Learning Java, 4th Edition

HelloJava

In the tradition of introductory programming texts, we will begin with Java’s equivalent of the archetypal “Hello World” application, HelloJava.

We’ll end up taking four passes at this example before we’re done (HelloJava, HelloJava2, etc.), adding features and introducing new concepts along the way. But let’s start with the minimalist version:

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

This five-line program declares a class called HelloJava and a method called main() . It uses a predefined method called println() to write some text as output. This is a command-line program, which means that it runs in a shell or DOS window and prints its output there. That’s a bit old-school for our taste, so before we go any further, we’re going to give HelloJava a graphical user interface (GUI). Don’t worry about the code yet; just follow along with the progression here, and we’ll come back for explanations in a moment.

In place of the line containing the println() method, we’re going to use a JFrame object to put a window on the screen. We can start by replacing the println line with the following three lines:

    JFrame frame = new JFrame( "Hello, Java!" );
    frame.setSize( 300, 300 );
    frame.setVisible( true );

This snippet creates a JFrame object with the title “Hello, Java!” The JFrame is a graphical window. To display it, we simply configure its size on the screen using the setSize() method and make it visible by calling the ...