Action Listener Review

• Many components call an action listener in response to a user action (JButton, JMenuItem, JTextField, etc). An action listener is any object that implements the ActionListener interface.
• The ActionListener interface requires that the class define an actionPerformed(ActionEvent e) method.
• Call the component's addActionListener(...) method, passing a parameter which is an object implementing ActionListener.
• Common ways to implement action listeners follow.

1. One shared action listener (this) - Not great.

The class which defines the graphical user interface implements ActionListener. The component uses this for the action listener.

myButton.addActionListener(this);

Example p 234.

This doesn't scale -- what if there is more than one component that needs an action listener?

2. Anonymous inner class listener (so-so)

JButton b1 = new JButton("Hello");
b1.addActionListener(
    new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            // do something for button b1
        }
    }
);

• Convenient.
• Often mixes listeners into other code. (bad)
• Can't share listeners.

3. Named inner class listener (better)

ActionListener myHelloListener = new HelloListener():

JButton b1 = new JButton("Hello");
b1.addActionListener(myHelloListener);
. . .
class HelloListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        // do something for button b1
    }
}

• Relatively convenient.
• Separates listeners from other code. (good)
• Can easily share listeners.

4. Subclassing AbstractActionListener (very good)

AbstractAction exitAction = new AbstractAction() {
    public void actionPerformed( ActionEvent e ) {
        if (gCurrentSkeema.isDirty()) {
            int response = JOptionPane.showConfirmDialog(xc_window, 
                                       "Exit without saving changes?");
            if (response == JOptionPane.YES_OPTION) {
                System.exit(0);
            }
        } else {
            System.exit(0);  // Not dirty, can quit.
        }
    }
};

More on this later.

End