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The class file
Compiled Java programs are stored in "bytecode" form in a file with an extension of .class where the name of the file is the same as the name of the controlling class (or other class) in the program.
The main method is static
The main method in the controlling class of an application must be static , which results in main being a class method.
Class methods can be called without a requirement to instantiate an object of the class.
When a Java application is started, the Java Virtual Machine or JVM (an executable file named java.exe) finds and calls the main method in the class whose name matches the name of the class file specified on the command line.
Running an application
For example, to start the JVM and run a Java application named hello1 , a command such as the following must be executed at the operating system prompt:
null
This command instructs the operating system to start the JVM, and then instructs the JVM to find and execute the java application stored in the file named hello1.class . (Note that the .class extension is not included in the command .)
This sample program is a Java application named hello1.java .
When compiled, it produces a class file named hello1.class .
When the program is run, the JVM calls the main method defined in the controlling class .
The main method is a class method.
Class methods can be called without a requirement to instantiatean object of the class.
The program displays the following words on the screen:
Hello World
I will explain this program code in fragments. A complete listing of the program is provided in Listing 5 .
The code fragment in Listing 1 shows the first line of the class definition for the controlling class named hello1 . (I will discuss class definitions in detail in a future module.)
| Listing 1 . Beginning of the class named hello1. |
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class hello1 { //define the controlling class |
The code fragment in Listing 2 begins the definition of the main method. I will also discuss method definitions in detail in a future module.
| Listing 2 . Beginning of the main method. |
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public static void main(String[] args){ |
The fragment in Listing 3 causes the string Hello World to be displayed on the command-line screen.
The statement in Listing 3 is an extremely powerful statement from an object-oriented programming viewpoint. When you understand how itworks, you will be well on your way to understanding the Java version of Object-Oriented Programming (OOP).
I will discuss this statement in more detail later in a future module.
| Listing 3 . Display the string Hello World. |
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System.out.println("Hello World"); |
Listing 4 ends the main method and also ends the class definition for the class named hello1 .
| Listing 4 . End of the class named hello1. |
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}//end main
}//End hello1 class |
The complete program listing
As mentioned earlier, a complete listing of the program is provided in Listing 5 near the end of the module.
This program illustrates several general aspects of Java programming.
Overall skeleton of java program
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