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Java Tutorial: Printing a String

20050123,20071221

Here's our first program, printing a string.

/*
  print out a string
*/

class PrintMeApp {
  public static void main(String[] args) {
    System.out.println("O toiler through the glooms of night in peril and in pain!");
    }
}

Save it to a file and name it “PrintMeApp.java”. Then compile it by running “javac PrintMeApp.java”. That will create a file named “PrintMeApp.class”. It is the compiled code. To run the program, type “java PrintMeApp” on the console. Then it should print the string.

If you have problems getting this to work, see: Java Tutorial↗.

Note that in Java, lines enclosed between “/*” and “*/” are ignored. Also, text to the rigth of “//” is also ignored.

Some Syntax Details

Note the “class”. A Class is like a boxed set of functions and data.

The PrintMeApp is the class's name. the “public” specifies that the class can be called by other classes/functions.

The “static” means that this class needs not be called without having to creat a instance of it first.

The “main” is a function of this class. Functions defined inside a class is called the class's “method”. “main” is the primary method of any program. That is, there is one “main” method in any java program.

The things inside the parenthesis declare that “main” takes several argument of strings.

Do not worry about these details. We'll cover them each later. If you are not clear about the general concepts of Object Oriented Programing, you should now read: What are OOP's Jargons and Complexities?.

General Structure of Java Code

In general, ALL java source code have the following structure:

/*
  comment
*/

// Declaring this file to be a package.
// All java source code belongs to some package.
// Only trivial, testing code does not declare a packge.
// When a source file does not declare a package, it is said be a “unnamed” package
package mary.games.sounds;

// load packages you need
import java.awt.Cursor;
import ...
...


// a bunch of class definitions
class MyClass1 {
    ...
}

class MyClass2 {
    ...
}

... // more classes


// The main class.
// The file name must be the same as this class name
class MyMainClass {
    public static void main(String[] args) {
        //body...
    }
}

Note that one of the class's name must be the same as the file name, else it won't compile. For example, if your file name is “MyMainClass.java”, then one of your class must be named “MyMainClass”.

For now, you don't need to understand excatly how package works, or how exactly those keywords like “public”, “static”, “void”, “main” ... means. We'll cover them in this book. For now, just remember that all java source code have the above structure, and one of the class must have the same name as the file's name.

Note: the above example is meant to give a general outline of the form of a source file. It omits some details. For example, you can also have one or more “interface” definitions in a source file, similar to the class definitions above. Many class definitions will contain variables and methods and constructers. Also, a source file usually do not have the “main” method.

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Page created: 2005-01.
© 2005 by Xah Lee.
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