Java Reference
In-Depth Information
Although the skeleton is quite short, it can be compiled and run. It produces an empty
window. However, it also produces the following output on the console:
When you close the window, this message is displayed on the console:
Of course, in a real program, the life-cycle methods would not normally output anything
to
System.out
. They do so here simply to illustrate when each method is called. Further-
more, as explained earlier, you will need to override the
init( )
and
stop( )
methods only if
your application must perform special startup or shutdown actions. Otherwise, you can use
the default implementations of these methods provided by the
Application
class.
Let's examine this program in detail. It begins by importing four packages. The first is
javafx.application
, which contains the
Application
class. The
Scene
class is packaged
in
javafx.scene
, and
Stage
is packaged in
javafx.stage
. The
javafx.scene.layout
package
provides several layout panes. The one used by the program is
FlowPane
.
Next, the application class
JavaFXSkel
is created. Notice that it extends
Application
.
As explained,
Application
is the class from which all JavaFX applications are derived.
JavaFXSkel
contains four methods. The first is
main( )
. It is used to launch the application
via a call to
launch( )
. Notice that the
args
parameter to
main( )
is passed to the
launch( )
method. Although this is a common approach, you can pass a different set of parameters to
launch( )
, or none at all. One other point:
launch( )
is required by a free-standing applica-
tion, but not in other cases. When it is not needed,
main( )
is also not needed. However, for
reasons already explained, both
main( )
and
launch( )
are included in the programs in this
chapter.
When the application begins, the
init( )
method is called first by the JavaFX run-time
system. For the sake of illustration, it simply displays a message on
System.out
, but it
would normally be used to initialize some aspect of the application. Of course, if no initial-
ization is required, it is not necessary to override
init( )
because an empty, default imple-
mentation is provided. It is important to emphasize that
init( )
cannot be used to create the
stage or scene portions of a GUI. Rather, these items should be constructed and displayed
by the
start( )
method.
After
init( )
finishes, the
start( )
method executes. It is here that the initial scene is cre-
ated and set to the primary stage. Let's look at this method line-by-line. First, notice that
