Getting Ready for Java Programming

Using GCJ

Red Hat Linux comes with GCJ-the GNU Compiler for the Java Programming Language. GCJ should be installed if you selected the Development Tools package during Red Hat Linux installation.

You can use the GCJ to compile and run simple Java programs. For example, to build a simple Java program that prints Hello, World!, follow these steps from a text console or a terminal window (I explain Java programming further in the latter part of this chapter):

1. Use a text editor such as vi or Emacs to create a file named HelloWorld.java and add the following lines to that file:

   public class HelloWorld {
       public static void main(String [] args) {
           System.out.println("Hello, World!");
       }
   }
   

2. Compile and create an executable program named HelloWorld by running gcj (that's the name of the GNU Java compiler's executable) with the following command line:

   gcj --main=HelloWorld -o HelloWorld HelloWorld.java

3. Run the program with the following command:

   ./HelloWorld

   The program should display the following line of output:

   Hello, World!

You can compile more complex Java programs using GCJ, but you run into a roadblock because GCJ does not yet include all the Java libraries.

Downloading and Installing Java 2 SDK

To develop Java applications, you need the Java 2 Software Development Kit. You can download the latest version of this kit by following the links at the following website:

http://java.sun.com/j2se/downloads.html

Secret

Sun has two Java development products-Java 2 Standard Edition (J2SE) and Java 2 Enterprise Edition (J2EE). The J2SE Software Development Kit (SDK)-also known as Java 2 SDK-includes the Java compiler, APIs, and other tools you need to develop Java applications and applets.

J2EE is meant for developing Java applications that run on Web servers. Such server-side applications include servlets-Java applications that run under the control of a Web server and perform computations or provide access to databases. J2EE includes the servlet APIs as well the APIs needed to support Web services and application servers. Web services refer to software systems with public interfaces defined in the Extensible Markup Language (XML). Other applications can discover information about a Web service by using well-defined protocols and then make use of whatever services the Web service provides. Application servers refer to the tools and utilities that can be used on a Web server to run Java servlets, maintain session and state information, access databases, perform database transactions, process new content such as XML, and generate dynamic HTML documents. J2EE relies on J2SE.

You should download the J2SE SDK because it has all the tools you need to develop Java applications. If you also plan to develop servlets and other server-side Java applications, you need the J2EE as well.

At its J2SE download website (http://java.sun.com/j2se/downloads.html), Sun Microsystems provides the J2SE SDK for the following systems:

• Linux (32-bit Intel processors)

• Linux (64-bit Itanium processors)

• Microsoft Windows 95/98/Me/200/NT/XP

• Sun Solaris SPARC-both 32 bit and 64 bit

• Sun Solaris x86

• 64-bit Windows for Itanium

  For Red Hat Linux, you should download the 32-bit Linux version of the J2SE SDK.

To download and install J2SE SDK for Linux, follow these steps from your Red Hat Linux system:

1. Make sure that your system is connected to the Internet. Start the Web browser and point the Web browser to the following URL:

   http://java.sun.com/j2se/downloads.html

   Click the link for the latest version of J2SE SDK.

2. Select the SDK download link for the item labeled 'Linux RPM in self-extracting file.' Read the license agreement and, if you agree, click Accept.

3. Click the link for the J2SE SDK file, and save it a convenient directory on your Red Hat Linux system (for example, /usr/local). The SDK is typically close to 40 MB in size, so the download can take a while if you have a slow dial-up Internet connection. After the download is completed, you end up with a file whose name ends in .rpm.bin. This file is a shell script that you must execute to extract the RPM file for J2SE SDK.

4. From a terminal window, change directory to where you saved the J2SE SDK file and type the following command to make that file executable:

   chmod +x j2sdk*.bin

5. Execute the file by typing the following command:

   ./j2sdk*.bin

   This displays the license agreement a page at a time. After reading each page, press Enter to move forward. After the last page, you see the following prompt:

   Do you agree to the above license terms? [yes or no]

6. Type yes, and press Enter. This starts the unpacking and ends with an RPM file.

7. Install the RPM file with the following command:

   rpm -ivh j2sdk*.rpm

   This command installs the Java 2 SDK in the /usr/java directory under a subdirectory whose name depends on the version you are installing. For example, when I installed J2SE SDK version 1.4.1, everything is installed in the following directory:

   /usr/java/j2sdk1.4.1_01

Set the PATH environment variable to include the directory where the Java 2 SDK binaries are located. For example, if you install the Java 2 SDK in the /usr/java/j2sdk1.4.1_01 directory, add the following lines at the end of the /etc/profile file:

PATH=$PATH:/usr/java/j2sdk1.4.1_01/bin

Taking Stock of the Java 2 SDK

The Java 2 SDK comes with a number of files organized into several directories. The top-level directory contains the files and subdirectories shown in Table 26-1.

The tools you'll use most are in the bin subdirectory. Here are some of the tools that you'll use when creating Java applications:

• appletviewer-Java applet viewer program to load and run Java applets that are designed to be embedded in an HTML document (you can also load applets in a Java-capable browser by embedding the applet in an HTML document using the <applet> tag).

• java-Java interpreter that runs standalone Java programs specified by a class name (it expects to find the class in a file with same name as the class, but with a .class extension). Standalone Java applications are Java classes with a main method (in Java, functions are called methods).

• javac-Java compiler that converts Java source code into 'byte code' (that's the instruction set of the Java Virtual Machine). The compiler takes source code in a .java file and generates compiled code in a .class file.

• jdb-Java debugger with a command syntax like that of the Linux gdb debuggers.

• javadoc-Java API documentation generator that extracts documentation from source files with special comments that begin with /** and ends with a */.

• javah-Java C file generator that generates C .h and .c files for a Java class. Such C files are needed to implement the native methods that make the APIs work on a particular operating system.

• javap-Java class file disassembler that takes a class name and prints a human-readable description of the class.

Of these programs, the three that you'll use most are the following:

• You'll use javac a lot because that's the Java compiler program. The javac program converts a Java source file (you'll see some examples in the next section) into a binary file that contains Java byte code-the instruction set of the Java Virtual Machine.

• If you write a standalone Java application-one that can run without being embedded in a Web browser, you'll use the Java interpreter program, java, to run the application.

• If you write Java applets-miniapplications that run inside a Web browser, you'll use the appletviewer program to test the applets. Of course, you can also test applets using a Java-capable Web browser such as Mozilla (after you install the Java plug-in).