Java Reference
In-Depth Information
Figure 7-8.
A font's ascent and descent are relative to its baseline.
Everyfontisassociatedwithvariousmeasurements.The
ascent
isthatportionofthe
font'scharactersabovethebaseline;the
descent
isthatportionofthesecharactersbelow
thebaseline.Extraspaceaddedbetweenlinesoftextisknownas
leading
.Whenadded
together,ascent,descent,andleadingformthefont's
height
.Lastly,the
advance
roughly
specifies the baseline location where the next character should appear.
AWT's
java.awt.FontMetrics
class encapsulates this measurement inform-
ation. You can obtain an instance of this class by calling the
Graphics
class's
FontMetrics getFontMetrics()
method, which returns the font metrics for
the current font. Among its various methods, you will find the
int
stringWidth(String str)
method (which returns the total advance width for
showing
str
's characters in the current font) useful for centering a string horizontally.
Althoughyoucanpaintonanycomponent(includingacontainer)bysubclassingthe
component classandoverriding
paint()
,youshouldtrytoavoiddoingso,toavoid
confusingtheuserorsomeonewho'sreviewingyourcode.Instead,youshouldtakead-
vantage of AWT's
Canvas
class, which is intended for this purpose.
Touse
Canvas
,youmustextendthisclassandoverride
paint()
.Youalsoneedto
specifyitspreferredsizesothatyoucanviewthecanvasonthescreen.Accomplishthis
task by overriding
getPreferredSize()
to return a
Dimension
object contain-
ingthecanvas'sextents,orbyinvoking
setPreferredSize()
witha
Dimension
object containing the preferred size (as demonstrated in
fixGUI()
).
I'vecreateda
Geometria
applicationthatdemonstrates
Canvas
.(Although
Geo-
metria
is just a skeleton that presents a
Canvas
-based splash-screen component, it
couldbeturnedintoafull-blownapplication forteaching basicgeometry.)
Listing7-3
excerpts this application's
SplashCanvas
class.
Listing 7-3
. Creating a splash screen
