Swing II - Absolute Java - page 1054

Display 18.16 paintComponent Demonstration (part 2 of 2)

R ESULTING GUI

The program in Display 18.17 has a private instance variable wink of type boolean .
When the value of wink is false , the paint method draws an ordinary happy face.
When the value of wink is true , the paint method draws the face the same except that
the left eye is just a straight line, which looks like the eye is closed. The variable wink is
initialized to false .
When the button labeled Click for a Wink is clicked, this sends an action event
to the method actionPerformed . The method actionPerformed then changes the
value of the variable wink to true and invokes the method repaint . This use of the
method repaint is new, so let us discuss it a bit.
Every JFrame (in fact, every Component and every Container ) has a method
named repaint . The method repaint will repaint the screen so that any changes
to the graphics being displayed will show on the screen. If you omit the invocation
of repaint from the method actionPerformed , then the variable wink will change
to true , but the screen will not change. Without an invocation of repaint , the face
will not change, because the method paint must be called again with the new value of
wink before the change takes effect. The method repaint does a few standard things
and, most importantly, will also invoke the method paint , which redraws the screen.
Be sure to note that you should invoke repaint and not paint .
Now we explain why, when wink has the value true , the method paint draws the
face with the left eye changed. The relevant part of the code is the following, which
draws the left eye:

repaint

if (wink)
g.drawLine(X_LEFT_EYE, Y_LEFT_EYE,
X_LEFT_EYE + EYE_WIDTH, Y_LEFT_EYE);

else
g.fillOval(X_LEFT_EYE, Y_LEFT_EYE, EYE_WIDTH, EYE_HEIGHT);