Java Reference
In-Depth Information
Calling
getValue()
with the argument as the
NAME
key that is defined in the
Action
interface
returns the
String
object that was stored for the
FileAction
object when it was created. If the name
for the current object matches that of a particular
FileAction
object, then that must be the action to
which the event applies, so we know what to do. We have one
if
or
else-if
block for each action,
and we will code these one by one.
Many of these operations will involve dialogs. We need to get at the file system and display the list of
directories and files to choose from, for an Open operation for instance. It sounds like a lot of work, and
it certainly would be, if it weren't for a neat facility provided by the
JFileChooser
class.
Using a File Chooser
The
JFileChooser
class in the
javax.swing
package provides an easy to use mechanism for
creating file dialogs for opening and saving files. You can use a single object of this class to create all the
file dialogs you need, so add a member to the
SketchFrame
class to store a reference to a
JFileChooser
object that we will create in the constructor:
private JFileChooser files; // File chooser dialog
There are several
JFileChooser
constructors but we will discuss just a couple of them here. The
default constructor creates an object with the current directory as the default directory but that won't
quite do for our purposes. What we want is for the default directory to be the
DEFAULT
_
DIRECTORY
,
which we defined in the
Constants
interface, so we'll use the constructor that accepts a
File
object
specifying a default directory to create a
JFileChooser
object in the
SketchFrame
constructor. Add
the following statement to the constructor following the statements that we added earlier that ensured
DEFAULT
_
DIRECTORY
actually existed on the hard drive:
files = new JFileChooser(DEFAULT
_
DIRECTORY);
Any dialogs created by the
files
object will have
DEFAULT
_
DIRECTORY
as the default directory that
is displayed. We can now use the
files
object to implement the event handling for the
F
ile
menu.
There are a considerable number of methods in the
JFileChooser
class, so rather than trying to
summarize them all, which would take many pages of text and be incredibly boring, let's try out the
ones that we can apply to Sketcher to support the
F
ile
menu.
File Save Operations
In most cases we will want to display a modal file save dialog when the
S
ave
menu item or toolbar
button is selected. As luck would have it, the
JFileChooser
class has a method
showSaveDialog()
that does precisely what we want. All we have to do is pass it a reference to the
Component
object that
will be the parent for the dialog to be displayed. The method returns a value indicating how the dialog
was closed. We could display a save dialog in a
FileAction()
method with the statement:
int result = files.showSaveDialog(SketchFrame.this);
