Let's now examine the
|Lesson 8 || Closer look at HelloWorld|
|Objective|| Understand the elements of the HelloWorld program|
HelloWorld program in more detail. This program
consists of a single class named
HelloWorld that contains a single method named
main(). Following is the source code for the
program. Move your mouse cursor over the source code to see a more detailed description of each element.
main() method is a special method in Java, as it serves as the starting point for a Java program. All Java programs must have a
class with a
main() method. When you run a program with the Java interpreter, you specify the name of the class that contains your
main() method. The interpreter then starts running the program with the instructions in the
main() method of the
All of the Java programs you will be writing in this course will have a
main() method that begins with:
public static void main(String args)
This portion of the
main() method is referred to as the method header and is followed by a pair of curly brackets "
enclosing the instructions for the method. You're probably wondering what all of the strange and symbols words such as
args are for. Move your mouse cursor over the following source code to see a more
detailed description of each part of the
main() method header.
An in-depth understanding of each part of the
main() method header isn't critical at this point. What is critical is that the
main() method of your programs contain these parts. In more general terms, the method header of
HelloWorld indicates that the method can be accessed by any other class; it is associated with the class
HelloWorld; it does
not return a value; and it is passed as a list of text strings.
Displaying a message
When you run the
HelloWorld program, the message "Hello World!" is displayed in your command prompt window. This is accomplished
main() method by the statement:.
It's difficult to describe in detail how this statement works without using a fair amount of object-oriented programming terminology, but
essentially what is happening is that some existing instructions that come with the Java 2 SDK are being used to display the message. More
specifically, a class that comes with the Java 2 SDK named
System has a variable named
out that refers to an object that has
a method named
println(). When the first statement in the
main() method is executed (i.e., when the first instruction is
carried out), the
println() method is executed (i.e., the instructions in the
println() method are carried out) to display
the message "Hello World!"
In the next lesson you will learn how comments and indentation make programs easier to understand.