Introducing Swing - Java: A Beginner's Guide - page 735

add( ) method has several versions. The general form of the one used by the program is
shown here:

Component add(Component comp )

By default, the content pane associated with a JFrame uses a border layout. This version
of add( ) adds the component (in this case, a label) to the center location. Other versions of
add( ) enable you to specify one of the border regions. When a component is added to the
center, its size is automatically adjusted to fit the size of the center.
The last statement in the SwingDemo constructor causes the window to become visible.

Ask the Expert

Q : I have seen Swing programs that use a method called getContentPane( ) when

adding a component to the content pane. What is this method and do I need to
use it?

A : This question brings up an important historical point. Prior to JDK 5, when adding a
component to the content pane, you could not invoke the add( ) method directly on a
JFrame instance. Instead, you needed to explicitly call add( ) on the content pane of
the JFrame object. The content pane can be obtained by calling getContentPane( )
on a JFrame instance. The getContentPane( ) method is shown here:

Container getContentPane( )

It returns a Container reference to the content pane. The add( ) method was then called
on that reference to add a component to a content pane. Thus, in the past, you had to use
the following statement to add jlab to jfrm :

Here, getContentPane( ) first obtains a reference to the content pane, and then add( )
adds the component to the container linked to this pane. This same procedure was also re-
quired to invoke remove( ) to remove a component and setLayout( ) to set the layout
manager for the content pane. You will see explicit calls to getContentPane( ) frequently