Java Reference
In-Depth Information
EXAMPLE:
A Simple Window
Display 17.2 contains a Swing program that produces a simple window. The window
contains nothing but a button on which is written
"Click to end program."
If
the user follows the instructions and clicks the button with his or her mouse, the
program ends.
The
import
statements give the names of the classes used and which package they
are in. What we and others call the
Swing library
is the package named
javax.swing
.
The
AWT library
is the package
java.awt
. Note that one package name contains an
"x"
and one does not.
This program is a simple class definition with only a
main
method. The first line
in the
main
method creates an object of the class
JFrame
. That line is reproduced as
follows:
JFrame firstWindow =
new
JFrame();
This is an ordinary declaration of a variable named
firstWindow
and an
invocation of the no-argument constructor for the class
JFrame
. A
JFrame
object is a
basic window that includes a border and the usual three buttons for minimizing the
window down to an icon, changing the size of the window, and closing the window.
These buttons are shown in the upper-right corner of the window, which is typical,
but if your operating system normally places these buttons someplace else, that is
where they will likely be located in a
JFrame
on your computer.
The initial size of the
JFrame
window is set using the
JFrame
method
setSize
,
as follows:
firstWindow.setSize(WIDTH, HEIGHT);
In this case,
WIDTH
and
HEIGHT
are defined
int
constants. The units of measure
are pixels, so the window produced is 300 pixels by 200 pixels. (The term
pixel
is
defined in the box entitled “Pixel.”) As with other windows, you can change the size
of a
JFrame
by using your mouse to drag a corner of the
JFrame
window.
The buttons for minimizing the window down to an icon and for changing the
size of the window behave as they do in any of the other windows you have used. The
minimization button shrinks the window down to an icon. (To restore the window,
click the icon.) The second button changes the size of the window back and forth
from full screen to a smaller size. The close-window button can behave in different
ways depending on how it is set by your program.
The behavior of the close-window button is set with the
JFrame
method
setDefaultCloseOperation.
The line of the program that sets the behavior of the
close-window button is reproduced next:
firstWindow.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
(continued)