Java Event Handling

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Java Event Handling

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Event Handling

• Happens every time a user interacts with a user
  interface. For example, when a user pushes a
  button, or types a character.

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A Typical Situation Scrollbar
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Event Representation

• Each event is represented by an object that gives
  information about the event and identifies the
  event source.
• In Java, every event is a subclass of EventObject.

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Abstract class EventObject

• getSource returns the object on which the event
  occured.
• Each subclass adds its own specialized methods.

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Event Sources

• Event sources are usually components, but they
  can be other kind of objects too.
• An event source can have multiple event listeners
  registered on it.
• Examples of event sources are e.g. buttons and
  windows.

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Event Listeners

• An event listener is an object that wants to be
  notified when an event has occured on a
  component.
• The common interface that all event listener
  interfaces must extend is EventListener.

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Event Listeners

• For example, to listen for ActionEvents the class
  must implement the interface ActionListener, that
  in turn is an implementation of EventListener.
• An example is shown on the next slide

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An ActionListener
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Event Adapters

• To highten the ease of use, most eventlistener
  interfaces have corresponding adapter classes
  that have all of the interface methods
  implemented.
• Must be subclassed.

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A Complex Listener with Adapter
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Three Required Parts

• public class MyClass implements ActionListener
• someComponent.addActionListener(aMyClass)
• public void actionPerformed(ActionEvent e)

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An Example
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Threads and Event Handling

• All event handling code executes in a single
  thread, the event-dispatching thread.
• Ensures that one event handler finishes before
  the next one can start.

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Components and Events

• Every component support component, focus, key,
  mouse and mouse-motion listeners.
• Fires only events for which listeners has
  registered an interest in.

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Java Event Handling and the Observer Pattern

• Javas current event handling was introduced in
  JDK 1.1.
• It is a specialized version of the Observer
  pattern.

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Java Event Handling and the Observer Pattern

• The observer pattern is used when an object wants
  to be informed of a state change in another
  object.
• In java, the observer object listens for events
  to happen.

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Java Event Handling and the Observer Pattern
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Java Event Handling and the Observer Pattern
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Java and the Observer Pattern

• Java has included the Observer-Observable pair of
  objects that concurs to the Observer pattern, it
  is therefore easy to implement the pattern
  behaviour on your own.

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Events in the Converter

• ActionEvent
• Listener must implement the interface
  ActionListener.
• ChangeEvent (Swing) or AdjustmentEvent
• Listener must implement the interface
  ChangeListener or AdjustmentListener.
• ItemEvent
• Listener must implement the interface
  ItemListener.

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ActionEvent

• Generated when the user presses return in the
  textfield.
• EventSource calls the method actionPerformed() in
  the listeners and sends the event as a parameter.

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ChangeEvent

• Generated when the user changes the slider.
• Listeners must implement the method
  stateChanged(), that is invoked when the target
  of the listener has changed its state.

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AdjustmentEvent

• Generated when the user changes the value of the
  scrollbar (AWT).
• Has several specialized methods
• getAdjustable()
• getAdjustmentType()
• getValue()

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AdjustmentEvent

• Listeners must implement the method
  adjustmentValueChanged() that is invoked when the
  srollbar changes its value.

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ItemEvent

• Generated when the user selects another unit in
  the choicebox.
• Has some specialized methods
• getItem()
• getItemSelectable()
• getStateChanged()

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The ItemEvent Structure
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ItemEvent

• The event is passed to every ItemListener object
  which registered to receive such events using the
  component's addItemListener() method.
• When an item-selection event occurs, the listener
  object's itemStateChanged() method is invoked.