Swing I - Absolute Java - page 972

in a JFrame . Thus, one of the main functions of JPanel objects is to subdivide a
JFrame (or other container) into different areas.
For example, when you use a BorderLayout manager, you can place components
in each of the five locations BorderLayout.NORTH , BorderLayout.SOUTH ,
BorderLayout.EAST , BorderLayout.WEST, and BorderLayout.CENTER. But what if
you want to put two components at the bottom of the screen in the BorderLayout.SOUTH
position? To do this, you would put the two components in a panel and then place the
panel in the BorderLayout.SOUTH position.
You can give different layout managers to a JFrame and to each panel in the JFrame .
Because you can add panels to other panels and each panel can have its own layout manager,
this enables you to produce almost any kind of overall layout of the items in your GUI.
For example, if you want to place two buttons at the bottom of your JFrame GUI,
you might add the following to the constructor of your JFrame GUI:
setLayout( new BorderLayout());
JPanel buttonPanel = new JPanel();
buttonPanel.setLayout( new FlowLayout());
JButton firstButton = new JButton("One");
buttonPanel.add(firstButton);
JButton secondButton = new JButton("Two");
buttonPanel.add(secondButton);
add(buttonPanel, BorderLayout.SOUTH);
The next Programming Example makes use of panels within panels.
EXAMPLE: A Tricolor Built with Panels
When first run, the GUI defined in Display 17.11 looks as shown in the first view.
The entire background is light gray, and there are three buttons at the bottom of the
GUI labeled "Green" , "White" , and "Gray" . If you click any one of the buttons, a
vertical stripe with the color written on the button appears. You can click the buttons
in any order. In the last three views in Display 17.11, we show what happens if you
click the buttons in left-to-right order.
The green, white, and gray stripes are the JPanes named greenPanel ,
whitePanel , and grayPanel. At first the panels are not visible because they are
all light gray, so no borders are visible. When you click a button, the corresponding
panel changes color and so is clearly visible.
Notice how the action listeners are set up. Each button registers the this
parameter as a listener, as in the following line:
redButton.addActionListener( this );
Because this line appears inside of the constructor for the class PanelDemo, the this
parameter refers to PanelDemo, which is the entire GUI. Thus, the entire JFrame
(continued)