Building graphical user interfaces - Objects First with Java: A Practical Introduction Using BlueJ - page 376

11.4.6 Centralized receipt of events

In order to make our ImageViewer object be the single listener to all events from the menu, we
have to do three things:

1 We must declare in the class header that it implements the ActionListener interface.

2 We have to implement a method with the signature

public void actionPerformed(ActionEvent e)

This is the only method declared in the ActionListener interface.

3 We must call the addActionListener method of the menu item to register the

ImageViewer object as a listener.

Numbers 1 and 2—implementing the interface and defining its method—ensure that our object
is a subtype of ActionListener . Number 3 then registers our own object as a listener for the
menu items. Code 11.3 shows the source code for this in context.

Code 11.3
Adding an action
listener to a menu item

public class ImageViewer
implements ActionListener

{

// Fields and constructor omitted.

public void actionPerformed(ActionEvent event)
{

System.out.println( "Menu item: " + event.getActionCommand());

}

/**
* Create the Swing frame and its content.
*/
private void makeFrame()
{

frame = new JFrame( "ImageViewer" );
makeMenuBar(frame);

// Other GUI building code omitted.
}

/**
* Create the main frame's menu bar.
* @param frame The frame the menu bar should be added to.
*/
private void makeMenuBar(JFrame frame)
{

JMenuBar menubar = new JMenuBar();
frame.setJMenuBar(menubar);