Swing I - Absolute Java - page 946

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; 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
chapter. 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
endButton 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 programmed for action.