Objectives of This Session

2. Objectives of This Session • Explain the Event handling mechanism & demonstrate it using example • List the categories of Events & Listener interfaces • Demonstrate the above example using an independent event handler class • Identify the need for adapter classes & demonstrate it using an independent event handler example

3. Event Handling • GUI applications are event-driven applications. • They generate events when the user of the program interacts with the GUI. • The underlying OS is constantly monitoring these events. • When a event occurs, the OS reports these events to the programs that are running. • The application will handle the event using a appropriate “Event Handler”.

4. Delegation Event Model • Source generates an event & sends it to one or more listeners. • Listener simply waits until it receives an event. • Once received, listener processes the event & returns. • Advantage : application logic that processes event is clearly separated from the UI logic that generates those events. • An UI element is able to delegate the processing of an event to a separate piece of code ( Event handler)

5. Delegation Event Model • Event: is an object that describes a state change in a source. • Event Source: is an object that generates an event. • A source must register listeners for listeners to receive notifications about a specific type of event. • Event Listener : is an object that is notified when an event occurs. • It must be registered with a source. • It must implement methods to receive & process these notifications.

6. Event Handling import java.awt.*; import java.awt.event.*; class MyFrame extends Frame implements ActionListener { TextField t1; Button b1; MyFrame() { t1 = new TextField(20); b1 = new Button(“Click”); add(t1,”North”); b1.addActionListener(this); add(b1,”South”); }

7. Event Handling public void actionPerformed( ActionEvent e ) { t1.setText(“Click pressed”); } public static void main(String args[]) { Frame f = new MyFrame(); f.setSize(100,100); f.show(); } }

8. Event Object AWT Event Action Event Adjustment Event Component Event Item Event Text Event Container Event Focus Event Input Event Paint Event Window Event Key Event Mouse Event Inheritance diagram of the AWT Event Classes

9. Event Listener Interfaces ActionListener AdjustmentListener ComponentListener ContainerListener FocusListener java.util.EventListener ItemListener KeyListener MouseListener MouseMotionListener TextListener WindowListener

10. Event Handling Summary

11. Event Handling Summary

12. Event Handling Summary

13. Delegation Event Model with Another Class import java.awt.*; class MyFrame extends Frame { TextField t1; Button b1; MyFrame() { t1 = new TextField(20); b1 = new Button(“Click”); add(t1,”North”); b1.addActionListener(new ButtonHandler(this)); add(b1,”South”); }

14. Delegation Event Model with Another Class class ButtonHandler implements ActionListener { MyFrame f ; ButtonHandler(MyFrame mf) { f=mf; } public void actionPerformed( ActionEvent e ) { // code for processing button press. } }

15. Adapter Classes • Many of the listener interfaces have more than one method. • Thus, if a particular interface is implemented, all the methods of that interface should also be implemented. • To simplify this task, listener interfaces with more than one method come with adapter classes. • Adapter classes implement all the methods of an interface. • You can extend the adapter class to specify the desired reaction to some methods.

16. Without Adapter Classes class MyFrame extends Frame implements WindowListener { public void windowActivated(WindowEvent e){} public void windowDeactivated(WindowEvent e){} public void windowIconified(WindowEvent e){} public void windowDeiconified(WindowEvent e){} public void windowOpened(WindowEvent e){} public void windowClosed(WindowEvent e){} public void windowClosing(WindowEvent e){ System.exit(0); }

17. Without Adapter Class public static void main(String args[]) { Frame f = new MyFrame(); f.setSize(100,100); f.show(); f.addWindowListener(f); } }

18. Using Adapter Class import java.awt.*; import java.awt.event.*; class MyFrame extends Frame { MyFrame() { addWindowListener(new WindowHandler()); } }

19. Adapter Classes class WindowHandler extends WindowAdapter { { public void windowClosing(WindowEvent e) { System.exit(0); } }