Graphical user interfaces - Data Structures and Problem Solving Using Java - page 933

defined in JComponent ). The setBackground and setFont methods are used to
change the background color and font associated with a Component . They
require a Color and Font object, respectively. Finally, the show method makes a
component visible. Its typical use is for a JFrame .

B.2.2 Container

In the AWT, a Container is the abstract superclass representing all components
that can hold other components. An example of an AWT Container is the
Window class, which represents a top-level window. As the inheritance hierar-
chy shows, a Container IS-A Component . A particular instance of a Container
object will store a collection of Component s as well as other Container s.
The container has a useful helper object called a LayoutManager , which is a
class that positions components inside the container. Some useful methods are

A Container is the
abstract super-
class representing
all components that
can hold other
components.

void setLayout( LayoutManager mgr );
void add( Component comp );
void add( Component comp, Object where );

Layout managers are described in Section B.3.1. A container must first
define how objects in the container should be arranged. This is done by using
setLayout . It then adds the objects into the container one-by-one by using add .
Think of the container as a suitcase, in which you can add clothes. Think of the
layout manager as the packing expert who will explain how clothes are to be
added to the suitcase.

B.2.3 top-level containers

As Figure B.2 shows, there are two types of Container objects, namely

The basic contain-
ers are the top-
level Window and
JComponent . The
typical heavy-
weight compo-
nents are JWindow ,
JFrame , and
JDialog .

1. The top-level windows, which eventually reaches JFrame

2. The JComponent , which eventually reaches most other Swing

components

JFrame is an example of a “heavyweight component,” while all Swing
components in the JComponent hierarchy are “lightweight.” The basic differ-
ence between heavyweight and lightweight components is that lightweight
components are drawn on a canvas entirely by Swing whereas heavyweight
components interact with the native windowing system. As a result, light-
weight components can add other lightweight components (for instance, you
can use add to place several JButton objects in a JPanel ), but you should not
add directly into a heavyweight component. Instead you obtain a Container
representing its “content pane” and add into the content pane, thus allowing