action : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
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action

a method that is invoked when some specific type of event occurs, such as a mouse click or keystroke. It returns true if it was able to completely handle the event. If it cannot handle the event, it may invoke super.action to see if the superclass method can deal with it. If that too fails it can return false to indirectly request that the event object be given to the parent object’s handleEvent method to deal with it. Normally an action method will return false. Unless it returns false, the event percolation will stop dead, and the event will not return back to the native GUI via the original peer object.

The Action interface in Swing is a glorified ActionListener. You typically extend AbstractAction and attach it to a JMenuItem to field its ActionEvents. You can use that same same Action to field ActionEvents from JButtons. If you do an Action.setEnabled( false ), that disables all the attached widgets in sync. The actionPerformed method of an individual Action can use getActionCommand to do a Hashtable lookup to find the code to execute. Alternatively, you can use the first character of the actionCommand as a case index. Alternatively, you can embed a binary 16-bit unsigned integer as the first character of an actionCommand and use that as an array index. Typically you use one Action per JMenuItem, and redirect all (possibly predigested) ActionEvents to a common ActionListener for dispatching.

Why use Actions?

You don’t need yet another inner class for every ActionListener, just another Action object. Inner classes have a 3K RAM overhead each.
You can use the same event handling logic for both a JButton and a JMenuItem. If you are going to have a menu item, a button on a UI screen, and a button on a toolbar all do the same thing, you have one and only one method that they all call. There is no chance of them getting out of sync. You could do this without Actions by creating a common ActionListener, but now it is more convenient.
You can enable or disable a collection of widgets that share a common Action with one call to action.setEnabled ( true ). You can change properties of all attached widgets with a single call, e.g.
```
action.putValue( "ToolTipText", "Fermez le fichier" );
```
Terser code. new JButton( Action ) or new JMenuItem( Action ) creates the JButton/JMenuItem, using properties in the Action (Text, Icon, Enabled, ToolTipText, ActionCommand, and Mnemonic), and attaches the Action to the JButton/JMenuItem as an ActionListener, and attaches the JButton/JMenuItem to the Action as a PropertyChangeListener.

Why not use Actions?

If you spell one of the properties incorrectly, the compiler won’t complain and neither will the runtime. Your property will just be ignored.
Not all methods of JButton and JMenuItem are supported via Action. You will then have your logic split between the Action and the JButton/ JMenu.

Learning More

Sun’s Javadoc on the Swing Action interface class : available:

on the web at java.Sun.com
in the JDK 1.6.0_06 or in the older JDK 1.5.0_15 or the even older JDK 1.4.2_17 on your local J: drive.

Sun’s tutorial on Action

ActionListener
event
JavaDeskTop Action Tutorial
trampoline

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