It is traditional in introductory programming courses to write and explain a simple program that prints the text "Hello World" on the computer screen.

This module continues that tradition.

This module introduces you to Java programming by presenting and discussing a traditional Hello World program.

Two approaches

Java programs can be written and executed in several different ways, including the following:

It is also possible in many cases to write applets, which can be run in a stand-alone mode from the command line, or can be run under control of a Java-capable browser. An example of such an applet will be presented in a future module.

Applets vs. applications

Programming an "application" in Java is significantly different from programming an "applet." Applets are designed to be downloaded and executed on-line under control of a browser.

Restrictions on applets

Their functionality of an applet is usually restricted in an attempt to prevent downloaded applets from damaging your computer or your data. No such restrictions apply to the functionality of a Java application.

Class definitions

All Java programs consist of one or more class definitions. In this course, I will often refer to the primary class definition for a Java application as the controlling class .

The main method

A stand-alone Java application requires a method named main in its controlling class .

An Applet does not require a main method. The reason that a Java Applet does not require a main method will be explained in a future module.

Getting started

Image 1 shows the steps for compiling and running a Java application.

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:

java hello1

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 instantiate an 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.)

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.

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 it works, 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.

    System.out.println("Hello World");

Listing 4 ends the main method and also ends the class definition for the class named hello1 .

  }//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

The overall skeleton of any Java program consists of one or more class definitions.

All methods and variables must be defined inside a class definition. There can be no freestanding methods or global variables.

File names and extensions

The name of the controlling class should be the same as the name of the source file that contains it.

Files containing source code in Java have an extension of java .

The main method

The controlling class definition for an application must contain the main method.

The primary class file

The file produced by compiling the file containing the controlling class has the same name as the controlling class, and has an extension of class .

Many class files may be produced

The java compiler produces a separate file for every class definition contained in an application or applet, even if two or more class definitions are contained in the same source file.

Thus, the compilation of a large application can produce many different class files.

What are jar files?

A feature known as a jar file can be used to consolidate those class files into a single file for more compact storage, distribution, and transmission. Such a file has an extension of jar .

The main method is static

The controlling class for a Java application must contain a static method named main .

When you run the application using the JVM, you specify the name of the class file that you want to run.

The JVM then calls the main method defined in the class file having that name. This is possible because a class method can be called without a requirement to instantiate an object of the class.

The main method defined in that class definition controls the flow of the program.

I encourage you to copy the code from Listing 5 . Compile the code and execute it. Experiment with the code, making changes, and observing the results of your changes. Make certain that you can explain why your changes behave as they do.

This section contains a variety of miscellaneous information.

A complete listing of the program discussed in this module is provided in Listing 5 .

/*File hello1.java Copyright 1997, R.G.Baldwin
This is a Java application program .

When compiled, this program produces the class named:

hello1.class

When the Java interpreter is called upon the application's
controlling class using the following statement at the
command line:

java hello1

the interpreter starts the program by calling the main
method defined in the controlling class. The main method is
a class method which can be called without the requirement
to instantiate an object of the class.

The program displays the following words on the screen:

Hello World

*********************************************************/
class hello1 { //define the controlling class
  //define main method
  public static void main(String[] args){
    //display text string
    System.out.println("Hello World");
  }//end main
}//End hello1 class.

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