Swing Components and the Containment Hierarchy

Swing application creates four commonly used Swing components:

i)  a frame, or main window (JFrame)

ii) an applet, called JApplet

iii) a panel, sometimes called a pane (JPanel)

iv) lots of other types of containers such as JScrollPane, JTabbedPane, JInternalFrame etc

The hierarchy of classes used is as follows –

Every top-level container indirectly contains an intermediate container known as a content pane.  The content pane contains, directly or indirectly, all of the visible components in the window's GUI.

1.         Component

A component is an object having a graphical representation that can be displayed on the screen and that can interact with the user. Examples of components are the buttons, checkboxes, and scrollbars of a typical graphical user interface.  It contains basic methods such as setting the size, background/foreground colors that are associated with every GUI element.

2.         Container

A container is a component that has the ability to hold other components.  It also takes of alignment and sizing of components that are present in it.  This is done using Layout manager classes. 

3.         Window

A Window object is a top-level window with no borders and no menubar. The default layout for a window is BorderLayout.

4.         Frame

A Frame is a top-level window with a title and a border.

5.         Panel

Panel is the simplest container class. A panel provides space in which an application can attach any other component, including other panels.  The default layout manager for a panel is the FlowLayout layout manager.

6.         Applet

An applet is a small program that is intended not to be run on its own, but rather to be embedded inside another application. The Applet class must be the superclass of any applet that is to be embedded in a Web page or viewed by the Java Applet Viewer.

7.         JFrame

JFrame is an extended version of java.awt.Frame that adds support for the JFC/Swing component

architecture.  Like all other JFC/Swing top-level containers, a JFrame contains a JRootPane as its only child. The content pane provided by the root pane should, as a rule, contain all the non-menu components displayed by the JFrame.

8.         JApplet

JApplet is an extended version of java.applet.Applet that adds support for the JFC/Swing component architecture.

9.         Canvas

A Canvas component represents a blank rectangular area of the screen onto which the application can draw or from which the application can trap input events from the user.  It cannot be used to add/remove components.

The different types of swing components such as – JList, JButton etc. are derived from JComponent.

 Drawing in AWT/Swing

1.         paint ( Graphics g )

This method is called when the contents of the component should be painted; such as when the component is first being shown.  The Graphics class is the abstract base class for all graphics contexts that allow an application to draw onto components that are realized on various devices, as well as onto off-screen images.  It contains methods such as setColor, drawRect, fillOval etc. 

2.         paintComponent ( Graphics g)

Swing programs should override paintComponent() instead of overriding paint().  Each time a window needs to be redrawn, no matter what the reason, the event handler notifies the component. This causes the paintComponent methods of all components to be executed.  When a component calls its own paintComponent method, it draws in its own window.  But this may affect other components in the main component.  That’s why you need to call super.paintComponent method.