Events, Event Sources, and Event Listeners

• User interface events include key presses, mouse moves, button clicks, and so on

• Most programs don't want to be flooded by boring events

• A program can indicate that it only cares about certain specific events

• Event listener:

  • Notified when event happens

  • Belongs to a class that is provided by the application programmer

  • Its methods describe the actions to be taken when an event occurs

  • A program indicates which events it needs to receive by installing event listener objects

• Event source:

  • Event sources report on events

  • When an event occurs, the event source notifies all event listeners

• Example: Use JButton components for buttons; attach an ActionListener to each button

ActionListener interface:
 public interface ActionListener
 {
    void actionPerformed(ActionEvent event);
 }

• Need to supply a class whose actionPerformed method 
   contains instructions to be executed when button is clicked

• event parameter contains details about the event, such as the
   time at which it occurred

• Construct an object of the listener and add it to the button:  
   ActionListener listener = new ClickListener();  
   button.addActionListener(listener);

Click here for an example.

Which objects are the event source and the event listener in the ButtonViewer program?

   Answer: The button object is the event source. The listener  object is the event listener.

Why is it legal to assign a ClickListener object to a variable of type ActionListener?

   Answer: The ClickListener class implements the ActionListener interface.