Java Applications

Every Java application contains a class that defines a method called main(). The name of this class is the name that you use as the argument to the Java interpreter when you run the application. You can call the class whatever you want, but the method which is executed first in an application is always called main(). When you run your Java application, the method main() will typically cause methods belonging to other classes to be executed, but the simplest possible Java application program consists of one class containing just the method main(). As you will see below, the main() method has a particular fixed form, and if it is not of the required form, it will not be recognized by the Java interpreter as the method where execution starts.

You can see how this works by taking a look at just such a Java program. You need to enter the program code using your favorite plaintext editor, or if you have a Java development system with an editor, you can enter the code for the example using that. When you have entered the code, save the file with the same name as that used for the class and with the extension .java. For this example the file name will be OurFirstProgram.java. The program consists of a definition for a class I have called OurFirstProgram. The class definition contains only one method, the method main(). The first line of the definition for the method main() is always of the form:

public static void main(String[] args)

The code for the method appears between the pair of curly braces. This version of the method has only one executable statement:

System.out.println(“Krakatoa, EAST of Java??”);

So what does this statement do? Let’s work through it from left to right:

[1] System is the name of a standard class that contains objects that encapsulate the standard I/O devices for your system - the keyboard for command-line input and command-line output to the display. It is contained in the package java.lang, so it is always accessible just by using the simple class name System.

[2] The object out represents the standard output stream - the command line on your display screen - and is a data member of the class System. The member, out, is a special kind of member of the System class. Like the method main() in our OurFirstProgram class, it is static. This means that out exists even though there are no objects of type System. Using the class name, System, separated from the member name out by a period - System.out - references the out member.

[3] The bit at the rightmost end of the statement, println(“Krakatoa, EAST of Java??”),calls the println() method that belongs to the object out, and that outputs the text string that appears between the parentheses to your display. This demonstrates one way in which you can call a class method - by using the object name followed by the method name, with a period separating them. The stuff between the parentheses following the name of a method is information that is passed to the method when it is executed. As we said, for println() it is the text we want to output to thebcommand line. You can compile this program using the JDK compiler with the command

javac OurFirstProgram.java

or with the -classpath option specified:

javac –classpath . OurFirstProgram.java

If it didn’t compile, there’s something wrong somewhere. Here’s a checklist of possible sources of the problem:

[4] You forgot to include the path to the jdk1.5.0\bin directory in your PATH, or maybe you did not specify the path correctly. This will result in your operating system not being able to find the javac compiler that is in that directory.

[5] You made an error typing in the program code. Remember Java is case-sensitive, so OurfirstProgram is not the same as OurFirstProgram, and of course, there must be no spaces in the class name. If the compiler discovers an error, it will usually identify the line number in the code where the error was found. In general, watch out for confusing zero, 0, with a small letter o,or the digit one, 1, with the small letter l. All characters such as periods, commas, and semicolons in the code are essential and must be in the right place. Parentheses, (), curly braces, {}, and square brackets, [], always come in matching pairs and are not interchangeable.

[6] The source file name must match the class name exactly. The slightest difference will result in an

error. It must have the extension .java. Once you have compiled the program successfully, you can execute it with the command:

java –ea OurFirstProgram

The -ea option is not strictly necessary since this program does not use assertions, but if you get used to putting it in, you won’t forget it when it is necessary. If you need the -classpath option specified:

java –ea –classpath . OurFirstProgram

Assuming the source file compiled correctly, and the jdk1.5.0\bin directory is defined in your path, the most common reason for the program failing to execute is a typographical error in the class name, OurFirstProgram. The second most common reason is writing the file name, OurFirstProgram.class, in the command, whereas it should be just the class name, OurFirstProgram.