Java Reference
In-Depth Information
colors.add(greenItem);
colors.add(blueItem);
Now our
Color
menu checks are set automatically so we can forget about them.
Using Actions
One difficulty with the code that we have added to support the menus is that it is very menu specific.
What I mean by this is that if we are going to do a proper job on the Sketcher application, we will
undoubtedly want it to have a toolbar. The toolbar will surely have a whole bunch of buttons that
perform exactly the same actions as the menu items we have just implemented, so we will be in the
business of doing the same thing over again in the toolbar context. Of course, the only reason I brought
it up, as I'm sure you anticipated, is that there is another way of working with menus, and that is to use
an
action
object.
An action object is a bit of a strange beast, and it can be quite hard to understand at first so we will take
it slowly. First of all let's look at what we mean by an 'action' here, as it is a precise term in this context.
An action is an object of any class that implements the
Action
interface. This interface declares
methods that operate on an action object, for example storing properties relating to the action, enabling
it and disabling it. The
Action
interface happens to extend the
ActionListener
interface so an
action
object is a listener as well as an action. Now that we know an
Action
object can get and set
properties, and is also a listener, how does that help us in implementing the Sketcher GUI?
The answer is in the last capability of an
Action
object. Some Swing components, such as those of type
JMenu
and
JToolBar
, have an
add()
method that accepts an argument of type
Action
. When you add
an
Action
object to these using the
add()
method, the method creates a component from the
Action
object that is
automatically
of
the right type
. If you add an
Action
object to a
JMenu
object, a
JMenuItem
will
be created and returned by the
add()
method. On the other hand, when you add exactly the same
Action
object to a
JToolBar
object, an object of type
JButton
will be created and returned. This means that you
can add the very same
Action
object to both a menu and a toolbar, and since the
Action
object is its own
listener you automatically get both supporting the same action. Clever, eh?
First, we should look at the
Action
interface.
The Action Interface
In general, properties are items of information that relate to a particular object and are stored as part of
the object. Properties are often stored in a map, where a key identifies a particular property, and the
value corresponding to that property can be stored in association with the key. The
Properties
class
that is defined in the
java.util
package does exactly that. The
Action
interface has provision for
storing seven basic standard properties that relate to an
Action
object:
A
name -
a
String
object that is used as the label for a menu item or a toolbar button.
❑
A
small icon
- an
Icon
object to be displayed on a toolbar button.
❑
A
short description
of the action - a
String
object to be used as a tooltip.
❑
An
accelerator key
for the action - defined by a
KeyStroke
object.
❑
