Interfaces and event handling

Interfaces

• An interface is a collection of constants and abstract methods. An interface is treated as a parent by a class which implements it. As with abstract classes, all of the abstract methods in an interface must be overridden by classes which implement the interface.
• A class may implement more than one interface. This feature gives Java some of the functionality of multiple inheritance.
• Like classes, interfaces can be used as data types.

Events

• A graphical user interface consists of Components such as menus, buttons, labels, text areas, canvases, and windows. Classes for the components are built into the AWT or Swing packages.
• Within each component, particular Events can occur in response to actions by the user. There is a class for each type of event. Among these are ActionEvent, WindowEvent, MouseEvent, ItemEvent, and KeyEvent. When the user takes an action on a component, an event object is automatically created.
• In order to respond appropriately to events, a Java program makes use of Listeners, objects which get passed events of a particular type when they occur in a component. There are different interfaces for each type of listener. Among these are WindowListener, MouseListener, ActionListener, ItemListener, and KeyListener. Each listener has astract methods which specify how the program is to respond when a particular type of event occurs. Each component type has methods to add listeners of particular types to it. When the user takes an action on a component which has a listener listening to it, the appropriate method is called in the listener with the event object as its argument.

Event handling

To implement event handling for a component:

1. Create a listener for the type of event by implementing one of the listener interfaces. The listener can be the component itself, another component which contains the component, or a separate listener object (using a defined class for this).
2. Add the listener to the component using the appropriate method, for example, addMouseListener.
3. Tell the listener how to respond to different events using the appropriate method(s), for example, actionPerformed, mouseClicked, windowOpening.
   • Because these are abstract methods in the listener interface, each must be defined in the listener class, even if nothing happens.
   • Each of these methods takes an event of the appropriate type as its single parameter. For example, mouseClicked takes MouseEvent and actionPerformed takes an ActionEvent.
   • The event parameters allow you to access features that you may need to decide how the system should respond to the event. For example, MouseEvents have methods getX() and getY() which return the x and y coordinates of the MouseEvent within the component where it occurs.
   • These methods are automatically called when an event of the relevant type happens in the component that has the listener associated with it; you don't have to call them.

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/events.html 1 Nov 2004