Java graphics - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 166

Swing containers - JFrame , JApplet , JDialog , JWindow - are conceptually
constructed of several such overlapping panes . The panes organize the display
of the components, the interception of events, the z-ordering of components, and
various other tasks. If you ever want to build a custom component, it is necessary
to understand the details of the panes system. However, for most GUI building
you only need to deal with the content pane.
We note that for J2SE 5.0, calling getContentPane() is no longer
required for Swing components. This was accomplished by enhancing the add() ,
remove() , and setLayout() methods so that they forward all calls to the
content pane automatically for the JFrame , JDialog , JWindow , JApplet
and JInternalFrame components. In the above example, you would thus only
need to invoke add (button) to place the button on the applet panel. We con-
tinue to use the content pane explicitly in this topic for those readers who have
not yet upgraded their Java environment to the 5.0 platform.

6.5.1 JPanel and JButton
We typically build an interface with one or more instances of JPanel .A JPanel
is a container that holds other components. The following applet creates a subclass
of JPanel called ActionButtonsPanel that holds two JButton objects. An
instance of this JPanel is then added to the applet's content pane. Figure 6.3
shows the resulting display.

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

/** Demonstrate the Creation of a JPanel subclass. **/
public class ButtonsPanelApplet extends JApplet
{

public void init () {
Container content - pane = getContentPane ();
// First create a panel of buttons
ActionButtonsPanel buttons - panel =
new ActionButtonsPanel ();

content - pane.add (buttons - panel);

}
} // class ButtonsPanelApplet

// JPanel subclass with two buttons
class ActionButtonsPanel extends JPanel
{

Figure 6.3 The
program Buttons-
PanelApplet puts
two JButton
components on a
JPanel .

ActionButtonsPanel () {
// Create two buttons