GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 338

Let's review the important points introduced in this section:
￿ The preceding program has exactly one class: RectangleProgramOne .
￿ The

contains the constructor

class RectangleProgramOne

RectangleProgramOne and the main method.
￿ You created the new class RectangleProgramOne by extending the
existing class, JFrame . Therefore, JFrame is the superclass of
RectangleProgramOne , and RectangleProgramOne is a subclass of
JFrame .
￿ Whenever there is a superclass-subclass relationship, the subclass inherits all
the data members and methods of the superclass. The methods setTitle ,
setSize , setVisible ,and setDefaultCloseOperation are methods
of the class JFrame , and these methods can be inherited by its subclasses.
The next few sections describe how to create GUI labels, text fields, and buttons, which
can all be placed in the content pane of a window. Before you can place GUI compo-
nents in the content pane, you must learn how to access the content pane.

Getting Access to the Content Pane
If you can visualize JFrame as a window, think of the content pane as the inner area of the
window (below the title bar and inside the border). The class JFrame has the method
getContentPane that you can use to access the content pane of the window. However, the
class JFrame does not have the necessary tools to manage the components of the content
pane. The components of the content pane are managed by declaring a reference variable of
the Container type and then using the method getContentPane , as shown next.
Consider the following statements:

Container pane; //Line 1
pane = getContentPane(); //Line 2
The statement in Line 1 declares pane to be a reference variable of the Container type.
The statement in Line 2 gets the content pane of the window as a container, that is, the
reference variable pane now points to the content pane. You can now access the content
pane to add GUI components to it by using the reference variable pane .
The statements in Lines 1 and 2 can be combined into one statement:
Container pane = getContentPane(); //Line 3

If you look back at Figure 6-2, you will see that the labels, text fields, and buttons are
arranged in five rows and two columns. To control the placement of GUI components in
the content pane, you set the layout of the content pane. The layout used in Figure 6-2 is
called the grid layout. The class Container provides the method setLayout ,as
described in Table 6-2, to set the layout of the content pane. To add components such
as labels and text fields to the content pane, you use the method add of the class
Container , which is also described in Table 6-2.