the end (<TT>sample.js</TT>,

for example). This is because several applications, including MIME, already recognize

the <TT>.js</TT> filetype and can handle them properly.</P>

<P>We don't have the space here to go into details about JavaScript programming,

but many good books have been published on the subject, including Web Programming

with Java and Teach Yourself Java in 14 Days (both Sams.net, 1995).

<H3 ALIGN="CENTER"><A NAME="Heading7<FONT COLOR="#000077">Summary</FONT></H3>

<P>We've taken a quick look at Java and JavaScript, both of which are available for

Linux platforms in both server and client versions. Programming both Java and JavaScript

requires a bit of past programming experience, but if you've programmed before, you

can use them both to add a lot of features to Web pages and HTML documents. Give

them a try! After all, the software is free.









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