The Java Lesson 2: Anatomy of a simple Java program

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A sample Java program

The following program is a slight variation of the program that is part of the original Test project. When executed, it will display the message "Hello World!" on the system output device (the computer console):

/**
* This class defines a program that displays a message on the console.
*
* Written by: Jon Huhtala
*/

public class App {

// This required method is the starting point for processing.

public static void main(String[] args) {
System.out.println("Hello World!"); // Display the message.
}
}

Testing the sample program

Assuming you have a Test project and have already opened a Notepad window and a Command Prompt window, the procedure to test this sample program is as follows:

1. "Copy" the source statements (shown in blue above).

2. "Paste" the source statements into your Notepad window being sure to replace the entire contents of the window if it contained previous code.

3. Save the source file under the name "App.java" in your Test project.

4. Compile the "App.java" source file by entering the following command in the Command Prompt window:

javac App.java

5. Run the "App.class" bytecode file by entering the following command in the Command Prompt:

java App

Program analysis

Studying this sample program line-by-line will start you on the path to learning Java. The sections that follow will help you analyze the sample program. For now, it is not important that you understand everything completely. Through constant usage, you will eventually know what it all means.

Comments in Java

• Are for documentation purposes only. The compiler ignores them when generating bytecode.

• Three comment techniques are supported

1. Javadoc. Begin with /** and end with */. Everything between is treated as a comment.

2. Single-line. Begin with // and automatically continue to the end of the line.

3. Multi-line. Begin with /* and end with */. Everything between is treated as a comment.

• Comments make programs easier to maintain. Good programmers document well enough that a person not familiar with the program can read the comments to know what each section of code is doing.

The program's class

• Every Java program must be defined as a class. The statement

public class App

is the class header. It declares that a class named App is to be defined and that it is to be publicly available for access from other places. In order for the Java Virtual Machine to find and use the program's class, always make it public. Other access modifiers (private, protected, and default or package access) will be presented later.

• The words public and class are Java keywords and have special meaning. The class name is up to the programmer but must adhere to the restrictions on Java identifiers to be covered in the next lesson.

• The name of the class must be the same as the name of the source file. If the class name is App, it must reside in a file named App.java

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