Java Chapter 2 - Hello World

2.1 First Java Program

Below is the source code for a "Hello World" Java program. You can use any text editor to create it. Save the file as hello.java

hello.java

// This is a Java Hello World Program

public class hello
{
   public static void main (String[] args)
   {
      System.out.println("Hello World!");
   }
}

Output
Hello World!

Compiling and Executing

Open a command prompt and go to the directory where you saved your hello.java file. If you saved this file in the Java directory under the E: prompt, then do the following:

E:\Java> javac hello.java // this creates an intermediate file named hello.class
E:\Java> java hello // this executes class file using the Java run-time

Important: The file name you use must be the same as the class name (hello).

Analyzing Hello World

All Java programs must be part of a class. The above program is within the class named hello.

The Java application must have a main() method which is the entry point. The content between the parenthesis, (String[] args), allows you to pass command-line values to the application. The word static is used to insure that the method will be accessible even though no instances of the class exist. The word void means that the main method does not return any value.

2.2 Printing Text

You can print text to the console screen with System.out.print or System.out.println. The latter puts a newline at the end of the output. The table below shows special characters (escape sequences) used in printing text.

Code	Output	Comments
System.out.print("First Line\nSecond Line");	First Line Second Line	\n is a newline
System.out.print("1\t2\t3");	1 2 3	\t is a tab
System.out.print("C:\\");	C:\	You need to backslashes to print one
System.out.print("\"To be or not to be\"");	"To be or not to be"	Since quotations are used to contain the text, you use \" to print a quotation

2.3 Comments

Comments allow you to write messages (document) your program. Comments are not compiled and do not affect how your program executes. There are two types of comments that can be used in Java programs:

// Line comments can be added at the end of any line

/* Block comments are good for
commenting long passages or for
commenting out a section of
code for testing. */

Exercises

1. Create a program named face.java that prints the following:

/"""""\
| o o |
| & |
\ --- /