Swing I - Java - page 888

EXAMPLE: (continued)
In this case the argument JFrame.DO_NOTHING_ON_CLOSE is a defined constant
named DO_NOTHING_ON_CLOSE , which is defined in the JFrame class. This sets the close-
window button so that when it is clicked nothing happens (unless we programmed
something to happen, which we have not done). Other possible arguments are given in
Display 17.3.
The method setDefaultCloseOperation takes a single int argument, and each of
the constants described in Display 17.3 is an int constant. However, do not think of
them as int values. Think of them as policies for what happens when the user clicks
the close-window button. It was convenient to name these policies by int values.
However, they could just as well have been named by char values or String values or
something else. The fact that they are int values is an incidental detail of no real
importance.
Descriptions of some of the most important methods in the class JFrame are given
in Display 17.3. Some of these methods will not be explained until later in this chap-
ter. A more complete list of methods for the class JFrame is given in Appendix 5.
A JFrame can have components added, such as buttons, menus, and text labels. For
example, the following line from Display 17.2 adds the JButton object named end-
Button to the JFrame named firstWindow :
firstWindow.add(endButton);
The description of how the JButton named endButton is created and programmed
will be given in the two subsections entitled “Buttons” and “Action Listeners and
Action Events” a little later in this section.
We end this subsection by jumping ahead to the last line of the program, which is
firstWindow.setVisible( true );
This makes the JFrame window visible on the screen. At first glance this may seem
strange. Why not have windows automatically become visible? Why would you create
a window if you did not want it to be visible? The answer is that you may not want it
to be visible at all times. You have certainly experienced windows that disappear and
reappear. To hide the window, which is not desirable in this example, you would
replace the argument true with false .
JFrame
An object of the class JFrame is what you think of as a window. It automatically has a border
and some basic buttons for minimizing the window and similar actions. As you will see, a
JFrame object can have buttons and many other components added to the window and pro-
grammed for action.