Displaying a drawing - An Introduction to Graphical User Interfaces with Java Swing - page 47

Displaying a
drawing

5

In this chapter we shall learn how a program can display a drawing in a panel. For
now, the drawing is predefined and cannot be altered by the user.

To display graphics we use panels as canvases. The whole rectangular area of a
panel can be used to draw on, no matter whether it is currently visible or not.
The central means for drawing is method paintComponent of class JPanel . This
method redraws the panel. It is automatically called by the runtime system when
necessary; for example, when the GUI is resized. All the commands to draw some-
thing into the panel should therefore be put into that method. More precisely we

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override paintComponent to add our own commands.
It is important never to call paintComponent directly in order to initiate a
redrawing. To do that, one calls the repaint() -method of the panel. This is done
to avoid conflicts between redrawing and other operations of the runtime system.
A call to repaint does not immediately repaint a component. Instead it tells the
runtime system that one would like the component to be redrawn. The system
calls paintComponent after other pending actions have been completed; see also
Chapter 20.

■ 5.1 Method paintComponent

Method paintComponent of class JPanel has the following syntax:

public void paintComponent(Graphics g)

It receives a parameter g of type Graphics . This is an abstract class from the AWT
library. A Graphics object connects the Java drawing commands to the actual
drawing mechanism of the current computer. The class Graphics also provides
methods to draw into a panel. As we will never call paintComponent directly,
we do not have to construct a Graphics object; this is done by the operating
system.
To add our own drawings to a panel we extend paintComponent by adding
graphics commands. To this end we override paintComponent .Ofcourse we do