The JavaTM Tutorial

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Trail: Creating a GUI with JFC/Swing
Lesson: Using Other Swing Features

## How to Use Actions

> If you have two or more components that perform the same function, consider using an `Action` object to implement the function. An `Action` object is an `ActionListener` that provides not only action-event handling, but also centralized handling of the text, icon, and enabled state of tool bar buttons or menu items. By adding an `Action` to a `JToolBar`, `JMenu`, or `JPopupMenu`, you get the following features:
> - A new `JButton` (for `JToolBar`) or `JMenuItem` (for `JMenu` and `JPopupMenu`) that is automatically added to the tool bar or menu. The button or menu item automatically uses the icon and text specified by the `Action`.
> - A registered action listener (the `Action` object) for the button or menu item.
> - Centralized handling of the button or menu item's enabled state.
> Here's an example of using an `Action` to create a tool-bar button and menu item that perform the same function:

> >     Action leftAction = new <a class that implements Action>(...);
> >     JButton button = toolBar.add(leftAction);
> >     JMenuItem menuItem = mainMenu.add(leftAction);

> For a button or menu item to get the full benefit of using an `Action`, you must create the component using the `add(Action)` method of `JToolBar`, `JMenu`, or `JPopupMenu`. Currently, no API beyond `addActionListener(ActionListener)` exists to connect an `Action` to an already existing component. For example, although you can add an `Action` object as an action listener to any button, the button won't be notified when the action is disabled.

> To create an `Action` object, you generally create a subclass of `AbstractAction` and then instantiate it. In your subclass, you must implement the `actionPerformed` method to react appropriately when the action event occurs. Here's an example of creating and instantiating an `AbstractAction` subclass:

> >     leftAction = new AbstractAction("Go left",
> >                                     new ImageIcon("images/left.gif")) {
> >         public void actionPerformed(ActionEvent e) {
> >             displayResult("Action for first button/menu item", e);
> >         }
> >     };

> Here's a picture of a demo application that uses actions to implement three features.

> > Try this:
> > 1. Compile and run the application. The source file is `ActionDemo.java`. You will also need three image files.
> >    See Getting Started with Swing if you need help compiling or running this application.
> > 2. Choose the top item from the left menu (Menu > Go left).
> >    The text area displays some text identifying both the event source and the action listener that received the event.
> > 3. Click the leftmost button in the tool bar.
> >    The text area again displays information about the event. Note that although the source of the events is different, both events were detected by the same action listener: the `Action` object with which the components were created.
> > 4. Choose the top item from the Action State menu.
> >    This disables the "Go left" `Action` object, which in turn disables its associated menu item and button.

> Here is what the user sees when the "Go left" action is disabled:

> Here's the code that disables the "Go left" action:

> >     boolean selected = ...//true if the action should be enabled;
> >                           //false, otherwise
> >     leftAction.setEnabled(selected);

> After you create components using an `Action`, you might well need to customize them. For example, you might want to set the tool-tip text for a button. Or you might want to customize the appearance of one of the components by adding or deleting the icon or text. For example, `ActionDemo.java` has no icons in its menus, no text in its buttons, and tool tips for its buttons. Here's the code that accomplishes this:

> >     button = toolBar.add(leftAction);
> >     button.setText(""); //an icon-only button
> >     button.setToolTipText("This is the left button");
> >     menuItem = mainMenu.add(leftAction);
> >     menuItem.setIcon(null); //arbitrarily chose not to use icon in menu

### The Action API

> The following tables list the commonly used `Action` constructors and methods. The API for using `Action` objects falls into two categories:
> - Creating and Using an Action
> - Creating an Action-Controlled Component

> Creating and Using an Action
> Constructor or Method | Purpose
> --- | ---
> `AbstractAction() AbstractAction(String) AbstractAction(String, Icon)` | Create an `Action` object. Through arguments, you can specify the text and icon to be used in the components that the action controls.
> `void setEnabled(boolean) boolean isEnabled()` | Set or get whether the components the action controls are enabled. Invoking `setEnabled(false)` disables all the components that the action controls. Similarly, invoking `setEnabled(true) enables the action's components.`

> Creating an Action-Controlled Component
> Method | Purpose
> --- | ---
> `JMenuItem add(Action) JMenuItem insert(Action, int)` (in `JMenu` and `JPopupMenu`) | Create a `JMenuItem` object and put it in the menu or popup menu. See the discussion in this section and in How to Use Menus for details.
> `JButton add(Action)` (in `JToolBar`) | Create a `JButton` object and put it in the tool bar. See the discussion in this section and in How to Use Tool Bars for details.

### Examples that Use Actions

> The following examples use `Action` objects.

> Example | Where Described | Notes
> --- | --- | ---
> `ActionDemo` | This section | Uses actions to bind buttons and menu items to the same function.
> `TextComponentDemo` | General Rules for Using Text Components | Uses text actions to create menu items for text editing commands, such as cut, copy, and paste, and to bind key strokes to caret movement. Also implements custom `AbstractAction` subclasses to implement undo and redo. The text action discussion begins in Concepts: About Editor Kits.

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