Java Reference
In-Depth Information
PITFALL: Forgetting to Invoke
setDefaultCloseOperation
If you register a window listener to respond to window events from a
JFrame,
you
should also include an invocation of the method
setDefaultCloseOperation,
typically in the
JFrame
constructor. This is because the default or other behavior set
by
setDefaultCloseOperation
takes place even if there is a window listener. If you
do not want any actions other than those provided by the window listener(s), you
should include the following in the
JFrame
constructor:
setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
If you do not include any invocation of
setDefaultCloseOperation,
the default
action is the same as if you had included the invocation
setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);
which hides the
JFrame
when the close-window button is clicked. (The actions of any
registered window listener are also performed.)
■
The
WindowAdapter
Class
In Display 18.2, we gave empty bodies to most of the method headings in the
WindowListener
interface. The abstract class
WindowAdapter
is a way to avoid all
those empty method bodies. The class
WindowAdapter
does little more than provide
trivial implementations of the method headings in the
WindowListener
interface. So, if
you make a window listener a derived class of the class
WindowAdapter,
then you have
only to define the method headings in the
WindowListener
interface that you need.
The other method headings inherit trivial implementations from
WindowAdapter.
(
WindowAdapter
is unusual in that it is an abstract class with no abstract methods.)
For example, in Display 18.3 we have rewritten the inner class
CheckOnExit
from
Display 18.2, but this time we made it a derived class of the
WindowAdapter
class. This
definition of
CheckOnExit
is much shorter and cleaner than the one in Display 18.2,
but the two implementations of
CheckOnExit
are equivalent. Thus, you can replace the
definition of
CheckOnExit
in Display 18.2 with the shorter one in Display 18.3 . The file
WindowListenerDemo2
on the accompanying website contains a version of Display 18.2
with this shorter definition of
CheckOnExit
.
The class
WindowAdapter
is in the
java.awt.event
package and so requires an
import
statement such as the following:
extra code
on website
import
java.awt.event.WindowAdapter;
You cannot always define your window listeners as derived classes of
Windowdapter.
For example, suppose you want a
JFrame
class to be its own window listener. To
accomplish this, the class must be a derived class of
JFrame
and so cannot be a derived
