Java Concepts for iSeries Programmers

by Kevin Vandever

Java Basic Terms

Before we get too deeply into it, I will define some very general, Java-related terms that you will hear and see over and over again as you delve into the Java world:

Object-Oriented Programming Concepts

Programming languages are programming languages, right? Once you learn the syntax of a new language, all the rest of the rules apply, such as looping, conditioning, and mathematical operations. The same is true for Java. It is a programming language that does the same types of things that other programming languages do, so I am not going to cover that stuff here. However, there is something that makes Java different than most other programming languages, and that is object-oriented programming. It is very important that you understand OO concepts, if you really want to maximize Java's capabilities. With that said, I am going provide some more terms (sorry!) and a little explanation of each:

I'll Have Some Java, Please.

OK, now that you're armed with terminology, you're ready to code. First, however, you must install Java on your Windows, Unix, or Linux platform. To do this, check out Sun's download site to select the version you want to install and the platform you want to install to. For detailed installation instructions, visit the installation site for your particular platform. For example, the windows install instructions can be found at http://java.sun.com/j2se/1.3/install-windows.html. Chances are that Java is already installed on the iSeries, but if it's not, you can install it yourself by copying the JAR files to the Integrated File System on your iSeries, or by loading the appropriate IBM CD and installing from there.

Let's Code a Java App

Now, I know many of you have seen the "Hello World" application, used to demonstrate how Java works. That particular Java application simply prints out "Hello World" to the screen. Well, I'm not going to do that. I like to be different, daring. Instead, I am going to show you how to code a more appropriate Java application, one that prints "Happy Valentine's Day" instead. Actually, I am going to show you that Java code, but I will also include a Java program, available for download, that was published in our first issue and used to call an RPG program. The following is the Java code necessary to print "Happy Valentine's Day":

/**
 * The ValentineApp class displays 
 * "Happy Valentine's Day!" to the standard output.
 */
class ValentineApp {
    public static void main(String[] args) {
        System.out.println("Happy Valentine's Day!"); //Display the string.
    }
}

If you look closely at the comments, you'll see they are shown two ways. There are actually three ways to show comments in Java. The first is /** text */. This way allows you to block a bunch of text together and show it as comments. However, this method carries another advantage. When the javadoc program--which is a Java application included with every Java Development Kit, and which can be run from any DOS command line to create instant documentation of your Java code--is run against this class, it will take any comments signified by /** text */ and turn them into HTML java documentation. If you simply want to comment a block of text without creating HTML documentation from it, you can use the second comment type, /* text */. The third way to comment text is used for short or single line comments. In that case, you just use the double slash (//) followed by the text.

The first line of real Java code is the class definition. A class is always defined by the class name and then variables and methods enclosed within curly braces:

class name  {
 variables and methods 
}

The next line is the main method of the class. Every class must contain the main method, which is used to decide what the Java application will do. The main method in Java is much like your mainline code in an RPG program. You can have logic in your main method, as I do in mine, or simply call other methods from the main method. There is stuff to the left and to the right of main method identifier, main. To the left, you will see public static void. Public means that this class can be called from any object. You can also use private, which means only this class can call the method; protected, which means the class and subclass, as well as any class within the package, can call the method; and package, which says that the method can be called from the class and from within the package, but not from the subclass. There's a little more to it than that, but for now understand that there are four different entries that can control method access.

Static defines the method as a class method. Class methods and variables have memory allocated for them once, when the class is first called, no matter how many instances of that class have been created. This is much like ILE service programs. The memory is allocated once, and the service program is bound by reference to those programs that need it. If you don't use static, the method would be an instance method. Memory for instance methods and variables is allocated for every instance of that class. This is how modules are sometimes used in ILE. They are bound by copy, meaning that every program that binds-by-copy to a module will make a copy of that module at compile time.

Void is the last entry to the left of main. Void means that nothing is going to be returned by this method call. If you wanted to return a value from the method, you would define the return variable instead of using void.

The stuff within the parentheses, to the right of main, is the parameters that can be passed into the main method. In this case, the main method accepts a string array (as signified by the brackets) called args. This means that you can pass multiple command line arguments, separated by a space, and they will get stored in the string array, args. In this case, we do nothing with command line argument, so they are ignored if passed.

The last thing to discuss in my Java application is the actual print to standard output. The form of the statement is class.variable.method (parameter to pass to method). So, we are calling the println method, passing the string we want to print, using the out variable, which defaults to standard output, from the System class.

Compiling and Running Java

There are a couple of things to keep in mind before compiling and running your application. If you are planning to run your app in a Windows environment, you must first tell the operating system, and possibly your development environment, how to find your Java environment. I coded and compiled my Java application using CODE/400 on Windows 2000. Before I could successfully do this, however, I had to set my PATH environment variable, to tell CODE/400 where to find my Java environment. If you use WebSphere development tools, you can add the following to your PATH environment variable:

SET PATH=%PATH%;C:/WDT400/Studion35/bin

Your setup might be different, or you might not use CODE/400 as your development environment, so make sure to find where your javac command is stored before modifying your PATH environment variable. The next thing you'll want to do is set your CLASSPATH environment variable, to tell it where your toolbox JAR files, or any other classes that are not part of the JDK or JRE, reside. In the Valentine example, setting the PATH is enough, because it only uses core Java classes. However, I've also included a Java application that is used to call an RPG program. (You can find the details about that program in "Calling a Program Using the iSeries Toolbox for Java".) To successfully compile and run that application, you will need JAR files from the Java tookit. Again, if you use the WebSphere development tools, specifically CODE/400, those JAR files will reside in C:\WDT400\java. So the entry in my CLASSPATH environment variable would be the following:

SET CLASSPATH=%CLASSPATH%;C:\WDT400\java\jt400.jar;C:\WDT400\java\data400.jar

On the iSeries, the CLASSPATH is configured automatically when you install the Java Development Kit and the iSeries Toolbox licensed programs. If you have other Java classes that you want to access, or you've copied the toolbox or JDK to your iSeries instead of installing from CD, you can modify the CLASSPATH either in the Qshell environment (STRQSH) or from a command line using the Work with Environment Variables (WRKENVVAR) command, or in a CL program using the ADDENVVAR command.

Compiling and Running the ValentineApp Class

To compile your Java application from a DOS command line, open a DOS window and type javac, followed by the name of the Java source code, in this case ValentineApp.java:

javac ValentineApp.java

Case is important; you must type it exactly as it is named, followed by the .java extension.

If you use CODE/400 to compile this java app, click on the Code/400 ACTIONS menu item, then select Compile Local. You can prompt it or not; it doesn't matter for this example. When the code has compiled cleanly, click on ACTIONS, then select Run Local, and you'll see the output from this program.

You can also compile and run this program on the iSeries. To do so, you must be in the Qshell environment (STRQSH). Once there, type javac from the Qshell command entry screen. To run your application, simply type java ValentineApp (don't include the .java extension).

After you've successfully compiled your application, you're ready to go; just type java ValentineApp, and you should receive your valentine greeting.

Tip of the Iceberg

I've given you a lot of information, but it's only the beginning. As you can see, it's pretty easy to get an application up and running, but to take advantage of a full-fledged OO application, you must really understand OO concepts. You must also learn about constructors and dig a little deeper into class variables and methods, versus instance variables and methods. And when all that's done, you can concentrate on Java syntax, because that's the easy part.

Kevin Vandever is a lead IT engineer for Boise Cascade Office Products in Itasca, Illinois, and co-editor of Midrange Programmer, OS/400 Edition. He can be reached at kvandever@itjungle.com.