2EE, J2SE, and J2ME Platforms

When selecting the version of the Java SDK to download, you need to choose between the J2EE, J2SE, and J2ME platforms. Each of these three editions contains a different set of packages, or APIs, to develop Java programs. The first two characters, “J2,” stand for Java platform 2, which includes versions 1.2 and later of the Java SDK, and the fourth character, “E,” simply stands for Edition. The third character, “E,” “S,” or “M,” determines which libraries the download includes.

J2EE is the Enterprise Edition that contains the most libraries, and supports building enterprise- class server-side applications. J2SE is the Standard Edition that contains the tools to develop Java applets and applications. J2ME is the Micro Edition that contains a highly optimized run-time environment for deploying applications to consumer products such as cellular phones, Personal Digital Assistants (PDAs), and car navigation systems. If you are unsure which platform to choose, select the J2SE platform as it contains all of the tools you need to start learning Java.

Installing and Testing the Installation

Installing the Java SDK is as simple as running the downloaded executable file and following the installation prompts for both Windows and Unix platforms. For Unix platforms, first turn on the executable bit using the chmod +x command before running the downloaded executable file, and upon the completion of the installation, add the <INSTALL>/bin directory to your class path.

The class path is an environment variable named CLASSPATH where Java programs search for dependent objects. For Unix installations, the class path is typically set in a run code script such as .profile or .cshrc, and for Windows installations the class path is set using the System Properties within the Control Panel.

To test the installation, create a small Java program named Hello.java using your favorite text editor. Do not worry if you do not understand all of the pieces of this program at this time; this step is only to verify the installation of the Java SDK.

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

At a command prompt, compile the program into Java bytecodes using the Java compiler javac.

prompt> javac Hello.java

If there are no errors, the compiler writes a class file named Hello.class in the same directory as your source code file. Test your JVM by running the Java interpreter java using the new class file. Note that you do not include the .class extension when running the interpreter.

prompt> java Hello
hello, world

Running the class file on any JVM produces the same results. For example, if the class file was created on a Windows platform, you can perform a binary transfer of the file to a Unix platform and run the same java command using the same class file, and the program will run perfectly.

Checking the Java Virtual Machine Version

You can verify the version of the JVM installed by passing the –version parameter to the interpreter:

prompt> java –version
java version "1.3.1_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_01)
Java HotSpot(TM) Client VM (build 1.3.1_01, mixed mode)

There is no equivalent parameter for the Java compiler. 

Integrated Development Environments

As shown in the installation test, all that you need to develop and compile a Java program is a text editor and the Java compiler. Once you become comfortable developing Java applications, you will most likely want to use an Integrated Development Environment (IDE) to simplify program development. IDEs provide useful features such as syntax highlighting, quick access to different source files, wizards to create template code for common program types, an interactive debugger, and source code control.

IDEs will even point out errors in your program, such as not catching exceptions or mismatched braces, before you attempt to compile the program.