Java: Example - CircleMain.java

/**
 * Program:  CircleMain.java - Uses the CirclePanel component
 * @version 2002-00-00
 * @author Mickey Mouse
 */

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

/////////////////////////////////////////////////////////////////// CircleMain
class CircleMain extends JFrame{
    //================================================ instance variables
    CirclePanel drawing = new CirclePanel();               // Note 1
    
    //======================================================= constructor
    CircleMain() {
        //--- Get content pane, set layout, add components
        Container content = this.getContentPane();
        content.setLayout(new BorderLayout());
        
        content.add(drawing, BorderLayout.CENTER);        // Note 2
        
        this.setTitle("Circles");     
        this.pack();       // finalize the layout
    }//end constructor
    
    //============================================================== main
    public static void main(String[] args) {
        JFrame myWindow = new CircleMain();
        myWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myWindow.setVisible(true);
    }//end main
}//endclass CircleMain

Notes

1. The drawing is declared as an instance variable, which can be referenced in all methods. There is no need for that here since no one every references after it has been built. However, it is very common to interact with a drawing during execution of a program, and an instance variable is useful for later interaction.
2. Because the variable doesn't need to be an instance variable or indeed any variable, this could be rewritten as

   content.add(new CirclePanel(), BorderLayout.CENTER);