Creating Windows - Beginning Java 2 SDK - page 668

Using Containers

A container is any component with the Container class as a base; so all the Swing components are
containers. The Container class is the direct base class for the Window class and it provides the ability
to contain other components. Since the Container class is an abstract class, you cannot create
instances of Container . Instead it is objects of the subclasses such as Window , JFrame , or JDialog
that inherit the ability to contain other components.

Note that a container cannot contain an object of the class Window , or an object of
any of the classes derived from Window . An object of any other class that is derived
from Component can be contained.

The components within a container are displayed within the area occupied by the container on the
display screen. A dialog box, for example, might contain a JList object offering some choices:
JCheckbox objects offering other options and JButton objects representing buttons enabling the user
to end the dialog or enter the selections - all these components would appear within the boundaries of
the dialog box. Of course, for the contained components to be visible the container must itself be
displayed, as the container effectively 'owns' its components. The container also controls how its
embedded components are laid out by means of a layout manager .

Before we look at what a layout manager is, and how the layout of the components in a container is
determined, let's look into the basic methods defined in the Container class, and therefore available
to all containers.

You can find out about the components in a container object by using the following methods defined in
the Container class:

Method Description

int getComponentCount() Returns a count of the number of components

contained by the current component.

Component getComponent
(int index)

Returns the component identified by the index
value. The index value is an array index so it must
be between 0 and one less than the number of
components contained, otherwise an
ArrayIndexOutOfBoundsException will be
thrown.

Component[]getComponents()

Returns an array of all the components in the
current container.

If we have a Container object, content , perhaps the content pane of a JFrame window, we could
iterate through the components in the Container with the following statements: