Event Handling Techniques

Question

Hi All,
     I have encountered some code which takes a somewhat different approach to event handling that I have used previously.
This is a basic example of the principles:
private final static String = "CMD_ONE";
private final static String = "CMD_TWO";
private final static String = "CMD_THREE";
EventHandler eventHanlder = new EventHandler();
JButton one = new JButton();
one.addActionCommand(CMD_ONE);
one.addActionListener(eventHandler);
JButton two = new JButton();
two.addActionCommand(CMD_TWO);
two.addActionListener(eventHandler);
JButton three = new JButton();
three.addActionCommand(CMD_THREE);
three.addActionListener(eventHandler);
class EventHandler implements ActionListener
     public void actionPerformed(ActionEvent ev)
          if(ev.getActionCommand().compareTo(CMD_ONE) = 0)
               System.out.println("User hit the ONE button");
          if(ev.getActionCommand().compareTo(CMD_TWO) = 0)
               System.out.println("User hit the TWO button");
          if(ev.getActionCommand().compareTo(CMD_THREE) = 0)
               System.out.println("User hit the TWO button");
My question is what could the motivation for this approach be? Is it to save on memory by having fewer EventListener objects instantiated (presumably).
Is this necessarily a better approach than having a dedicated EventListener for each separate component.
I always thought that the idea was to minimise the amount of processing for any event so that the UI was very reactive to user input (by having dedicated EventListeners). I can see that this approach could suffer
in a situation with the EventHandler being responsible for many components i.e. Having to evaluate a big list of expressions because of the number of delegated commands.
I would very much like to know if anyone has any strong views on this or if anyone knows the Sun guidance on this matter.
Thanks for any replies,
Jonathan.

Answer 1

I would recommend creating a single listener and add it to every label added to the graph. If you're interested in events that occur outside of the labels (in the background), add a listener to the container of the labels as well.
Unfortunately, you cannot simply add a listener to the container and get all the mouse events from the container's children because Swing's event dispatching system will only dispatch mouse events to a container if the click location was not within any of the container's children. This means that mouse listeners registered to a container will only receive mouse events within the container's background.