1. Mari Göransson - KaU - Datavetenskap - DAVD11 1 Java Event Handling --

2. Mari Göransson - KaU - Datavetenskap - DAVD11 2 Event Handling Happens every time a user interacts with a user interface. For example, when a user pushes a button, or types a character.

3. Mari Göransson - KaU - Datavetenskap - DAVD11 3 A Typical Situation: Scrollbar

4. Mari Göransson - KaU - Datavetenskap - DAVD11 4 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.

5. Mari Göransson - KaU - Datavetenskap - DAVD11 5 Abstract class EventObject getSource returns the object on which the event occured. Each subclass adds its own specialized methods.

6. Mari Göransson - KaU - Datavetenskap - DAVD11 6 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.

7. Mari Göransson - KaU - Datavetenskap - DAVD11 7 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.

8. Mari Göransson - KaU - Datavetenskap - DAVD11 8 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

9. Mari Göransson - KaU - Datavetenskap - DAVD11 9 An ActionListener

10. Mari Göransson - KaU - Datavetenskap - DAVD11 10 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.

11. Mari Göransson - KaU - Datavetenskap - DAVD11 11 A Complex Listener with Adapter

12. Mari Göransson - KaU - Datavetenskap - DAVD11 12 Three Required Parts public class MyClass implements ActionListener someComponent.addActionListener(a MyClass) public void actionPerformed(ActionEvent e){}

13. Mari Göransson - KaU - Datavetenskap - DAVD11 13 An Example

14. Mari Göransson - KaU - Datavetenskap - DAVD11 14 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.

15. Mari Göransson - KaU - Datavetenskap - DAVD11 15 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.

16. Mari Göransson - KaU - Datavetenskap - DAVD11 16 Java Event Handling and the Observer Pattern Java’s current event handling was introduced in JDK 1.1. It is a specialized version of the Observer pattern.

17. Mari Göransson - KaU - Datavetenskap - DAVD11 17 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.

18. Mari Göransson - KaU - Datavetenskap - DAVD11 18 Java Event Handling and the Observer Pattern

19. Mari Göransson - KaU - Datavetenskap - DAVD11 19 Java Event Handling and the Observer Pattern

20. Mari Göransson - KaU - Datavetenskap - DAVD11 20 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.

21. Mari Göransson - KaU - Datavetenskap - DAVD11 21 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.

22. Mari Göransson - KaU - Datavetenskap - DAVD11 22 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.

23. Mari Göransson - KaU - Datavetenskap - DAVD11 23 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.

24. Mari Göransson - KaU - Datavetenskap - DAVD11 24 AdjustmentEvent Generated when the user changes the value of the scrollbar (AWT). Has several specialized methods: –getAdjustable() –getAdjustmentType() –getValue()

25. Mari Göransson - KaU - Datavetenskap - DAVD11 25 AdjustmentEvent Listeners must implement the method adjustmentValueChanged() that is invoked when the srollbar changes its value.

26. Mari Göransson - KaU - Datavetenskap - DAVD11 26 ItemEvent Generated when the user selects another unit in the choicebox. Has some specialized methods: –getItem() –getItemSelectable() –getStateChanged()

27. Mari Göransson - KaU - Datavetenskap - DAVD11 27 The ItemEvent Structure

28. Mari Göransson - KaU - Datavetenskap - DAVD11 28 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.